cluster

The only time it's safe to put a star atop your tree

esiegel — Tue, 24 Dec 2013 13:38:36 +0000

The only time it's safe to put a star atop your tree

"Bethany: Is your house on fire, Clark?
Clark: No, Aunt Bethany, those are the Christmas lights." -National Lampoon's Christmas Vacation

Ahh, Christmas. It's easy to forget how much the invention of the light bulb in 1879 reduced the number of tree fires in people's homes. It was a mere three years later that people began decorating Christmas trees with strings of lights instead of candle flames, and as you can imagine, the reduction in open flames atop fresh kindling had its benefits, and caught on like wildfire. Trees now routinely sport previously unfathomable numbers of lights, limited only by your imagination and your (optional) good taste.

Image credit: National Park Service, via http://nps.gov/.

But what about putting a star atop one of these trees?

I don't mean one of those pretend, cheap, Earth-built stars; I mean the real deal. You know, a star: a massive, luminous sphere of plasma held together by its own gravity and held up by a fantastic amount of outward radiation pressure!

Image credit: NASA's Goddard Space Flight Center / SDO.

And if you decide to put one of those on top of a tree... well... if something like this happens, it's your own fault!

Image credit: Andrew Walsh, aka flickr user radiofree.

But there is at least one place in our galaxy where it's not only safe but also beautiful to place a star atop your Christmas tree, and -- with the right equipment -- you can see it for yourself this holiday season. Shortly after sunset tonight and all through the majority of the coming winter nights, you can find the great winter constellation Orion. If you draw an imaginary line from the "highest" stars of the hunter -- Bellatrix to Betelgeuse and well beyond -- you'll eventually come to the prominent star Gomeisa, just a little north of the brilliant Procyon.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Well, if you go to the place that's halfway between Gomeisa and Betelgeuse, you can move from that location towards ξ Geminorum, another prominent naked-eye star. And midway on your journey there, you can find the much less prominent (but still naked-eye) star S Monocerotis, a very (intrinsically) bright-and-blue star. If you view it through a telescope or very good binoculars, you should be able to find a spectacular sight for the holiday season: the Christmas Tree cluster!

Image credit: Alicia, aka flickr user capella_891.

Also known as NGC 2264, this collection of bright blue stars lies some 2,600 light-years away, and the very bright, blue star at the base is the aforementioned S Monocerotis, which is actually a supergiant variable star system some 8,000 times as luminous as the Sun! A rare O-class star -- the brightest and hottest of all stellar classes -- this will someday go supernova, likely outshining all other stars and planets in the night sky when it does.

It's also the base of the Christmas Tree, which has a beautiful blue star (HD 47887) and a little something extra as a tree-topper.

Image credit: Hap Griffin via http://www.machunter.org/hap_ngc2264.html.

The cluster is a collection of around 600 bright, new stars, and it's full of nebulosity in the form of neutral-and-ionized hydrogen, which gives it its characteristic combined blue-and-red hues. The ionized hydrogen, when it recombines to form neutral atoms, emits a characteristic red line at 656.3 nanometers, which explains the faint red hue. There's also some neutral dust that reflects the bright blue starlight of S Monocerotis, which is where the blue color comes from!

But you'll notice something unique at the top of the proverbial tree topper, and it's the other famous part about NGC 2264: the Cone Nebula!

Image credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA.

This nebula is actually a molecular gas cloud giving rise to newly forming stars, in the process of being evaporated from the intense stellar heat both coming from the interior -- due to the protostars inside it -- as well as from the ultraviolet radiation coming from the surrounding young, hot stars!

There's also a much larger molecular cloud complex at work here, which observations that specifically look for the ionized hydrogen signature can really bring out spectacularly.

Image credit: T.A. Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A. Wolpa (NOAO/AURA/NSF).

If you're having difficulty seeing the tree shape, I've taken the liberty of outlining it for you below using the ESO image featured at the beginning, below. (And if you really want an insanely large version, give this link a shot!)

Image credit: European Southern Observatory (ESO).

The most unique view you're likely to see, however, is one that your eyes will never pick up, and that's of what's present in the infrared! At wavelengths longer than the human eye can see, we find warm-and-cold dust, some of which is present in tendrils a mere hundred thousand years old!

Image credit: NASA / JPL-Caltech / P.S. Teixeira (CfA), MIPS / IRAC.

Can you find the tree-topper star and the Cone Nebula in there?

I've sifted through the image and pulled it out for you, and even though it's a glorious sight, the most surprising thing in there is not to be found in the nebula itself, but in the sheer number of stars and protostars actually present in this object, which clearly extends well into the thousands from views of this small region alone!

Image credit: NASA / JPL-Caltech / P.S. Teixeira (CfA), MIPS / IRAC.

So this Christmas, don't be afraid of putting an Earth-based star atop your tree, but if you've got aspirations for a real star atop one, do us all a favor and stick to the one atop the Christmas Tree cluster! Happy holidays, all, and I'll see you back here after Christmas!

esiegel Tue, 12/24/2013 - 08:38

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Random Stuff

Stars

Happy holidays Ethan, all the best!

Ethan rules!
Thank you!

Thanks for an excellent year of gems, Ethan.
Seasons greetings to all.

Messier Monday: The 'Little Sister' Cluster, M46

esiegel — Mon, 23 Dec 2013 17:03:47 +0000

Messier Monday: The 'Little Sister' Cluster, M46

"A sister is both your mirror - and your opposite." -Elizabeth Fishel

With 110 deep-sky wonders to choose from in the Messier catalogue, our long-running series on Messier Monday promises to keep us busy for some time to come! As we've finally passed the winter solstice here in the Northern Hemisphere, many new spectacular sights await skygazers in the early part of the night. As it's also the 1-year anniversary of when we adopted a little sister for our dog from the local humane society, I thought it would only be fitting to highlight the little sister to last week's Messier object.

Image credit: Greg Scheckler (http://gregscheckler.wordpress.com/blog/) with telescopy by Slooh.com.

Last week, we took a look at Messier 47, a relatively nearby, young open star cluster that rises shortly after sunset at this time of year, trailing a little bit behind Sirius, the brightest star in the night sky. But just a single degree away is this week's object -- Messier 46 -- which appears at first glance to be a dimmer, less spectacular 'sister' cluster.

Is that the truth? Let's show you how to discover it and then find out together!

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

As we approach the new year, Orion can be seen flying higher in the sky earlier and earlier in the night, trailed by Sirius and -- a little bit later (and lower) -- the slightly more southerly bright pair Adhara and Wezen. Right next to Sirius is the quite bright (at magnitude +2) star Mirzam.

If you draw an imaginary line from Mirzam to Sirius and another from Adhara to Wezen and extend them both, where they meet is approximately where Messier 46 lies.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

There are really only about three stars in that small region that are clearly visible to the naked eye, but through binoculars or a telescope, there are quite a few deep-sky objects in that region of sky. This is no surprise for a location that looks into the galactic plane like this, and the dominant deep-sky object is last week's Messier 47. Just a little bit south of that cluster, however, between the naked-eye stars 4 Puppis and HIP 37379, lies a dimmer, smaller cluster: this week's object, Messier 46!

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

It's easy to overlook this cluster when compared to the bright, blue beauties just to the north of it, but if you take a closer look at what might -- at first glance -- appear to be a non-spectacular object, you'll be glad that you did.

Image credit: W. Garrett Grainger, Jr. of http://www.beachastronomy.com/.

Standing out clearly against the galactic backdrop, these stars may all appear faint, but are they really? Messier described them so:

A cluster of very small stars, between the head of the Great Dog and the two hind feet of the Unicorn... one cannot see these stars but with a good refractor; the cluster contains a bit of nebulosity.

But even though Messier was wrong about this cluster in a whole slew of ways, it's easy to understand when you look back with 242 years of hindsight since its discovery.

Image credit: Kfir Simon of Pbase, at http://www.pbase.com/image/133973249.

Although it might seem wimpy compared to it's brighter sister cluster in the sky, Messier 46 isn't made up of small stars, nor does it have any nebulosity inside of it. Instead, it's one of the most distant open clusters in the Messier catalogue, located over 5,000 light-years away!

And one of the main reasons it appears to have so many stars prominently displayed in there is because the brightest classes of stars -- the O and B stars -- have all run through their entire life cycle and died by this point.

Image credit: Unknown, retrieved from http://www.docdb.net/show_object.php?id=ngc_2437.

That leaves a cluster dominated by A-class stars, which are still blue and bright, but much less so than its shorter-lived brethren. All clusters, when they're born, are thought to contain stars of all types, with the brightest and most massive stars running out of fuel fastest. From the stars that still remain in M46, we can tell it's about 300 million years old, an intermediate age for a star cluster.

A closer inspection -- one that brings out more colors and the fainter stars -- can reveal that there are around 500 stars identifiable in this cluster, including many that are dimmer and redder (in addition to a small number of giant stars) than the Sirius-like A-stars.

Image credit: Roy Uyematsu of http://www.roystarman.com/.

There could, of course, be even more stars located in this 26-light-year-wide spherical region; infrared observations -- as they often do -- highlight a somewhat different stellar population than what we see with visible light images.

Image credit: Two-Micron All-Sky Survey (2MASS) / IPAC.

In addition, you may be noticing what looks like a familiar shape: a ringed nebula around one of the stars! In fact, there is a planetary nebula there, a remnant of a dying sun-like star going through its death throes, blowing off its outer layers and contracting its core down to a white dwarf.

But, is it connected to the cluster, or is it merely a foreground object?

Image credit: Daniel López, IAC.

It's hard to tell from just the optical image, isn't it? Although you might intuit that the apparent reddening of stars around the planetary nebula is indicative that the stars are behind the nebula, that doesn't tell you whether this is a foreground object or whether this nebula is merely towards the "near side" of the cluster.

If we want to know for sure, we have to measure the redshift of many individual stars and the planetary nebula.

Image credit: Canada-France-Hawaii Telescope/Coelum Observatory.

You see, open star clusters are very tenuously bound, and stars contained inside one all have approximately the same redshift, as even small changes in a star's velocity would cause it to escape from the cluster. Well, the stars in the cluster are receding from us at 41.4 km/sec, but the nebula recedes at 77 km/sec, so it's not a part of the cluster after all!

Our best estimates, in fact, place it at a mere 2,900 light-years distant, much less than the estimated 5,400 of today's cluster!

Image credit: Wikimedia Commons user Fabian RRRR, via the Hubble Legacy Archive (ESA/NASA).

The shame of the more distant star clusters is that they're often obscured by the disk of the galaxy, and that it's increasingly more difficult to bring out the fine details in them. But, it is easier to fit the entire cluster into a high-magnification field-of-view, which makes it very easy to view the entire thing at once!

This cluster is large, massive, and has all the right ingredients to make it to a billion years or more intact, a bonafide rarity for an open star cluster! Enjoy this less well-known sight all winter long, but feel free to start today, as we wrap up the year's penultimate Messier Monday! Including today, we’ve looked at the following objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M11, The Wild Duck Cluster: September 9, 2013
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
M32, The Smallest Messier Galaxy: November 4, 2013
M33, The Triangulum Galaxy: February 25, 2013
M34, A Bright, Close Delight of the Winter Skies: October 14, 2013
M36, A High-Flying Cluster in the Winter Skies: November 18, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M39, The Closest Messier Original: November 11, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M46, The 'Little Sister' Cluster: December 23, 2013
M47, A Big, Blue, Bright Baby Cluster: December 16, 2013
M48, A Lost-and-Found Star Cluster: February 11, 2013
M50, Brilliant Stars for a Winter’s Night: December 2, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M73, A Four-Star Controversy Resolved: October 21, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M75, The Most Concentrated Messier Globular: September 23, 2013
M77, A Secretly Active Spiral Galaxy: October 7, 2013
M78, A Reflection Nebula: December 10, 2012
M79, A Cluster Beyond Our Galaxy: November 25, 2013
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M101, The Pinwheel Galaxy, October 28, 2013
M102, A Great Galactic Controversy: December 17, 2012
M103, The Last ‘Original’ Object: September 16, 2013
M104, The Sombrero Galaxy: May 27, 2013
M106, A Spiral with an Active Black Hole: December 9, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013
M109, The Farthest Messier Spiral: September 30, 2013

Come back next week, where another deep-sky wonder -- and another tale that the Universe tells us about itself -- awaits you, only on Messier Monday at Starts With A Bang!

esiegel Mon, 12/23/2013 - 12:03

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The Largest Trainwrecks in the Universe

esiegel — Wed, 11 Dec 2013 17:46:49 +0000

The Largest Trainwrecks in the Universe

"Where you used to be, there is a hole in the world, which I find myself constantly walking around in the daytime, and falling in at night. I miss you like hell." -Edna St. Vincent Millay

It was just a little while ago that we were all speculating wildly -- and optimistically -- about Comet ISON, as it plunged towards the Sun from its origins in the very, very distant Solar System. As its perihelion date (the moment of closest approach to the Sun) drew near, you may have noticed something interesting about photos of the comet: it's tail appeared to get longer and longer!

