"This nebula had such a resemblance to a comet in its form and brightness that I endeavored to find others, so that astronomers would not confuse these same nebulae with comets just beginning to shine." -Charles Messier
Astronomers have been scouring the skies for new discoveries since long before the invention of the telescope. Why, just with the naked eye and some dark skies, anyone can discover about 6,000 stars, five major planets, the Milky Way, and the occasional very faint nebula.
Image credit: Miloslav Druckmuller.
And of course, if you're very fortunate and very diligent, you could have been among the first to discover whenever a new comet brightened and suddenly appeared! By time comets get relatively close to the Sun, their tails become bright and extended, and often easily visible to the naked eye.
But the telescope changed everything. Because now, for the first time, you could discover a comet when it was still very faint. When it was, in fact, still invisible to the naked eye, but where a careful observer would notice an object where there previously was none. Something, perhaps, like 2004's Comet Machholz.
Image credit: Charles and Pat Briley.
But one had to be very careful with such a venture. After all, if you're only looking for a dim, extended object, you can very easily fool yourself, as a myriad of 17th and 18th Century astronomers did.
For example, take a look at the image below. You might notice something intriguing in both the upper-right-hand corner and in the lower-left-hand corner.
Image credit: Ian Musgrave.
One of these guys is a comet! In the lower-left-hand corner, next to a bright star, is comet C/2008 Q3 Garrad. But in the upper-right hand corner? That's no comet; that fuzzball is a special type of object, known as a globular cluster!
Image credit: wikimedia commons.
Globular clusters can be incredibly dense, tightly packed objects, with up to millions of stars packed into a space just a few light years across, like Messier 75, above, one of the densest, or Type I, globular clusters.
Globular clusters come in less dense varieties also, all the way up to Type XII, like the much more diffuse Messier 68, below.
Image credit: Andrew Bray.
The earliest catalog of hey-look-out-that's-not-a-comet objects comes from Charles Messier, an 18th Century French astronomer. And one of the more curious objects he cataloged might look no different to you than your typical globular cluster. In fact, Messier 54, shown below, looks like your pretty run-of-the-mill globular cluster.
Image credit: NM Tech.
As a Type III globular cluster, it's one of the densest ones out there. It's also quite faint, and it requires a pretty spectacular-sized telescope to start to see the individual stars in it. Yes, Hubble can do it, but so can the 32" telescope at Misti Mountain Observatory:
Image credit: Misti Mountain Observatory.
Discovered in 1778, Messier 54 is a dense-but-unspectacular collection of many hundreds of thousands of stars, in a span just 150 light-years across. (By comparison, that's about 40-100 times as many stars as there are that close to our Sun.)
So you might be tempted to make a map of all of the globular clusters we've found in our galaxy, and ask which one is Messier 54?
Image credit: William E. Harris, McMaster U., and Larry McNish.
Surprisingly, the answer is: none of them! Unknown to Messier, this globular cluster was the first globular cluster found that's actually outside of our own galaxy! Unbelievably, this wasn't discovered until 1994, over two centuries after Messier 54 was discovered.
So, you ask, where is this guy?
Image credit: David Martinez-Delgado (MPIA) & Gabriel Perez (IAC).
87,000 light years away, in the Sagittarius Dwarf Elliptical Galaxy, which is currently being cannibalized by our Milky Way! This puts it more than three times as far away as the center of our Milky Way galaxy is, and explains both why it's so faint for its size and why you need such a large telescope to resolve individual stars in it. Who knew, back in the 1700s, that this diffuse ball of stars was actually part of a miniature galaxy currently being devoured by our own?! Yet here we are, over 200 years later, finally crediting Charles Messier with a discovery he likely never imagined he made: finding the first globular cluster outside of our own galaxy!
Image credit: Flickr user Donbo7011.
And finally, I know that many of you out there are wondering about black holes. Large galaxies like us have very large, supermassive black holes. Individual, massive stars can collapse at the end of their lives to form stellar-mass black holes. But what do you find at the cores of these globular clusters? Anything? Something different? Well, unlike our galaxy (and other large galaxies like it) with a supermassive black hole millions or billions of times the mass of our Sun, Messier 54 likely contains an intermediate mass black hole, of about 10,000 solar masses, at its core. (This is recent news!) And if you want to know how it got there, welcome to the cutting edge, where you get to choose between three leading theories! (My money's on #1, but I wouldn't be surprised by #2, either.)