If you compare this to earlier photos, you might think it had something to do with flying too close to the Sun, à la the Icarus and Daedalus myth. But it has a lot more to do with the physics of gravity than with anything melting due to heat!

Don't believe me? Take a look at this video, captured by NASA and ESA's great solar observatories, and pay particular attention to the speed of the comet.

Did you notice anything different about the speed of the comet (or what's left of it) at different times in the video? If you're looking at the same thing I am, it might appear that it moves somewhat slowly as it approaches the Sun, speeds up, and then slows down again as it moves away.

Is this a trick of the angle from which we view the comet?

Image credit: National Air and Space Museum, Smithsonian Institution.

Not a chance; that's just gravity. You see, you might be familiar with escape velocity, or the speed at which you'd need to travel at in order to escape the gravitational pull of a massive, centrally located object.

From the surface of the Earth, that's something like 25,000 miles-per-hour (40,000 km/hour).

Image credit: Pearson / Prentice Hall.

But a lesser-known application of the same physical laws and properties tells us that if you dropped an object from almost zero velocity in the vicinity of that same gravitationally massive source, it will fall towards that center-of-mass, reaching that exact escape velocity if it collides with the surface of the massive object dominating the system.

For something falling into the Sun from rest an arbitrary distance away, it would be moving at a whopping 617 km/s, or about 0.2% the speed of light, when it hits the Sun's surface. Comet ISON didn't quite get there, but it did get all the way up to 377 km/s, or about 12 times as fast as the Earth orbits the Sun. In general, the closer an object operating only under the influence of gravity gets to the center-of-mass, the faster it moves; Kepler's famous second law is a special case of this.

Image credit: Wikimedia Commons user Gonfer.

But the Sun is hardly the most massive thing we know of, and speeds well in excess of this occur naturally, for huge astrophysical systems, all the time.

Consider a typical, Milky-Way sized galaxy; for our Solar System to escape from it, we'd need to achieve a speed of about 550 km/s, and we're already some 25,000 light-years from the galactic center! That means if we dropped an object -- say a much smaller galaxy -- from an arbitrary distance away, it would be moving at about 550 km/s when it collided with us.

More spectacularly, however, two massive galaxies attract each other in the depths of space, creating a trainwreck with cosmic motions on the order of double that -- or at around 1,000 km/s -- when they finally collide!

Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University). (Click for an insane version, if you dare!)

This is, in fact, the future fate of our own galaxy; our somewhat bigger sister, Andromeda, is moving towards us at around 40 km/s, from a gigantic distance of a little more than 2 million light years away. By time we meet in a few billion years, the two galaxies will move at a maximum speed approaching that figure -- 1,000 km/s -- relative to one another!

But two galaxies merging like this is hardly large or unusual on the scale of the cosmos.

Much more interesting is when we have large collections of galaxies smashing into one another all at the same time! Two spectacular examples are Seyfert's Sextet,

Image credit: Hubble Legacy Archive, NASA, ESA; Processing: Judy Schmidt.

and Stephan's Quintet,

Image Credits: X-ray: NASA/CXC/CfA/E. O'Sullivan Optical: Canada-France-Hawaii-Telescope/Coelum.

both of which will have their constituent galaxies travel in speeds exceeding that value, relative to one another!

But why settle for groups of five-or-six galaxies, when we have clusters containing thousands?

Image credit: © Fabian Neyer / Antares Observatory, via http://www.starpointing.com/ccd/virgodeeplarge1.html.

The Virgo Cluster, shown here, has galaxies whizzing about at around a full 1% the speed of light relative to one another, and many of these galaxies are huge, giant ellipticals some ten times (or more!) the mass of our Milky Way!

You think that's big?

Now, imagine two of these giant clusters falling into and colliding with each other!

Image credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.;
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

That's what we've got for the Bullet Cluster (above), cluster Abell 520 (below),

Image credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University).

and cluster MACSJ0025 (at bottom), among many others. These composite images show the individual galaxies in the optical, dark matter (a proxy for total mass) in blue, and shocked, hot X-ray gas in pink (or green, for Abell 520).

Image credit: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University).

With maximum relative speeds reaching 4,000 km/s, or around 1.4% the speed of light, these are the largest-scale, fastest-moving giant objects in the Universe, and as our telescopic reach extends farther and farther back, we're only finding progressively more impressive ones! Dark matter plays a huge role here, increasing the hefty masses of these objects by a factor of five or so over what they'd weigh if they were made of normal matter alone, and the highest speeds we see are raised by a factor of about 140% as a result!

So that's a little glimpse at the greatest cosmic trainwrecks in the Universe, at speeds you might never have imagined for something so large!

esiegel Wed, 12/11/2013 - 12:46

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If Dark Matter was actually made of weakly interacting matter, wouldn't a galaxy that had gone through a collision be devoid of Dark Matter?

The DM associated with the colliding galaxies would have escape velocity when the galaxies collided and would escape the galaxies. That we haven't found any galaxies that don't have any DM, or even any galaxies where the ratio of DM to regular matter deviates from the norm.

Doesn't this rule out WIMPs and similar particles as DM possibilities?

@Denier #1: I think you're missing something in the kinematics. When two galaxies (or clusters) collide, it is the galaxies _along_with_ their DM haloes which collide. The galaxies generally stay gravitationally bound within their much larger haloes. The gas and dust of the two galaxies interact and get tidally disrupted by the encounter (see that great collage at the top of the article). The DM haloes get much less disrupted. If the two systems merge, forming a single bound system, then the two haloes also "merge", overlapping to form a single large halo.

On a separate note, we _do_ have many observations of galaxies with wildly different DM ratios. These are mostly dwarf and satellite galaxies, and they run the gamut from being nearly all dark matter (as much as 90% or more!) to being almost entirely baryonic.

@Michael Kelsey #2: I agree that there has to be something I'm missing if collisions don't result in ejecting all DM. With the way Ethan described it, it sounded like everything has escape velocity when the galaxies collide, but the regular matter loses escape velocity via the interactions you describe. If DM doesn't interact then it wouldn't ever lose escape velocity. The disparity in interaction would cause regular matter to have a sub-escape velocity while DM had escape velocity and the two would be permanently separated.

It is good to read that observations do report varying ratios. I had always thought all elliptical galaxies had to have the exact same ratio to correctly model their spin. I did not know this was not the case.

I understand escape velocity from our sun is greater than escape velocity from the Milky Way *from our position*, since we're far from the centre (and so if we can escape the sun, escaping the Milky Way is a piece of cake). I was going to ask what the escape velocity would be from the centre of our galaxy, but I guess that's very ill-defined: at the very edge of the black hole at the centre, escape velocity is essentially light speed, right? So escape velocity can be anything up to light speed depending on how far you begin your journey from the event horizon of the black hole. Given that fact, is there some privileged escape velocity we could figure out for a given galaxy, or are escape velocities inescapably (sorry) dependent on position?

One other thing: how does 5 times the mass (in a DM-heavy galaxy) end up giving 140% of the velocity? I would have thought 5 times the mass in each gives a gravitational attraction between two galaxies that is 25 times bigger, and an acceleration that is 5 times bigger. Obviously you can't translate acceleration to velocity without knowing initial velocity and elapsed time--so is the 140% you quoted just an accident of how far apart they started and how fast they were moving before they got caught in each others' grip? It seems like that figure could have been much higher.

DM interacts gravitationally, denier.

UncleMonty @5,

Quick answer: 5 times the mass extra results in a velocity that's is sqrt(5+1) as great, which is ~240% as great, or 140% greater. Hope that helps!

@Wow #6 (following Denier #3): I think that was the point of Denier's question. He framed it in terms of individual galaxies, but our best observational data comes from clusters, and Denier's question is quite valid based on that data.

Consider the "Bullet Cluster", which is a post-collision in progress between two middle-sized galaxy clusters. Observationally, we see three very different things, simultaneously:

1) The two groups of visible galaxies mostly passed "through" each other with some tidal distortion, but not disruption, of individual galaxies. That's a consequence of the individual galaxies being relatively small, with relatively large spaces between them.

2) The mostly non-visible ionized gas is highly visible in X-rays, in the form of a wide and deep hot shock wave in the middle region between where the two clusters passed. That's a consequence of the intergalactic medium (IGM) being continuous through each cluster, and interacting relatively strongly via pressure and density.

3) The majority of the mass, observed via distributions of strong and weak lensing of distant background galaxies, is found on the far sides of the X-ray shock described in (2). These two big mass "blobs" (read "haloes") more or less still encompass the visible groups of galaxies (which, as you recall from (1), mostly just passed through one another).

So what's happened in the Bullet Cluster is that the DM (3) really did just pass through, and the visible "small" (by comparison) galaxies went along for the ride. The more uniformly distributed baryonic stuff (the IGM) in the two clusters, crashed and burned, and got left behind :-)

The Bullet Cluster is just our first example of this; other cluster collisions have been found, and follow more or less the same pattern.

Now, with all of that description, Denier's question about what would happen in a galaxy-galaxy collision is quite valid! Naively, you'd expect the DM haloes to do what they did in the Bullet Cluster, and zip through each other, leaving behind an extra-large elliptical galaxy (the result of the merger), without much of a DM halo.

But that isn't what we expect, because the conditions aren't actually identical. The collision speeds will be lower, and the galactic DM haloes are themselves embedded in a larger cluster halo, so they are more likely to capture each other gravitationally, and (over ~ a billionish years) virialize into one big halo with the merged elliptical embedded in it.

Denier: We are seeing exactly the kind of separation you'd expect from weakly interacting matter in several of the cases above.

In broad strokes there are 3 kinds of matter to think about in these galactic collisions: Stars, dust/gas, and Dark Matter.

Stars are extremely dense compared to the other two types, and because of the great distance between them are extremely unlikely to collide with each other -- the Milky Way/Andromeda crash in the future is likely to only produce a few, if any, stellar collisions. Stars do interact with the diffuse gas of the interstellar medium, but that doesn't slow the star significantly because the star is so dense compared to the medium.

The dust/gas of the two galaxies does collide, and so tends to pile up where the collision occurs.

WIMPs or similar types of Dark Matter don't interact with much of anything including itself, so it carries on unimpeded.

So what you'd expect is that in a galactic collision the dust and gas would be stripped off from the galaxy, leaving just the stars and dark matter to continue on. So what you'd expect to see is a large cloud of gas, heated by the collision, with stars and dark matter separate.

And that's what we see! Galaxies with lots of stars, but very little dust, and much more mass than the stars alone can explain. And nearby, where the presumable collision occurred, a large cloud of heated gas

As to the point about escape velocity -- it's important to note that an object will only reach exactly escape velocity falling into a gravity well if it began at infinity. When that is not the case, the object can get going very fast, but only fast enough to reach the same distance it was before gravity slows it down to a stop and it comes back once again.

Well, Michael Kelsey, you beat me to it, and with a better explanation to boot. Very nice. Now where is that 'delete post' button?

When you say "dust," what particle sizes are you talking about? Does this include anything large enough to constitute a hazard to life-bearing planets in the manner of large asteroids?

And, will the increase in ambient gamma radiation be a hazard to life-bearing planets?

By the time Andromeda collides with the Milky Way, humans or our evolutionary descendants will either have become an interstellar species, or will not have done so and thereby gone extinct via the impacts of increase in solar luminosity.

Assume an interstellar civilization spread across many star systems, that has mastered asteroid defense on each of its homeworlds, and has the means to migrate out of any given system before its local star becomes a hazard. Assume that such a civ is spread out across a region of space greater than 5,000 LY, such that some fraction of it will survive an unexpected gamma ray burst anywhere in its range. What other existential threats are likely to be faced by such a civilization between that point and the heat death of the universe?

(Yo Ethan - Consider that a possible question for a column. What purely natural existential threats are faced by an interstellar civ between time X and end-of-universe, and how might they be met? One more condition: no miracles or magic, no Singularity, no "upload" to the Borg or "post-biological life", no travel at faster than 0.2 c, etc.)

@G #11: Quoting from the Wikipedia article, "It is for the most part a type of small dust particles which are a few molecules to 0.1 µm in size." Grain size can be determined quite well spectroscopically.

Observations of the solar system suggest that population scales inversely with grain size according to a power-law relationship (so that large grains are exponentially less common than small ones). [Yes, I need a good citation for this, and I'm having trouble finding one. Sigh...]

Limits on MACHO searches in the galactic halo imply a very low density of astronomically small, but terrestrially large, objects (asteroid/planet or larger).

@Denier#1and#3 ... please reread and rethink Michael Kelsey #2. As so inferred above by Michael and CB, the DM DOES interact ... with itself and with normal matter through the Gravitational Force. Gravity is THE defining measure in these huge interactions. Think WEAK when discussing DM itself or it's interactions with either itself or normal matter. I believe someone mentioned WIMP.

@Michael Kelsey #8 (and all others who have chimed in): I greatly appreciate the time and effort put in to helping me understand. It is just that I don't have a grasp on how something with escape velocity, absent any interaction outside of gravity, becomes gravitationally bound. I had thought that escape velocity by its very definition meant that it has sufficient energy to escape being gravitationally bound.

Changing the size of the aggregate whole, or the speed required so long as escape velocity is always achieved, which Ethan's piece implies is a truth of physics.

I think it was nailed in the first sentence. I'm missing something in the kinematics.

Escape velocity does mean that.

Energy enough to escape isn't in everything. After all, the original cloud, being in the cloud rather than running away, doesn't have escape velocity. It doesn't have infall velocity either, until some energy is lost to friction (absent for DM), but that matter wasn't leaving the cloud either.

@Wow #15: I'm not quite sure I follow your point on how mass in a cloud obeys the laws of physics differently than mass in a clump, but my misunderstanding has greater consequences than just ejection during galactic collisions. With my current thoughts on kinematics, no galaxies would ever have a halo of DM because every particle of DM would always have the necessary escape velocity to prevent being gravitationally bound to anything.

Instead, what you'd have is a diffuse fog of DM that is everywhere with a density that varied inversely with the density of normal matter. In effect, space curved by the gravity of regular mass would act like the curved upper skin on an airplane wing. Particles of DM entering the gravity well of an object would be accelerated by gravitational attraction and lower the pressure density of the DM fog around the object.

It might mathematically look like a halo from inside a galaxy, but there would be no outer edge of the halo. It would just be diffuse DM fog all the way to the next galaxy.

People much smarter than I am have a different understanding, which means something has to be wrong with something I thought I knew. I'll just follow along and see if I can figure out where the error is.

"every particle of DM would always have the necessary escape velocity to prevent being gravitationally bound to anything."

Why?

Where do you get this assertion for every DM particle?

Denier: Ethan's statements about escape velocity were inexact for the purposes you're applying them to. It works fine for estimating about how fast objects will be going in one of these cosmic collisions (probably at least as well as we can directly measure their relative velocities).

An object that starts at rest relative to a source of gravity can only get as far away from that source as it started. If it has an initial velocity, its final velocity will have the same magnitude. That's basic Conservation of Energy assuming no other interactions but gravity, as we can for stars and DM.

But that's also assuming no other sources of gravity, too. Once there are 3 or more bodies, orbits become chaotic, and objects can gain energy relative to the center of mass at the expense of other objects. So one object can get tossed out of the system (meaning it has exceeded escape velocity), while another will move in tighter. And so you get a concentrating effect around the center of mass.

This is how globular clusters become ever-more dense, with densities in the center that are high enough that star collisions do occur, while simultaneously "evaporating" as stars are tossed out.

So it is with Dark Matter, which should actually be more dense closer to the centers of galaxies. Directly proportional to normal matter density, not inversely. With a small constant. You're right that because DM is still very diffuse, the 'fog' does extend far beyond the visible galaxy, out to other galaxies. Like Michael Kelsey said the entire galaxy cluster has a (non-uniform) halo of Dark Matter.

I hope that helps explain why you'd expect Dark Matter to clump even with zero non-gravitational interactions, and why you shouldn't get hung up on escape velocity in that context.

The flip side of the coin is that the reason DM is gravitationally bound to galaxies is because it is believed likely that the galaxies formed in DM over-densities in the first place. So in a sense it's the normal matter that became bound to the DM.

It also assumes you are talking about the matter most distant from the object. But that isn't all the matter, just the bit you took for calculating.

Nowhere does it say all dark matter (nor matter) has escape velocity.

Denier: This doesn’t this rule out WIMPs and similar particles as DM possibilities. Perhaps I can explain it like this:

If you fall towards the Earth you end up approaching it at 11km/s. If you've got two Earths falling towards each other, they each end up approaching at 11km/s, and their closing speed is 22km/s. The two Earths together make a bigger mass. If you were on one but jumped aside at the last moment, your 11km/s isn't enough to escape the bigger mass.

That said, there isn't really anything that "rules in" WIMPs and similar particles as DM possibilities. There's an assumption that dark matter consists of exotic particles, but no actual evidence. I think it's inhomogeneous vacuum energy myself, but sadly this doesn't get much airtime. See arXiv.

@John Duffield #20: It's not obvious to me that inhomogeneous cosmology does anything about replacing drark matter. All it does, as near as I can tell, is do a better job at modelling the spatially varying curvature due to (assumed!) matter distributions.

That means that it's great for getting a better handle on how large scale structure evolves, but doesn't do diddly squat for telling you want that structure contains.

As a particle physicist, and one currently involved in a direct-detection experiment (SuperCDMS), I quite agree with you that there is *NO* observational evidence (yet :-) ) for dark matter being WIMPs, axions, or anything else specific.

The arguments follow two branches, one based on essentially the consistency and completeness of physics (i.e., "everything else is made of quanta, so DM must be"), and the other based on looking under the lamp post (i.e., we know how to build an experiment to detect WIMPs/axions/whatever, so that's what we're going to look for).

If we find a confirmed signal, that will tell us we're on the right track, and can shift from "what is it" to "how does it work". If we find nothing (and the next generation of proposed experiments can get us below the minimum possible cross-section limits), then we know our "completeness" argument is wrong.

Michael, thanks for the quick reply @ 12, and apologies for my slow reply here.

Interesting that the objects follow a power law relationship as to sizes. Even though the human-relevant sizes are rare, we still need to build a viable space defense, starting now, so it's fully operational & well-tested on harmless objects before it encounters a potentially dangerous one.

A power law is inevitable, really.

One thing wallops into another and breaks up into two pieces, both necessarily smaller than the original, and expected value of half the size of the original.

@Wow #23: I have some hesitation about your "explanation" for the power-law distribution of asteroid sizes. You're not wrong, but I think you've oversimplified. A majority of small solar system objects are, we believe, primordial accretions (i.e., planetesimals), not the result of collisions.

I'm pretty sure that, as you say, a power law is almost inevitable if you have repeated collision events (though the 50-50 breakup case is actually quite unlikely, see the distribution of fission fragments from U-235, for example).

I'm not entirely sure you can claim the same mathematical inevitability for accretion. I suspect that the local environment and physics kick in early enough to prevent a multi-order-of-magnitude distribution (which you need in order to observe a power law vs. just linear).

We don't have any good accretion models to span the gap between ~cm size clumps of dust, up to ~km size planetesimals. Such a model could predict a power law, but it could just as easily not.

@ Denier #16 ... So, young man, it would seem to an untrained mind such as mine, that whoever you were discussing 'Escape Velocity' of a group of specific particles, or simply an individual particle with, he or she left you without insuring you had a solid grasp of the relationship between a particle at escape velocity, and the 'object' from which you infer it must be escaping.

It works as such ( or reasonably close for purposes of this conversation): if EV>11.18 km/s to escape a body with the mass of 1 Earth (approx 5.974E24 Kg), then EV must > 617.6 km/s to escape a body with the mass of the Sun (approx 1.989E30 Kg).

So please understand that a particles escape velocity is directly related to the mass of the object from which it is attempting (allegorically) to esape. Therefore, one must have at least a few defined variables to correctly discuss "Escape Velocity" - Mass (in grams) of the body to be escaped, the distance in meters between the two 'Objects' i.e; the Earth and the Particle (say a Proton), and figuring in for the Mass of both the Solar Syatem and the Mass of the Milky Way Galaxy, along with a few other seemingly unimportant values.

Again Sir, a particles "Escape Velocity" is only relevant when certain other 'things" are brought to bear such as the relative Masses and Distances of those objects being discussed.

@Tony #25: I'm pretty sure Denier understood that. The context for his question was specifically the topic of this blog -- the collisions of galaxies and galactic clusters. In that case, the masses involved are much, much larger than Earth's mass, as I am sure that you are keenly aware.

What is more, as I explained in my reply to him, the objects involved in these collisions are generally bound gravitationally into larger assemblages Thus, their collisional speeds are not really either object's own escape velocity, but rather the orbital speed they have in that larger assemblage.

"@Wow #23: I have some hesitation about your “explanation” for the power-law distribution of asteroid sizes. You’re not wrong, but I think you’ve oversimplified."

Indeed a few seconds thought would have ponderment about rates of collision in a power law realm already set up.

Simplification, however, lets you know what the first step is.

It gives the broad picture.

My simple explanation gives a power law of order 2. This is not the power law actually observed. But to get that other order, you need to do some calculation.

However, their method is generally something very similar, just with more effects included and accounted for.

And note: Fission isn't a breakup from collision.

And second note: no, if all states are equally valid results, the average is exactly half way. Down to a few score atoms, the indivisibility of matter doesn't, pun unintended, matter. It is why fission is no counter.

"I’m not entirely sure you can claim the same mathematical inevitability for accretion."

Never tried to do so.

@Wow #27: It sounded to me like you were suggesting that the power-law observed for small solar-system objects was due _entirely_ to collisional breakup (which would be an oversimplification). Sorry for misunderstanding.

You wrote that you didn't like my analogy between collisional fragmentation and fission. You're right that fission is not truly collisional in detail, but it can be modelled relatively well as a billiard-ball collision between a neutron and a large, fragile nucleus (sort of like a BB hitting a glass marble). As you say, "Simplification lets you know what the first step is. It gives you the broad picture."

I think you'll discover that in collisional fragmentation, all sizes are _not_ equally likely. The exact distribution depends on the relative masses of the colliding objects. Since there are more small than large ones, the majority of collisions will be smaller objects impacting larger objects. You can see this most clearly if you consider the size ratio, rather than absolute sizes: since the distribution is a power law, you should see most of the collisions being M -> 2M (or whatever the scale height is).

Suppose the collision is fast enough to cause full melting of both systems (i.e., rather than an impact crater). In that case, you have a net system 3M, and even if we take your "equal split", then you get two fragments of 1.5 M, which isn't half of the original target. In reality, what you'll usually get, just from angular momentum considerations, is a big blob with a small blob thrown off.

Like I said, I _don't_ think you were wrong, nor do I think your comments in #27 were wrong. I'm pointing out a few of the "more effects included and accounted for" which make this a relatively hard problem.

"You’re right that fission is not truly collisional in detail"

It's more that the chances of fission resultants are nowhere near even, Michael. Worse, not all lighter atoms are available in a fission scenario. More problematical are that they will often self-fission again, with the same problems of inhomogeneity.

To simplify to a collisional breakup would be to simplify it quite a long way beyond "simple enough".

@Wow #30: You wrote, "It’s more that the chances of fission resultants are nowhere near even." Yes, exactly so! In fact, the distribution is very clearly double-peaked, with maxima at around the 1/3 and 2/3 masses (for U-23x, around 85 and 170). Which was one of the reasons I brought it up as an analogy. In large body collisions, the same thing applies, though of course the distribution is different: you don't get a flat spread of possible fragment masses, but a peak around small-ish stuff, and another peak around big residues.

The underlying physics behind the two cases are vastly different, and the two distributions have quantitatively different shapes, but the _qualitative_ (i.e. argument from analogy) outcomes are not dissimilar.

@Wow: There's a new preprint out on arXiv (http://arxiv.org/abs/1401.1813) which is relevant to our discussion above on the size distribution of asteroids. Thought you might be interested, so I'm posting here.

In particular, their Figure 2 shows that the true distribution (magenta boxes) is only a (rough) power law above about 1 km diameter. Below that, there is a steep cutoff, with smaller bodies down by three orders of magnitude! I'm fairly suspicious that this is a consequence of observational bias, rather than being a real effect.

The different models they study for generating the distribution (the blue and green points) both track the data above 1 km (probably because they are tuned to do so), but follow the kind of power law you and I were asserting below that. Good to know my (our) physics intuition isn't completely screwed up :-)

I referenced this past article after seeing the recent third episode of "Cosmos". In this episode, there was a similar and even more outstanding simulation of Andromeda and our Milky Way colliding. The photos in this article and the linked ones in Wikipedia of multiple galaxy collisions are eye opening and even more amazing! Thank you Ethan.

Messier Monday: A Cluster Beyond Our Galaxy, M79

esiegel — Mon, 25 Nov 2013 17:29:06 +0000

Messier Monday: A Cluster Beyond Our Galaxy, M79

"Anyone who keeps the ability to see beauty never grows old." -Franz Kafka

The night sky is our window into the Universe beyond the Solar System, and in some cases, even beyond our own galaxy! Perhaps the oldest useful collection of deep-sky objects, the Messier catalogue showcases 110 of the most prominent night sky wonders, a full 42 of which are located beyond our own galaxy. Today, for Messier Monday, let's take a look at one of the true rarities of the Messier catalogue: a globular cluster that's not a part of our own galaxy!

Image credit: Al Kelly of http://www.kellysky.net/; images from Johnson Space Center Astronomical Society.

While the Messier catalogue boasts 29 globular clusters -- dense collections of hundreds of thousands of ancient stars -- only two of those are from outside of our own galaxy!

Globular clusters in general are found in the halos of galaxies, with the Milky Way containing around 200 of them and some giant ellipticals containing more than 10,000! As the famed winter constellation Orion rises higher and higher in the sky in the early part of the night, a new group of Messier objects becomes visible to skywatchers across the globe, and today we highlight one of the rare extragalactic globulars: Messier 79. Here's how to locate it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

By about 10 PM tonight -- and progressively earlier as the winter comes on -- Orion will be well over the eastern/southeastern horizon, with the young blue star Saiph lingering the lowest in the sky. If you follow the imaginary line from Betelgeuse to Saiph and curve slightly towards Rigel, you'll run into two prominent stars even lower in the sky: Arneb and Nihal.

These two stars are the brightest members of the constellation of Lepus (the hare; don't call it a bunny), and will guide you to Messier 79 all winter long.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

If you now draw an imaginary line from Arneb to Nihal and continue on about 1½ times that distance, you'll come to a fifth-magnitude (naked eye under decent conditions) star known as either HIP 25045 or HR 1771, depending on which bright-star catalogue is your favorite.

The good news is, if you can find this star, Messier 79 is only half-a-degree away, and if you train your telescope or binoculars in that region -- with good seeing -- you really can't miss it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

A faint, fuzzy cloud of stars, Messier recorded it one year after his assistant, Pierre Méchain, discovered it in 1780. It was recorded as a nebula without stars (as all globulars appeared in Messier's instruments), and described as:

this nebula is beautiful; the center brilliant, the nebulosity a little diffuse; its position was determined from the star Epsilon Leporis, of fourth magnitude.

If all you had was a small instrument, this is the view you might see.

It appears tiny on the sky, but that's not because it's a small object, not at all! When we take a deep look inside, what we find is simply stunning.

Image credit: AURA, NSF, NOAO.

This cluster is loaded with red giant stars, with plenty of blue stragglers lighting the way as well. Based on the discovery and observation of these very standard stars -- and the well-known color/magnitude relations -- we can pinpoint this object as being 41,000 light-years from Earth and 60,000 light-years from the center of our galaxy, making it one of Messier's most distant globulars.

But unlike the other globulars, this one is found in a very suspicious area of the sky.

Image credit: © The Worlds of David Darling, via http://www.daviddarling.info/.

Just below the plane of our Milky Way, the Canis Major Dwarf Galaxy is slowly being devoured by the gravitational pull of our galaxy on this soon-to-be-former member of the local group. As it stands right now, there's a great tidal tail of material being pulled off of this object, and everything that's bound to it -- including its globular clusters -- are slowly being absorbed into our own galaxy!

Image credit: R. Ibata (Strasbourg Observatory, ULP) et al., 2MASS, NASA.

The way to be sure that this cluster has the stars we think it does is to look at it in different wavelengths. If we want to find blue stars, all we should see are blue stragglers close to the core, and those should show up in the Ultraviolet portion of the spectrum.

Image credit: GALEX, via NASA.

And if we want to know about red giants, we should be looking in the infrared, where these prominent stars should shine brightest. In addition, we ought to be seeing a different concentration: red giants in a larger sphere, blue stragglers close to the center.

Image credit: Atlas / 2MASS (L) / UTIV (R).

That's exactly what we see, and very high-quality ground images can even bring this out.

Image credit: Digitized Sky Survey (DSS).

This is somewhat younger than many globular clusters found in our galaxy, at an estimated age of "only" 11.7 billion years. That might seem incredibly old, at around 85% the age of the Universe, but when you consider that many globulars are 90-95% as old as the Universe, this one is younger than many. This age determination is supported by the fact that it has 3-5 times as many heavy elements as some of the oldest globulars!

Image credit: Al Kelly of http://www.kellysky.net/.

With hundreds of thousands of stars in a radius of around 100 light years, this cluster would be a lot brighter if there weren't so much galactic dust in the way from the nearly-directly-intervening plane of our galaxy. As it is, things are just clear enough for the Hubble Space Telescope to have taken a quality look at the core of this object.

Image credit: Hubble Legacy Archive.

What you're looking at is raw, unprocessed data from the Hubble Legacy Archive of Messier 79. My image processing skills aren't good enough to really bring out the best of this object, but I will showcase that the core, here, contains a huge number of tremendously interesting stars...

Image credit: Hubble Legacy Archive.

and that if you were interested in taking a dive through the center at maximal resolution, this is the sort of stellar neighborhood you'd see! For comparison, if you put the Sun at the center of the image, below, the nearest star to us -- the Proxima/Alpha Centauri system -- would appear about 100 pixels below the bottom of this image.

Image credit: Hubble Legacy Archive.

That a lot of stars! And that's a glimpse into the Universe just slightly beyond our own galaxy, and a great way to wrap up another Messier Monday! Including today’s entry, we’ve explored at the following Messier objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M11, The Wild Duck Cluster: September 9, 2013
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
M32, The Smallest Messier Galaxy: November 4, 2013
M33, The Triangulum Galaxy: February 25, 2013
M34, A Bright, Close Delight of the Winter Skies: October 14, 2013
M36, A High-Flying Cluster in the Winter Skies: November 18, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M39, The Closest Messier Original: November 11, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M48, A Lost-and-Found Star Cluster: February 11, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M73, A Four-Star Controversy Resolved: October 21, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M75, The Most Concentrated Messier Globular: September 23, 2013
M77, A Secretly Active Spiral Galaxy: October 7, 2013
M78, A Reflection Nebula: December 10, 2012
M79, A Cluster Beyond Our Galaxy: November 25, 2013
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M101, The Pinwheel Galaxy, October 28, 2013
M102, A Great Galactic Controversy: December 17, 2012
M103, The Last ‘Original’ Object: September 16, 2013
M104, The Sombrero Galaxy: May 27, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013
M109, The Farthest Messier Spiral: September 30, 2013

Come back next week, where another deep-sky object -- and another unique story about our Universe -- awaits you here, only on Messier Monday!

esiegel Mon, 11/25/2013 - 12:29

Tags

canis major dwarf galaxy

cluster

extragalactic

extragalactic globular

I love these Messier Mondays! :-)

Another great article, magnificently illustrated and very well written - thankyou Ethan Siegel.

Have never before seen the likes of CG simulation showing the flow of canis galaxy into our own. Very cool.

Since we don't have an "outside" image of our own galaxy, we are thought that it looks like an average spiral galaxy. Actually it's much more exiting! :)

Thank you for that. Now my old mental image of our galaxy has been updated to include awesome galaxy merger.

Messier Monday: A High-Flying Cluster in the Winter Skies, M36

esiegel — Mon, 18 Nov 2013 18:32:31 +0000

Messier Monday: A High-Flying Cluster in the Winter Skies, M36

“Once you have tasted flight, you will forever walk the Earth with your eyes turned skyward, for there you have been, and there you will always long to return.” -Leonardo da Vinci

Welcome back to another exciting Messier Monday here on Starts With A Bang! As Comet ISON dives towards the Sun and a nearly perfect full Moon towers overhead, it's easy to forget about those wondrous deep-sky objects that are fixed, but the 110 prominent members of the Messier Catalogue are always on tap for dedicated skywatchers. Although the extended objects -- galaxies and nebulae -- are difficult to view with a bright Moon out, the star clusters, both open clusters and globulars, still make for spectacular viewing.

Image credit: Rolando Ligustri, via http://itelescope.net/.

Today, we're going to highlight one of the dimmer (and thus, often overlooked) open star clusters of the Messier Catalogue, still clearly visible with even a small telescope or binoculars, provided you know where to look: Messier 36. For those of you who are night owls with clear skies, you may have noticed the recent appearance of the famed winter constellation Orion; that will be your guide to finding this week's Messier object.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

The Moon is famous among skywatchers for not only being a towering, brilliant source of light, but also for being a terrible source of light pollution. Tonight's practically full Moon means that the night sky's immediate vicinity around it at a total wilderness site is about as polluted as the night sky in a downtown urban area of a large (Seattle-sized) city. But that doesn't mean you can't still see the absolute brightest stars; Orion rises at around 9 PM over the Eastern horizon and even farther above it is the constellation of Auriga, heralded by brilliant Capella, the third brightest star in the entire Northern Hemisphere, behind only Arcturus and Vega.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

Auriga makes an oval-like shape, with bright blue Alnath on the narrow end, opposite Capella and Menkalinan. (Alnath is technically in the constellation of Taurus, but it's by less than a single degree; I'm far from the only one who always thinks of it as part of Auriga.) If you draw an imaginary line from Alnath back towards Menkalinan, you'll come to the naked eye star χ Aurigae, which should still be visible even with the full Moon nearby.

And if you navigate from Alnath to χ Aurigae and head just a little bit farther in that same direction, you won't be able to miss a very nice cluster of stars -- Messier 36 -- in either a telescope or binoculars.

Image credit: me, using the free software Stellarium, available at http://stellarium.org/.

This open star cluster is often challenging if you don't know where to look, as it takes up a very small region on the sky (just a fifth of a degree) and is invisible to the naked eye under ideal dark-sky conditions. Nevertheless, it had been known for more than century before Messier catalogued it, noting it as a:

Cluster of stars in Auriga, near the star Phi: with an ordinary telescope... one has pain to distinguish the stars, the cluster contains no nebulosity.

And if you viewed it in an instrument of Messier's power, you might well reach the same conclusion.

Image credit: Bill Longo, via http://billlongo.com/messier36.php.

It seems like it barely stands out at all against the backdrop of other stars of similar magnitude. But what appears to be a small and unspectacular collection of dim stars from our point of view is actually an incredible sight, if we're willing to take a look inside.

Image credit: Andy E. Rostron of http://nightcamera.blogspot.com/.

A glittering array of mostly blue stars that's quite concentrated, Messier 36 has two particularly bright stars located at its center. Although they might not have stood out to Messier, even a 3" (80 mm) telescope is good enough to see that brilliant central pair!

And if you're willing to dive in with a really high-quality telescope, you can find out a whole lot more!

A quite young cluster with no red giant stars inside, the bluest star found in Messier 36 is of spectral class B2, just barely below the threshold of stars that end their lives in supernovae!

The reason this cluster appears so small and faint is because it's so far away; at a distance of 4,100 light years it's one of the top-10 most distant Messier open clusters, some ten times as far away as the Pleiades. At its current distance, it's 14 light years in diameter, making it almost the same physical size as the Pleiades, and based on the stars we find in there, it's only 25 million years old; our Sun is nearly 200 times older!

At least 60 individual stars have been identified in this cluster, although it's easily conceivable that there are hundreds -- if not as many as a thousand -- in there that are simply hidden from our view thus far. It's no wonder; like most open star clusters, Messier 36 is indeed located right in the galactic plane!

Still, you might ask yourself whether there was anything interesting to learn in the infrared. Is there dust? Red stars simply invisible to our visible eyes? Something else?

Image credit: Two Micron All-Sky Survey (2MASS).

It looks like -- if you didn't know better -- a shooting, red star is in there, doesn't it?

What's amazing is that, to the best we've been able to tell, there really is something interesting in there!

Image credit: NOAO / AURA / NSF.

What is that thing? If only there were a better image to find out! In the meantime, this is the best I've been able to find!

Image credit: NOAO / AURA / NSF.

It looks like a star whipping through the interstellar medium so quickly it's grown a tail, similar to the well-known star Mira, but until better data comes along, it will have to remain a mystery! With that said, it's a great sight for now and all through the entire winter, even when the Moon is at its brightest! And that will have to wrap up another Messier Monday! Including today’s entry, we’ve explored at the following Messier objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M11, The Wild Duck Cluster: September 9, 2013
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
M32, The Smallest Messier Galaxy: November 4, 2013
M33, The Triangulum Galaxy: February 25, 2013
M34, A Bright, Close Delight of the Winter Skies: October 14, 2013
M36, A High-Flying Cluster in the Winter Skies: November 18, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M39, The Closest Messier Original: November 11, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M48, A Lost-and-Found Star Cluster: February 11, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M73, A Four-Star Controversy Resolved: October 21, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M75, The Most Concentrated Messier Globular: September 23, 2013
M77, A Secretly Active Spiral Galaxy: October 7, 2013
M78, A Reflection Nebula: December 10, 2012
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M101, The Pinwheel Galaxy, October 28, 2013
M102, A Great Galactic Controversy: December 17, 2012
M103, The Last ‘Original’ Object: September 16, 2013
M104, The Sombrero Galaxy: May 27, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013
M109, The Farthest Messier Spiral: September 30, 2013

Come back next week, where another deep-sky object — and another unique story about the Universe — awaits you here, only on Messier Monday!

esiegel Mon, 11/18/2013 - 13:32

Tags

"t looks like a star whipping through the interstellar medium so quickly it’s grown a tail,"

Very comet-like.
Like Mira's tail but also like a comet.

If only there were a better image to find out! In the meantime, this is the best I’ve been able to find!

I presume the images were from different nights and same place thus not a comet or temporary object, right? No dates on the images there that I can see.

Certainly intriguing & would be great to discover its identity and details.

We are looking outward to the Perseus arm, so it is conceivable that this is a galaxy. But my money is on a red giant star in the line of site of a more distant nebula.

It's hard to be sure what that is, but it could be a local M dwarf which is being ejected from the cluster--that would account for its high velocity with respect to the local interstellar medium. And since the ejection would be recent (by cosmological standards), the tail would not have had much time to grow.

wow. interesting. could it be final stages of 2 stars merging?

or a black hole chucking some large chunks and some are ejected on the pole? a wish perhaps :)

It's just an exclamation point put there by the universe so the Hubble takes a look and lets us admire its beauty :)

Hey everyone, just wanted to give you a heads-up that you should say what you need to say over the next six hours or so here.

The site is undergoing maintenance / migration tomorrow, and may be down for extended periods of time. As such, I won't be able to post and you likely won't be able to comment until the end of the day Wednesday at the earliest. Sorry for the inconvenience, but hopefully all the loading / slowness issues we've experienced over the past couple of months will be a thing of the past.

See you back here soon; thanks for being such an excellent community!

@ ^ Ethan : Thanks for the warning.

A few more thoughts on our mystery red comet-like object here. (Like some of the other ideas here too.)

1. Could it be a proplyd (protoplanetary disk) that's "evapourating" / being blown away (photodissociation the term maybe?) by nearby hot massive stars - just like some proplyds in the Orion nebula?

Of course we can't really see any bright stars nearby and the tail doesn't seem to be pointing away from any but maybe there's one hidden from us by thick dust there - too thick even for the 2MASS, NOAO / AURA / NSF to reveal? Okay perhaps a bit unlikely.

2. Maybe its a young T-Tauri / FU Orionis type star illuminating a surrounding nebula like Hubble's Variable nebula (NGC 2261) that actually seems reasonably plausible and matching , perhaps making this one of the very last of the stars to form in M36?

3. Or what about a protoplanetary nebula or micro-quasar :

http://en.wikipedia.org/wiki/Microquasar#Microquasar

that's seen from an odd angle or suchlike?

What we really need is a spectrum of it! Don't know if anyone here is able to get one of it or more images?

I like some of the other suggestions here such as a background galaxy too. Occurs to me that a distant nebula could also be another possible explanation.

Without more information its going to be really hard to do more than brainstorms such suggestions. More data please!

PS. For comparison of this object with Hubble's Variable nebula (NGC 2261) see :

http://www.spacetelescope.org/images/opo9935c/

note the star at the centre is R Monocerotis in case that helps with searching. Plenty of other images for that should be findable too.

A spectrum, a spectrum my (oh I dunno, looks around) .. cup of tea for a spectrum! ;-)

(With apologies to Shakespeare, King Richard III.)

This object has been studied before. I used MAST

http://archive.stsci.edu/

to find HST observations, which led me via ADS

http://adsabs.harvard.edu/abstract_service.html

to a paper by Magnier et al.

http://adsabs.harvard.edu/abs/2013AJ....146...49M

The bottom line is: the object is a young star making the transition from Class 1 to Class 2; that means that the accretion disk is shrinking from "comparable to the central star in mass" to "much less than the central star in mass".

As #10 M Richmond noted, the object appears to be a Young Stellar Object known as IRAS 05327+3404 (aka "Holoea", Hawaiian for 'flowing gas') discovered by Magnier et. al. in 1996. Further studies of this object, with a K2 spectrum and possibly associated with the nearby star-forming region Sh2-235, can be found here:

http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?1999A%26A...346..441M&…

http://arxiv.org/abs/1305.6550

Messier Monday: The Closest Messier Original, M39

esiegel — Mon, 11 Nov 2013 18:02:20 +0000

Messier Monday: The Closest Messier Original, M39

"Books serve to show a man that those original thoughts of his aren't very new at all." -Abraham Lincoln

It might be Veterans Day / Armistice Day all around the world, but it's still Messier Monday here on Starts With A Bang! We may have been fighting wars for all of human history, but nearly all of the 110 deep sky objects that make up the Messier Catalogue go back long before that.

Image credit: Tenho Tuomi of Tuomi Observatory, via http://www.lex.sk.ca/.

Today, we take an in-depth look at one of the brightest and closest star clusters in the entire night sky, one that -- despite being visible to the naked eye -- went unrecorded until Messier himself catalogued it in 1764. Let's take a tour of Messier 39, starting with how to find it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Although one of the most prominent collections of stars in the night sky is known as the Summer Triangle -- consisting of the dominant stars (in its portion of the sky) Altair, Deneb and Vega -- it's actually visible after sunset well into autumn and even early winter for the Northern Hemisphere! The galactic plane passes through the summer triangle, and most of the open star clusters contained within our galaxy are found in the galactic plane, and Messier 39 is no exception to this rule.

To find it, draw an imaginary line from Vega (the brightest one) to Deneb (on the short side with Vega), and continue to head a little farther in that same direction.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

The first thing you'll run into is the easily visible naked-eye star, the orange supergiant ξ Cygni, which is nearly along the imaginary line extended along the Vega-to-Deneb path. If you think about Deneb as the bottom of a circle and move clockwise towards ξ Cygni, you'll hit three more (albeit slightly dimmer) naked eye stars as you reach the top of the circle -- ρ Cygni, π² Cygni, and Azelfafage (also π¹ Cygni), in order -- and Messier 39 lies just inside of them.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Although Messier, discovering it in 1764, gave it the very terse description:

Cluster of stars near the tail of the Swan; one can see them with an ordinary telescope...

this actually turns out to be one of the most remarkable objects in his entire catalogue! Let's take a look inside.

Image credit: John (J.C.) Mirtle of http://www.astrofoto.ca/.

You might first notice the prominent blue stars, and you should: there are about 30 prominent, blue-colored stars identified in M39, the bluest of which is of spectral type A0, the brightest and bluest of all the A-class stars. But one of the things that makes this cluster particularly interesting is that all of the bright stars are blue!

Image credit: Marcin Paciorek of http://www.astromarcin.pl/.

There are no red, orange or yellow giants, no prominent subgiants, and no stars in general found off of the main sequence!

For those of you who need a reminder, the main sequence is the line on which -- if you plot the colors vs. magnitudes of stars -- you'll find all stars burning hydrogen in their core.

Image credit: European Southern Observatory (ESO).

This is exceedingly rare! By this point in a star cluster's life, a star running out of fuel in its core takes some tens of millions of years to go from the main sequence and through the various subgiant and giant stages until it dies in a planetary nebula and white dwarf combination. With hundreds of stars expected in a typical star cluster, finding one without a single evolved star is exceedingly rare!

And yet, here Messier 39 is, doing exactly that.

Image credit: Jim Mazur's Astrophotography, via http://www.skyledge.net/.

With the background stars of the galaxy shining in a dim haze behind it, Messier 39 happens to be close, at a distance of just 800 light years, only the Pleiades and Praesepe star clusters are closer objects in the Messier catalogue. And yet, if we take a long exposure to really bring out the background stars, we don't really find many more that appear to be associated with these bright, blue ones! Even though -- based on what we see -- there are around 800 solar masses worth of material in there, we've only definitively identified around 60 stars in this cluster!

Image credit: Tonk of Cloudy Nights, via http://www.cloudynights.com/.

Only six of Messier's star clusters appear brighter to the eye, and the brightest of the blue stars here are just barely beyond the limit of human naked-eye vision under the darkest possible skies.

With long-exposure photography, the sight is nothing short of spectacular.

Based on the stars that are in there -- and assuming that higher mass ones are missing because they've evolved and disappeared -- we can estimate that this star cluster is around 300 million years old, but there are some uncertainties there.

After all, looking among what appear to be background stars, how many evolved white dwarfs from long-dead O-and-B-class stars are hanging out, invisible because of the incredible density of stars found in the galactic plane?

Image credit: flickr user The Killer Rabbit1.

You'd be very, very smart if you thought to look in the infrared; oftentimes, evolved, cooler stars that are less prominent in the visible will appear bright in the infrared!

Let's have a look at the best IR image out there of M39, thanks to 2MASS!

Image credit: The Two Micron All Sky Survey (2MASS) at IPAC.

This infrared image is what allowed us to go from about 30 (visible) stars up to around 60, but that's still a tiny number for an open star cluster! (And it's only seven light-years across; this star cluster would appear tiny if it weren't so ridiculously close!) Did something happen when this cluster was forming to end star formation early? It wouldn't be the only cluster like this, but the more of these objects I go through, the more of an appreciation I have for the sheer diversity of populations of stars that form naturally in the Universe!

My favorite of all the images that exist of M39 comes courtesy of NOAO, the National Optical Astronomy Observatories.

Image credits: Heidi Schweiker, WIYN, AURA, NSF, NOAO.

This image -- originally -- comes in an incredibly high resolution; if we take a "slice" through the center, here's what this incomparable star cluster looks like, silhouetted against the backdrop of the galactic plane!

Image credits: Heidi Schweiker, WIYN, AURA, NSF, NOAO.

This would make a lousy target for Hubble due to its incredibly large angular size, but that makes it all the more amazing as a target for amateur skygazers! And with that magnificent tour through one of the closest star clusters to us, that will wrap up another Messier Monday! Including today's entry, we've peered at the following Messier objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M11, The Wild Duck Cluster: September 9, 2013
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
M32, The Smallest Messier Galaxy: November 4, 2013
M33, The Triangulum Galaxy: February 25, 2013
M34, A Bright, Close Delight of the Winter Skies: October 14, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M39, The Closest Messier Original: November 11, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M48, A Lost-and-Found Star Cluster: February 11, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M73, A Four-Star Controversy Resolved: October 21, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M75, The Most Concentrated Messier Globular: September 23, 2013
M77, A Secretly Active Spiral Galaxy: October 7, 2013
M78, A Reflection Nebula: December 10, 2012
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M101, The Pinwheel Galaxy, October 28, 2013
M102, A Great Galactic Controversy: December 17, 2012
M103, The Last ‘Original’ Object: September 16, 2013
M104, The Sombrero Galaxy: May 27, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013
M109, The Farthest Messier Spiral: September 30, 2013

Come back next week, where another deep-sky object -- and another unique story about the Universe -- awaits you on Messier Monday!

esiegel Mon, 11/11/2013 - 13:02

Tags

" there are about 30 prominent, blue-colored stars identified in M39, the bluest of which is of spectral type A0, the brightest and bluest of all the A-class stars. "

For comparison, Vega is spectral class A0 (V or main sequence Sirian* dwarf)) and Sirius is spectral class A1 with other bright nearby A type main sequence Sirian* dwarf stars including Altair, Fomalhaut and four of the six stars making up Castor too.

Interestingly, Vega looks bluer and hotter than it really is because we see it pole-on :

http://en.wikipedia.org/wiki/Vega#Rotation

and thus at its hottest and bluest view.

I wonder whether any of the bright blue stars in M39 are similarly seen at their bluest and hottest angle and if so how many of them?

* Sirian used because "white dwarf" means something else entirely.

Isn't it Sirius B that is the dwarf star.

When colloquially called, the dwarf one is called "Sirius" too, but the visible one is not the dwarf.

But Sirius A is a main sequence, not dwarf.

They're using "dwarf" in the sense that is equivalent to "main sequence", or luminosity class V.

No idea what a "Sirian dwarf" is supposed to be, other than an invention to avoid confusion with the other, more commonly used meanings of "dwarf".

Which is just another reason why this sense of "dwarf" should be deprecated. Just say "main sequence".

Question here!
There are around 60 stars in this cluster. In the images, there are hundreds of stars within the confines of the obvious blue stars. How do we know which are in the cluster, and which are foreground/background stars. Has someone mapped the blue/red shifts of all the stars in the image? That would be impressive.

Google Hipparchos, Phil.

CB, Sirius has a white dwarf companion, Sirius B.

More confusion may arise because the Romans called Sirius (the visible one) "The little dog".

Yeah, I know, but there is only one sense in which Vega is a dwarf of any kind, and it's in the sense of "main sequence star", which they said, and also specified class V. Which is two ways in which that sense of "dwarf" is redundant.

But yeah, given the existence of Sirius B, using "Sirian dwarf" to *avoid* confusion with other meanings of "dwarf" is itself a confusing choice.

Vega is in Lyra, not Canis Majoris.

:-)

StevoR called both Vega and Sirius a "Sirian dwarf", aka "main sequence" aka "class V". The fact that they called Vega a dwarf (and also those other things in the same parenthetical) makes it fully clear that they did mean "dwarf" in the sense of "main sequence" which means they were not talking about Sirius B, because Sirius B is not the same kind of dwarf they were talking about.

To make it even more clear, Sirius B is a white dwarf and they specifically said they were not talking about white dwarfs.

So that's it. They were not talking about Sirius B because they were using a confusing (but real) definition of "dwarf".

I don't see what the location of Vega has to do with anything.

No, Steve was trying to shoehorn the same bullshit "argument" for Pluto to be a planet in here.

Pretty damn obvious, really.

The point of Vega was that I was talking about Sirius, not Vega, so there's no point of Vega, despite you bringing it up.

My point with Sirius was regarding the bunk claim of "sirian dwarf", the only match I see is for a fucking *Hamster*, was refutable and retarded.

They brought up Vega, not me, and in so doing made it perfectly clear what kind of dwarf they were talking about. Also they brought up Altair, Fomalhaut, also main-sequence class-V dwarfs.

But you want to talk Sirius. Okay. They were talking about Sirius A, not Sirius B. Sirius B is the white dwarf in the system, not the dwarf. Sirius A is the dwarf.

Except the point is that there's no such classification as "Sirian dwarf".

The only dwarf there is the white dwarf companion.

There's no such thing in astronomy as "Sirian Dwarf".

Also to be clear, "Sirian dwarf" wasn't a "claim". It was an attempt to make what definition of dwarf they were using more clear by distinguishing it from white dwarf.

Obviously it didn't work, and the failure is just a sign that this definition should be deprecated. Nevertheless it is technically correct to call Sirius A a dwarf, just like it is Sol.

"Sirian dwarf" isn't a thing in astronomy, they weren't claiming it was, they made it perfectly clear why they added that phrase (even if it didn't work).

"Dwarf" is a thing, and Sirius A is the dwarf.

Sirius B is the white dwarf.

"“Sirian dwarf” isn’t a thing in astronomy, they weren’t claiming it was, they made it perfectly clear"

Really?

What "they" are you wibbling on about, then?

Because Steve did no such thing.

There is no such thing as Sirian Dwarf unless you're into hamsters.

"Also to be clear, “Sirian dwarf” wasn’t a “claim”. It was an attempt to make what definition of dwarf they were using more clear by distinguishing it from white dwarf."

How about not making up a bullshit claim? Why was there an attempt to make a definition of dwarf that had absolutely no purpose other than to require that definition?

They didn't make a claim. And they didn't make a definition of dwarf, they tried to clarify an already-existing definition of dwarf (and failed). That definition of "dwarf" meaning "main-sequence" already exists.

Sirius A, Vega, Fomalhaut, and Sol are all dwarfs. That is valid terminology.

"They didn’t make a claim."

What definition of claim are you making here?

claim:5. A statement of something as a fact; an assertion of truth

Claim made: Vega is known as a Sirian Dwarf.

"Sirius A, Vega, Fomalhaut, and Sol are all dwarfs."

None of them are "Sirian Dwarfs". That is invalid terminology.

They did not claim it was known as that, they said they were using "Sirian" to distinguish "dwarf" from "white dwarf", the point of explaining such is that it *isn't* known that way. That attempt failed, partly because dwarf terminology is just confusing, partly because 'Sirian' is a confusing adjective to try to use for purpose of clarifying this case, and partly because you were unaware of the definition of "dwarf" they were trying to clarify in the first place so you didn't have the basis to understand what they were clarifying.

But as long as you now understand that you were wrong every time you said that the only dwarf in the system is Sirius B, because actually Sirius A is the dwarf (== main sequence == class V), while Sirius B is a white dwarf which is different, then I consider my contribution to this conversation done.

Please continue to berate StevoR for "claiming" that Vega is a Sirian Dwarf at your pleasure.

"They did not claim it was known as that"

No, he did.

@Wow

uh - WOW. the wikipedia entry for Hipparchos is kind of astounding. An astronomic education in itself. Definitely a sip from the fire hose.

:-)

It gets tiresome sometimes.

+1 CB

TWNLAA, Greg23.

-42.

"and partly because you were unaware of the definition of “dwarf” they were trying to clarify in the first place "

U gets one internets for fail, CB.

Messier Monday: A Four-Star Controversy Resolved, M73

esiegel — Mon, 21 Oct 2013 16:12:59 +0000

Messier Monday: A Four-Star Controversy Resolved, M73

"Controversy is only dreaded by the advocates of error." -Benjamin Rush

Welcome back to another Messier Monday! Each week, we take an in-depth look at one of the 110 deep-sky wonders of the Messier Catalogue, from distant galaxies to nearby star clusters, from nebulous star factories to ancient globular bunches, and from stellar remnants to the rare-but-interesting anomalies. There are only three such anomalous entries out of all 110 Messier objects, and today provides us with a fantastic opportunity to take an in-depth look at one of them.

Image credit: The Messier Objects by Alistair Symon, from 2005-2009.

A little bit of context: back in the late 18^th Century, when Messier was compiling his catalogue, professional telescopes were small in terms of aperture and light-gathering power. The primary mirrors had diameters of only 6-to-8 inches (15-20 cm) and the poor reflectivity of an old-fashioned speculum mirror, since the glass mirror didn't come into use until the 1850s. (Even a 3" amateur telescope today -- which you can buy for around $100 -- is probably superior to anything Messier ever used, although his skies were probably freer of light pollution than yours!)

As a result, objects -- even at maximum magnification -- appeared tiny to him and were often difficult to resolve; it's a wonder that 107 of these 110 objects actually do turn out to be genuine deep-sky wonders! But today, I'd like to highlight one of the three oddballs: Messier 73. Here's how to find it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

As the months progress onwards towards the December solstice, the Summer Triangle of Deneb, Vega and Altair head farther and farther towards the western horizon after sunset. Meanwhile, the bright star Fomalhaut hovers above the southern horizon. If you look halfway between Fomalhaut and Altair, there are a number of naked-eye stars that can help guide you towards Messier 73. In particular, there are four rather bright stars near the imaginary line connecting them: two north of the line and closer to Fomalhaut, and two south of the line and closer to Altair.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Sadalsuud and Deneb Algedi are the two bright ones closer to Fomalhaut, while α Capricorni (a double star) and Dabih are the two bright ones closer to Altair. In the middle of that region enclosed by those four stars, two slightly less bright -- but still naked eye -- stars can guide you to your destination: Albali (closer to Altair) and ν Aquarii (closer to Fomalhaut).

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

There are actually three interesting objects in this vicinity: the globular cluster Messier 72, the recently deceased star that formed the Saturn Nebula, and today's object, the difficult-to-find Messier 73. What should you look for, if you're poking around in this star field?

Image credit: Rick Beno of Conferring With the Sky Observatory, via http://www.conferringwiththesky.org/.

This appears to be a tiny cluster of four stars of approximately the same brightness (one is slightly dimmer), described by Messier as:

Cluster of three or four small stars, which resembles a nebula at first glance, containing very little nebulosity.

If you can locate it for yourself, it looks like this.

Image credit: REU program / NOAO / AURA / NSF.

Now, this is really interesting. On one hand, most of the open star clusters we see in the night sky contain hundreds or even thousands of stars, but only a handful of relatively bright ones that can be easily resolved in an instrument of the size-and-quality that Messier had at his disposal. One can hardly be upset at Messier for thinking -- especially if he thought he saw a little nebulosity around this object -- that there was probably a star cluster here.

Image credit: Bayfordbury Observatory, via their flickr account.

On the other hand, it's well-known that star clusters tend to be short lived. The vast majority of them are completely dissociated, with their component stars strewn through the galaxy, in less than a billion years. It stands to reason, however, that when a star cluster does dissociate, there ought to be a few small groups that remain together, bound by their own gravity.

Could this collection of four bright stars (with possibly more below the limits of Messier's instruments) be the very first such object to be discovered?

Or would this simply turn out to be a very statistically unlikely coincidence: four stars of almost identical brightness, standing out against the black backdrop of space, separated from one another by less than a twentieth of a degree (or 2.8 arc-minutes, to be precise)?

Image credit: Fred Espenak of http://astropixels.com/.

There were many throughout the centuries who derided this object's inclusion in the Messier catalogue, much like the other anomaly we've looked at here: the double star Messier 40. But this is no obvious mistake; it would take more than two centuries to figure it out. As recently as 2000, a high-resolution photometry study indicated that this was a 2-or-3 billion year old cluster remnant, which looked like it followed a characteristic Hertzsprung-Russell curve.

Image credit: L. P. Bassino, S. Waldhausen, and R. E. Martínez, Astronomy & Astrophysics 355, 138 (2000).

It wasn't until two years later that a high-resolution spectroscopic study was done on this cluster. By looking at the spectra of the six brightest stars near the center of the cluster (including the four main ones), they could tell whether they were gravitationally bound and whether they were the same rough age-and-metallicity. The results?

Image credit: M. Odenkirchen & C. Soubiran, A&A 383, 163 (2002).

The stars are all at different distances, moving with speeds that indicate they're unbound, have different atmospheres and are in no way related. This does, however unlikely it seems, confirm that this is merely a chance alignment of stars, and it also means that we have not yet discovered a smoking gun for a tightly-bound remnant of a once-open cluster.

But they're out there; it just happens that Messier 73 isn't one of them.

Image credit: Sloan Digital Sky Survey (SDSS), via WikiSky.

A chance, unlikely alignment of stars never seemed so interesting, and at last a 200+ year controversy can be laid to rest! It's just a collection of four stars that aren't connected at all, and yet, I can't help but be fascinated at the possibility that there are chance alignments out there that aren't chance at all, but are rather the once-majestic remnants of long-gone star clusters!

In any case, that brings us to the end of another Messier Monday! Including today's entry, we've looked at the following objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M11, The Wild Duck Cluster: September 9, 2013
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
M33, The Triangulum Galaxy: February 25, 2013
M34, A Bright, Close Delight of the Winter Skies: October 14, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M48, A Lost-and-Found Star Cluster: February 11, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M73, A Four-Star Controversy Resolved: October 21, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M75, The Most Concentrated Messier Globular: September 23, 2013
M77, A Secretly Active Spiral Galaxy: October 7, 2013
M78, A Reflection Nebula: December 10, 2012
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M102, A Great Galactic Controversy: December 17, 2012
M103, The Last ‘Original’ Object: September 16, 2013
M104, The Sombrero Galaxy: May 27, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013
M109, The Farthest Messier Spiral: September 30, 2013

Come back again next Monday, where another deep-sky wonder -- and another glimpse deep into the Universe -- awaits!

esiegel Mon, 10/21/2013 - 12:12

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Messier Monday: The Last 'Original' Object, M103

esiegel — Mon, 16 Sep 2013 16:02:31 +0000

Messier Monday: The Last 'Original' Object, M103

"Now, this is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning." -Winston Churchill

To kick off every week for nearly a year now, we've begun it with Messier Monday, where we take an in-depth look at the 110 deep-sky objects that make up the first elaborate catalogue of fixed night-sky wonders that could possibly be confused for transient comets. Originally, when first published, this catalogue was made up of 103 objects; the final 7 were added posthumously. Each one tells its own unique story, yet all of them tell a sliver of our own story, and give us a window into our place in the cosmos.

Image credit: Rich Richins, of all 110 Messier objects (in no particular order), from a 2009 marathon.

Today, let's take a look at the very last object in the original catalogue as it was published by Messier: the rich star cluster in Cassiopeia, Messier 103. After sunset (or, really, at any time) tonight, look towards the north from anywhere above the Earth's equator.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Polaris -- the bright star located almost exactly at the north celestial pole -- is visible at all times (with clear skies and horizons) from all locations in the northern hemisphere. The entire sky appears to rotate around this point, and it's flanked on opposite sides by two prominent configurations of familiar stars: the Big Dipper, shown off-screen to the left (except for the tip of the "cup", Dubhe), and Cassiopeia, the "W" to the right of Polaris in the image above. To locate Messier 103, look to the bottom of the first "V" in the "W" at the bright star Ruchbah.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Even on a night with a bright Moon (like tonight), even a pair of binoculars is revealing enough that if you look at Ruchbah, and head over just one degree towards the first star in the "W" of Cassiopeia (ε Cas), you can't miss M103, just slightly "below" the imaginary line that connects the two.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

This is one of the smallest open clusters in Messier's famed catalogue, just a tenth-of-a-degree (or 6') in diameter. Yet, located in the plane of the Milky Way (albeit far away from the galactic center), Messier 103 is a remarkable object for teaching us about the young clusters where the galaxy's newest stars reside.

Silhouetted against the dense star field of the galactic disk, there's one star -- a bright, blue triple star -- that clearly outshines all the others when you look towards this young cluster. Surprisingly, this star is definitively not a member of M103, but rather a foreground star known as Struve 131, a blue supergiant just a few hundred light-years away. But Messier 103 is located some 8,500-9,200 light-years away, making it (probably) the most distant open star cluster in the entire Messier catalogue!

Image credit: Lynn Easley, taken just last night, at http://www.astrobin.com/full/56654/?mod=none.

If we focus on the colors of the stars in this cluster -- ignoring the blue supergiant at the lower left, above -- you'll see a large number of bright, blue stars, and one bright red one in the center. It's the colors of these stars, more than any other property, that teach us the most about the cluster itself.

Newborn stars come in a wide variety of colors, brightnesses and masses: the most massive ones are the brightest and bluest, while the less massive ones shine less brightly and give off lower-energy, redder light. But the most massive ones also burn through their fuel the fastest, and when they run out of hydrogen fuel in their core, they expand into red giants, burning heavier elements until they run out of those, too, and die in planetary nebulae or -- if they're massive enough -- supernovae!

The brightest hydrogen-burning stars in M103 are still very young, blue and massive, but the red one in the center is, in fact, a red giant that left the main sequence only a few million years ago, at most.

Image credit: Hillary Mathis, N.A. Sharp / NOAO / AURA / NSF.

The abundance of intrinsically bright, blue stars, as well as the absence of the brightest class of blue stars -- the O-stars -- allow us to place the age of this cluster at a mere 22-25 million years, or just around 30% the age of the Pleiades.

There are at least 172 member stars in this cluster, although -- based on what we know of open clusters -- there may be as many as 1,000 stars or more in there. An interesting take on this cluster comes from 2MASS (the two-micron all-sky survey), which looks in the far infrared portion of the spectrum, preferentially highlighting redder stars and muting the intrinsically brighter, bluer stars.

Image credit: Two Micron All Sky Survey (2MASS) / UMass / IPAC / Caltech / NASA.

The best true-color "amateur" image I could find of this was taken by Jim Misti, although, given the fact that he used a 32" (0.8-meter) telescope, amateur is a loose term.

Image credit: Misti Mountain Observatory / http://www.mistisoftware.com/.

Beyond that, there is a Hubble space telescope image. Although my image processing skills leave a lot to be desired, I did want to point out that there are more than 1,200 identifiable (albeit, many may be spurious) sources here, and I've done my best to clean up the Hubble image so you can scroll through just a tiny portion of this cluster, although dead pixels still remain. It may not look impressive, but remember just what a tiny portion of the cluster the Hubble space telescope actually images; you're seeing faint stars as points-of-light that aren't even visible in any other image presented here!

Image credit: NASA, ESA, the Hubble Legacy Archive and me.

For those of you who want to see something better than that (and I don't blame you), here's a high-resolution look through the core of Messier 103 thanks to Jim Misti, which reveals just how rich this cluster -- as well as the outskirts of the galactic plane -- happen to actually be!

Image credit: Misti Mountain Observatory / http://www.mistisoftware.com/.

And that will do it for another Messier Monday! Including today’s entry, we’ve taken a look at the following Messier objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M11, The Wild Duck Cluster: September 9, 2013
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
M33, The Triangulum Galaxy: February 25, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M48, A Lost-and-Found Star Cluster: February 11, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M78, A Reflection Nebula: December 10, 2012
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M102, A Great Galactic Controversy: December 17, 2012
M103, The Last 'Original' Object: September 16, 2013
M104, The Sombrero Galaxy: May 27, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013

Come back next week for yet another one, as we won't stop until we've covered them all!

esiegel Mon, 09/16/2013 - 12:02

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Gotta catch 'em all!

See, that's the problem with open clusters. They're too open. If they were more globby, the Hubble picture would be great.

Or they need to at least have some nebulosity. A few scattered stars can look amazing if surrounded by whispy clouds.

Can someone point me at M103's suggestion box? I have some more ideas on how it could improve its image.

Messier Monday: The Top-Heavy Gumball Globular, M12

esiegel — Mon, 26 Aug 2013 18:13:28 +0000

Messier Monday: The Top-Heavy Gumball Globular, M12

"A little knowledge that acts is worth infinitely more than much knowledge that is idle." -Khalil Gibran

It’s time again for another Messier Monday! The Messier Catalogue was the original comprehensive and accurate catalogue of fixed, deep-sky objects visible to any dedicated (northern hemisphere) skywatcher with even the most primitive of astronomical equipment. Over the centuries, as our understanding of what we're looking at has improved, these 110 celestial wonders have provided classic examples of astronomical phenomena ranging from stellar corpses to new star-forming regions, from young clusters of just a few hundred stars in our galaxy to ancient collections of hundreds of thousands clustered around our galactic halo, from nearby spiral galaxies to giant ellipticals some 60 million light-years distant.

Image credit: Mike Keith of http://cadaeic.net/astro/PeriodicMessier.htm.

Each object has its own, unique story to tell about the Universe, and each week I try to highlight some of the most remarkable features and images about one that we haven't looked at before. Today, let's take an in-depth look at one of the most interesting of all the globular clusters and the twelfth object in the Messier catalogue, the Gumball Globular, M12. Here's how to go about locating it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Most of the Messier objects can be found by locating one of the major constellations or asterisms visible from the Northern Hemisphere at various times during the year: the Big Dipper, the Summer Triangle, Orion the hunter, or the Teapot in Sagittarius. For those of you hunting Messier 12, you'll have no such luck for this object.

The best I've been able to do is to locate the Summer Triangle, and then head south of Vega to the bright star Rasalhague, the brightest star in the ancient constellation Ophiuchus.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

To the south of Rasalhague, a rough "circle" of nine-to-ten bright stars should appear, with the blue star Marfik appearing just below the 3 o'clock position on that circle. And if you follow the barely visible naked-eye stars from Marfik into the circle, they'll make a hook-like pattern curling inwards in a counterclockwise fashion. Towards the tip of the hook, shown in the location against the background stars below, you'll find the Messier object known as the Gumball Globular, Messier 12.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

First discovered in 1764 by Messier himself, it was originally described as a:

Nebula discovered in the Serpent, between the arm and the left side of Ophiuchus: this nebula doesn't contain any star, it is round & its light faint; near this nebula there is a star of 9th magnitude.

Viewed through a small telescope or a pair of binoculars, it's virtually impossible to tell whether this object is, indeed, composed of stars or not.

Image credit: Mark Byland of the Litmus Paper Test, via http://tlpt.blogspot.com/.

This is true of most globular clusters; M12 is no exception to the rule here. Globulars tend to be collections of a few hundred thousand stars -- generally formed when the Universe was only a billion years old, give or take -- spread out over only 100 light-years or less, located in a galaxy's halo. Because they tend to be many thousands of light years away, it takes a more powerful telescope, usually with quite a long exposure (or series of exposures), to resolve individual stars.

Image credit: Wade Van Arsdale, Central Arkansas Astronomical Society, via http://www.compubuild.com/astro/.

And there are about 200,000 of them in there for M12, located some 16,000 light years away. You'll notice that there's a denser, more concentrated core and a more diffuse, extended halo around this globular cluster. Messier himself was only capable of seeing the core; he had no idea how far the true extent of this cluster reached. While the inner core is only about 14 light years in diameter, the entire cluster extends a remarkable 75 light years all the way from end-to-end.

Its properties actually make it one of the less dense globular clusters: on a scale from I to XII, this one is classified as a IX, which means it's only loosely concentrated towards the center.

The stars in it, at first glance, appear to be normal for a globular cluster.

Image credit: REU Program / NOAO / AURA / NSF.

There's no neutral gas or dust, there are only a handful (13, at last count) of variable stars inside, and most of the stars are very old (about 12.7 billion years, typically) and metal-poor (with only 7% of the heavy elements found in the Sun), two things that go together. The bright, orange-colored stars in the images you see tend to be stars that were once roughly the same mass as the Sun that have reached the end of their lives, and have expanded into their red giant phase of life.

But there's also something odd about this globular cluster.

Image credit: Michael Gariepy / Adam Block / NOAO / AURA / NSF.

It isn't the few blue stars that are located inside: these blue stragglers are common in globular clusters. Although typically only the very young stars are blue, they can also form when two older stars merge together, creating a larger, hotter, and bluer star. Globular clusters -- with hundreds of thousands of stars concentrated within just a few light years -- are hotbeds for blue stragglers to form.

No, the odd thing about Messier 12 is invisible to most eyes, even in non-visible wavelengths.

Image credit: Two Micron All-Sky Survey (2MASS).

You see, typically, when you have a collection of stars, whether they're an open cluster, a globular cluster, or even just stars in the field (like our Sun) that aren't associated with any particular grouping, they tend to follow particular patterns. In particular, the brightest stars tend to be a combination of the most massive and the most evolved, but they tend to be few in number. As you go to progressively lower masses and cooler temperatures, you get more and more stars, all the way down to the lowest-mass stars: the M-class red dwarfs, which make up roughly 3 out of every 4 stars.

So when we took some of the most powerful, highest-resolution telescopes we had and took a look deep inside the inner core of this cluster, what we found was really surprising.

Image credit: ESO, Guido De Marchi (ESA), Kristina Boneva & Haennes Heyer (ESO).

The image above was taken with the European Southern Observatory's Very Large Telescope; you can view a zoomed-in snippet of the very center of this image -- in full resolution -- below.

Image credit: ESO, Guido De Marchi (ESA), Kristina Boneva & Haennes Heyer (ESO).

The big surprise? Hardly any red dwarfs. Practically no M-class stars at all, compared to what's expected. Somehow, there are some 200,000 stars in here, and yet the most abundant ones -- the ones that there should be an extra 600,000+ of -- are nowhere to be found.

What gives? As Guido De Marchi explains:

It is however clear that Messier 12 is surprisingly devoid of low-mass stars. For each solar-like star, we would expect roughly four times as many stars with half that mass. Our VLT observations only show an equal number of stars of different masses.

The leading theory is that like many globulars, M12 passes through the galactic plane periodically. Unlike most of them, its orbit takes it very close to the galactic center, meaning that the lowest mass stars got preferentially kicked out, the same way if you kick a pebble and a boulder with the same force, the pebble tends to go farther.

Image credit: NASA / ESA / STScI / Hubble Space Telescope.

Our best estimates are even more severe than the naive estimate above: it's estimated that Messier 12 has lost about a million stars to the galaxy over its lifetime, and is expected to live only another 4.5 billion years before dissociating completely, making it one of the shortest lived globular clusters we know of!

As always, the best image comes courtesy of the Hubble Space Telescope, above, and I've provided you with a slice through this magnificent structure, below.

Image credit: NASA / ESA / STScI / Hubble Space Telescope.

Such an incredibly interesting object, and yet the one mystery I haven't been able to unravel is why it's called the Gumball Globular! Some mysteries are simply beyond the reach of modern astrophysics.

And that wraps up another Messier Monday! Including today's entry, we've taken a look at the following Messier objects:

M1, The Crab Nebula: October 22, 2012
M2, Messier’s First Globular Cluster: June 17, 2013
M5, A Hyper-Smooth Globular Cluster: May 20, 2013
M7, The Most Southerly Messier Object: July 8, 2013
M8, The Lagoon Nebula: November 5, 2012
M12, The Top-Heavy Gumball Globular: August 26, 2013
M13, The Great Globular Cluster in Hercules: December 31, 2012
M15, An Ancient Globular Cluster: November 12, 2012
M18, A Well-Hidden, Young Star Cluster: August 5, 2013
M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
M25, A Dusty Open Cluster for Everyone: April 8, 2013
M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
M30, A Straggling Globular Cluster: November 26, 2012
M33, The Triangulum Galaxy: February 25, 2013
M37, A Rich Open Star Cluster: December 3, 2012
M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
M40, Messier’s Greatest Mistake: April 1, 2013
M41, The Dog Star’s Secret Neighbor: January 7, 2013
M44, The Beehive Cluster / Praesepe: December 24, 2012
M45, The Pleiades: October 29, 2012
M48, A Lost-and-Found Star Cluster: February 11, 2013
M51, The Whirlpool Galaxy: April 15th, 2013
M52, A Star Cluster on the Bubble: March 4, 2013
M53, The Most Northern Galactic Globular: February 18, 2013
M56, The Methuselah of Messier Objects: August 12, 2013
M57, The Ring Nebula: July 1, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M65, The First Messier Supernova of 2013: March 25, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M71, A Very Unusual Globular Cluster: July 15, 2013
M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
M78, A Reflection Nebula: December 10, 2012
M81, Bode’s Galaxy: November 19, 2012
M82, The Cigar Galaxy: May 13, 2013
M83, The Southern Pinwheel Galaxy, January 21, 2013
M86, The Most Blueshifted Messier Object, June 10, 2013
M92, The Second Greatest Globular in Hercules, April 22, 2013
M94, A double-ringed mystery galaxy, August 19, 2013
M97, The Owl Nebula, January 28, 2013
M99, The Great Pinwheel of Virgo, July 29, 2013
M102, A Great Galactic Controversy: December 17, 2012
M104, The Sombrero Galaxy: May 27, 2013
M108, A Galactic Sliver in the Big Dipper: July 22, 2013

Come back next week, and if anyone has any history on why it's known as the Gumball Globular, I'd love to know!

esiegel Mon, 08/26/2013 - 14:13

Tags

M31 Please :)

I think the name "Gumball Cluster" comes from O'Meara's book, The Messier Objects. He came up with his own nicknames for several of these. I may be off on this (I do not have the book with me), but that's my recollection.

Yep, it was O'Meara. Kind of wish he hadn't done that.

What does the Universe really look like?

esiegel — Fri, 16 Aug 2013 16:13:51 +0000

What does the Universe really look like?

"On a cosmic scale, our life is insignificant, yet this brief period when we appear in the world is the time in which all meaningful questions arise." -Paul Ricoeur

Ask anyone who's looked up at a dark sky on a clear, moonless night, and you'll immediately hear tales about how incomprehensibly vast the Universe is.

Image credit: Randy Halverson, flickr user dakotalapse, from http://dakotalapse.com/.

But what you're looking at isn't much of the Universe at all. In fact, practically every point of light you see, including the vast swath of stars too dim to individually resolve, comes from within our own Milky Way galaxy. As we know from generations of telescopes, observatories, observations, as well as physicists and astronomers, the Universe goes far beyond that.

Image credit: NASA, ESA, R. Windhorst, S. Cohen, and M. Mechtley (ASU), R. O'Connell (UVa), P. McCarthy (Carnegie Obs), N. Hathi (UC Riverside), R. Ryan (UC Davis), & H. Yan (tOSU).

There are hundreds of billions of galaxies (at least) out there in our observable Universe, spread out, from our vantage point, over a sphere some 46 billion light-years in radius.

If we were to look at it, as human beings, we'd be limited by the biology of our eyes. Very well adapted for seeing in well-illuminated conditions, we'd do somewhat less well in intergalactic space; we'd only be able to see the closest and brightest of all light sources, which would most likely limit us to only a few dozen galaxies if we were plunked down in a random location.

Image credit: Knut Skaar of http://knutsastronomy.blogspot.com/, of Messier 109.

As it is, we're within our own galaxy, and so have thousands upon thousands of foreground stars that we have to ignore when we look deep into the Universe. We also are familiar with using tools like telescopes and/or cameras -- required to see even nearby, bright galaxies like Messier 109, above -- to help enhance our understanding of what's out there.

No wonder so many of us have dreams of voyaging across the Universe, seeing what's out there, of all the galaxies and how they clump and cluster together, of the different forms they take, and of what such an adventure would look like.

Image credit: Cosmic Flows Project/University of Hawaii, via http://www.cpt.univ-mrs.fr/.

Recently, the Cosmic Flows Project has put together a stunning video (narrated in French) that's a 17-minute tour through the local Universe within 300,000,000 light-years. It's a remarkable look at not only our Milky Way, our local group, our nearest supercluster (the Virgo supercluster, of which we're on the outskirts, and which contains about 100,000 galaxies), and the largest superclusters and voids found nearby! When you've got the time, you definitely want to watch the whole thing.

Video credit: Hélène Courtois, Daniel Pomarède, R. Brent Tully, Yehuda Hoffman, and Denis Courtois.

But you might look at this and wonder just how we figure this out. From our vantage point here on Earth -- or even in space from someplace within our Solar System -- there's a lot of information to filter through and figure out. The simplest thing you can do actually gets you very far: remember Hubble's Law, or the fact that not only is the Universe expanding, but the distance a galaxy is from us is directly proportional to its recession speed.

Image credit: Larry McNish of RASC Calgary Centre, via http://calgary.rasc.ca/.

It turns out that redshift is actually a somewhat easy property of a galaxy to measure, so if you know Hubble's law, you can infer how far away that galaxy is.

Well, kind of. Hubble's Law gives a very good approximation for distances on average, on large scales. But Hubble's law doesn't account for all of an object's redshift. There's also the very minor issue (that's sarcasm) of all the other matter in the Universe, and the gravitational effects it's had over the past 13.8 billion years.

Image credit: Wikimedia Commons user Brews ohare.

Matter has this annoying property that it clumps and clusters together, and that's because gravitational attraction causes it to move. Don't get me wrong, this is great for lots of things, but it's not great when you're trying to figure out how distant an object is based on its motion!

It creates distortions along the line-of-sight, known as redshift-space distortions.

Image credit: M.U. SubbaRao et al., New J. Phys. 10 (2008) 125015, via IOPscience.

As you can see, on the left, these distortions create apparent lines or streaks that point radially towards you. We call these features Fingers of God. These happen because galaxies that are clustered together move more rapidly, both towards and away from the center of the cluster, which spreads them out in redshift.

There's also a less noticeable effect, where clusters move relative to one another and fall into superclusters and filaments; these actually have the reverse effect on larger scales, creating flatter features on very large scales. There are some who call this the Kaiser effect (after Nick Kaiser), but I've always called them Pancakes of God.

Image credit: from the movie Thor (2011).

So, how do we overcome these redshift space distortions? Believe it or not, this is one of the times where simulations have helped us tremendously! Thanks to the way that structure forms over the history of the Universe, from its gravitational evolution, we can figure out exactly how, on all distance scales, clustered objects translate from redshift space, which is easy to measure, into real space, which is the Universe we actually live in.

Image credit: Lahav et al. and The 2dF Galaxy Redshift Survey Team (2002).

At this point, we understand clustering in our Universe -- as well as the dark matter and dark energy that it's dependent on -- to make this transformation with incredibly high degrees of confidence. So sure, we start in the same place: we measure the redshift of galaxies and plot them out accordingly.

Image credit: Two-micron all-sky survey (2MASS), IPAC / Caltech, Univ. of Massachusetts.

But then we use all the things we know about mass and matter and gravity to understand how these galaxies have clustered together, and to map out -- to the best of our abilities -- their peculiar velocities, or their velocity with respect to the Hubble flow. By subtracting those peculiar velocities out, we can get estimates for their real-space positions, and hence, for how far away in each direction each galaxy is.

Image credit: Tegmark, M., et al. 2004, ApJ, 606, 702. (FOGs are Fingers of God.)

So what would flying through the Universe -- the real space Universe -- actually look like? Not to human eyes, but to our eyes as they'd be if we had pupils the size of giant telescopes? Well enjoy this brilliant video by Miguel Aragon, Mark Subbarao and Alex Szalay of the Sloan Digital Sky Survey that puts it all together!

And this is "only" about 400,000 galaxies in their actual positions, or just 0.0003% of the galaxies in the Universe, at most.

And that's just a tiny glimpse into what the Universe really looks like!

esiegel Fri, 08/16/2013 - 12:13

Tags

large scale structure

The Cosmic Flows video (which I've seen before) is fascinating, but that SDSS fly-through is stunningly beautiful! I started to get the same feeling I get when I stare at the Hubble Deep Field.

Thanks for sharing.

Good blog. I watched videos. Wow. Great stuff. Thanks to all.

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The narrator is speaking English, she just has a French accent.

Thank you Ethan, I understand the Cosmic Flows video much better now!

I've read that if you picked a random spot in the Universe, the odds are you wouldn't be able to see a single galaxy or star with the naked eye. (David Deutsch said it, I think.)

" if you picked a random spot in the Universe, the odds are you wouldn’t be able to see..."

well.. if it's truly random then that spot could be in some other galaxy and you would see more or less the same thing you see from earth. If you happen to find yourself in intergalactic space, again depends where. If you're in some galaxy cluster, you would see some galaxies as point sources like stars. If you happen to land in a particularly big intergalactic void, then yes.. all you would see is nothing.

But such a big generalization, that the chances are you won't see anything, is wrong IMO.

"Hubble’s Law, or the fact that not only is the Universe expanding, but the distance a galaxy is from us is directly proportional to its recession speed."

I have a question that may make no sense whatsoever since my astrophysics degree got lost in the mail...

As I understand it: Galaxy A is receding from us at a certain rate. Galaxy B, which is twice as far away as Galaxy A, is receding from us at twice the rate of Galaxy A. Is the space between Galaxy B and Galaxy A expanding at a faster speed than the space between us and Galaxy A? Or does it seem like it's going faster because there's twice as much expanding space between us and Galaxy B, and it just looks faster from our perspective? Or is it all the same thing because of Relativity?

@9 william

the second one. there is twice as much. and vice versa. to someone in galaxy B we would appear to be moving away faster than galaxy A which is nearer to it. Basically every unit of space is expanding. everything appears to be moving away from everything else.

Mark,

I've heard that before, too.

It's quite wrong. It's true that the Universe is highly clustered into clumps and filaments, but -- if you removed our entire galaxy -- we'd be able to see a large number of galaxies. Andromeda and Triangulum would be the brightest, and other local group galaxies would be prominent as well, but there would also be many galaxies from beyond the local group, including at least two I can think of (including one of our Messier Mondays) more than 10 million light years distant.

So, there are plenty of locations from where not a single galaxy would be visible, but if you plunked yourself down at a random location, far fewer than 50% of those places would have that property.

So there you have it. The universe looks like--a mammogram!

This is why I believe in a Supreme Being

re #13 is it because you don't understand and do not wish to know your knowledge is limited and work to reduce the limitations. Hence will decide to dump the idea "I have no idea" into "Goddidit" and therefore drop the idea that maybe you could find out about things if you spent a little effort?

This is why I believe in a Supreme Being

What an of thing to say, when nothing here presents evidence for such a being.

And thank you to my tablet for changing 'odd' to 'of'. I should have caught that.

@ Sinisa;

Thanks! That's what I was thinking but as I said, my astrophysics degree got lost in the mail. ;)

Has the science of cosmology and astrophysics become absolutely positively 100% accurate were the theories are concrete undeniable evidence? While Classical Physics and Quantum Mechanics still duke it out? Which to give super detailed reports about measurements would be required. Personally, I think not, and don't buy the hype! Scientists want to pretend they have all the answers. I seriously doubt they understand the question!

It would be more than advisable to get acquainted with the Electromagnetic theory; "Electric Universe" for short. It answers more questions than the gravitational model and will ultimately replace Newtonian physics as well as much of Einstein's assumptions on how our universe operates. Just Google "Electric Universe Theory" and be prepared to be amazed...

Re #19: How are classical physics and quantum mechanics "duking it out"? If you mean that our two main theories in physics, general relativity and the standard model are incompatible, then yeah, we know that. However, they are hardly "duking it out". They are both right, just there are domains in which they give different answers.

It's a similar situation as Newtonian physics. Newtonian physics is perfectly right as long as you stay within its domain. We've launched interplanetary probes and made moon landings using only Newtonian physics. It works well within its proscribed domain. Similarly, GR and SM both work well within their domains, which in the case of GR includes just about any cosmological observation.

Obviously, both GR and SM cannot be absolutely complete. However, any new theory to replace them must yield predictions in line with them for observations within their domains. That is, we can explain the observations of cosmology with GR, and even when it's replaced, the new theory must give the same predictions. So yes, we are pretty confident in our cosmological models, at least unless new data becomes available. If so, then we would change the model.

Joe, that crock has been peddled here before.

We were amazed.

Just not in the way you'd hoped...

@ blade:

If you think scientists pretend to know all the answers, then you're only listening to pretend scientists.

@ Joe:

EU proposes that the sun is powered not by fusion, but by an interstellar DC current. Test this hypothesis. Calculate the minimum current strength required to explain the observed output of the sun, assuming a perfect conversion of input energy to output energy. Then calculate the strength of the induced magnetic field of such a current at 1 AU. Hint: It will be much stronger than the earth's magnetic field.

Then step outside with a compass and prove the theory false.

Electric Universe for short

I've heard of Electric Boogaloo and Electric Avenue, but Electric Universe is new. How large a surge protector is needed?

I'm gonna take you to Electric Avenue.

Cool article and interesting to think about. It might seem bizarre now, but with quantum processors we will eventually accomplish some incredible things, like run computer simulated universes (as modeled above) that are indistinguishable from our own “real” universe, even complete with simulated minds. There is even a new book out that discusses the implications of all this (i.e., “On Computer Simulated Universes”) and introduces concepts such as the 'Computer Simulated Universes Evolutionary Hypothesis'. With many active simulations, there would be a wide range of physical properties differing from universe to universe. Universes with more positive physical traits to support life would produce better environments for more advanced civilizations to evolve to the point where they themselves would create their own computer simulated universes. And this process would continue. So over a long period of time, universes would evolve with the physics more favorable for life. The book argues that universes, over time, might have been naturally selected for particular physical properties, with an end result of creating more and more habitable and longer-lived universes. This line of reasoning explains how the laws of physics might actually evolve relying on a process somewhat similar to human or species evolution.

i love this article because it has a lot of diagrams that show us what the universe is like for real and i especially like the graph aswell i think people should use this article more often because the universe is an interesting thing to know. i i find it interesting because everything on here is what i am looking for

bellooo i love universe hence i love these pictures