Update 06/29/2011: Guess who sent me an email? Larry McNish, the creator of the "Globular Clusters within 50,000 Light Years" image above in this post. In the interest of showing where M54 actually is in relation to the Sun, the galaxy, and the other globular clusters, he has created and sent me three images of globular clusters out to 100,000 light years from the galactic center. Enjoy the views below!
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I'd like to know more about the primoridal black hole idea out of that selection of hypotheses. I can picture those helping form globular clusters, which could then develop into protogalaxies via collision/mergers, then galaxies, with some bound together tightly enough and coming towards others at the right angle to end up in galactic halos instead.
But while I can picture that, I wonder if it wouldn't cause more problems than solving answers. Has someone done modeling on it?
OK, very nice science and explanation.
Can someone help me understand a few globular cluster knowns or unknowns?
-- Are all globular clusters always associated with a galaxy?
-- Are globular clusters located in the halo area of galactic dark matter?
-- What dark matter observations rules out globular clusters as a candidate?
-- What is the interrelationship between star, globular cluster and galaxy formation?
My money is on scenario 4, the collapse of a stellar mass and then a long boring accretion of surrounding mass and then mergings with simular accreated steller mass and then accrection of lesser mass and then every so often rip open a star that wanders too close.(just for fun)
I mean like a tiger rips into a deer, no mercy, spilling photons and cosmic rays all over the place,ewwww gory.
And of course, the continued expansion of space slows the coalesence to the rate we know as gravity.
Messier 54 must merge with the Milky Way, resistance is futile!
OKThen-- globular clusters aren't dark. They are clusters of stars (and yes, found in galactic halos. Not sure if they can say always, as the stars are old and I don't know if we would see ones off in intergalactic space). That's not what dark matter (which literally is dark, doesn't give off light because it doesn't react with electromagnetic force) is. Globular clusters are very old, and likely were involved in protogalaxies that collided to make galaxies. They are in the halo rather than the disk because the collisions brought them in at different angles from the accretion disk.
Samantha
Thanks.
Right globular clusters aren't dark because they are full of stars; yet this posts title is The First Globular Cluster Outside of Our Galaxy. So outside of the Milky Way galaxy, globular clusters must be pretty dim (invisible, dark, unobserved). Thus apparently, we can't see them circling around the Coma bullet galaxy; correct me if I am wrong. On the other hand let me argue with myself, wiki says, "almost every large galaxy surveyed has been found to possess a system of globular clusters." So do we see these "systems of globular clusters" associated with distant galaxies or do we infer them? I'm not trying to be difficult; I am simply interested if and how they have been observed in deep space away from galaxies and around galaxies other than the Milky Way. In particular the above fuzzball Messier 54 seems to be associated with the Milky Way in the same way that Pelham, NY is associated with the NY City. Just as Pelham is clearly not farmland and is part of the megametropolis NY City; Messier 54 seems not to be in intergalactic space and seems clearly part of the megagalaxy Milky Way. Am I wrong? I mean maybe this is the first globular cluster seem optically maybe they've been seen all the time with x-ray of some other way; please someone clarify the state of observation versus the state of theory. Thank you.
You seem to say that globular clusters ARE ONLY in the halo around galaxies; which seems to be the same halo in which dark matter is hypothesized. So your answer to my second question is Yes. But then since globular clusters and dark matter seem to occupy the same area; I am assuming that there is some theoretical reason why the "hypothesized dark matter" is not globular clusters. I assume that there is an answer based on some evidence or reasoning. I'm just asking what is it? Hence my third question: What dark matter observations rules out globular clusters as a candidate?
"Globular clusters are very old, and likely were involved in protogalaxies that collided to make galaxies. They are in the halo rather than the disk because the collisions brought them in at different angles from the accretion disk." Can you point me to a reference for these ideas? All I can find is words like "the formation of globular clusters remains a poorly understood phenomenon". Hence, my 4th question: What is the interrelationship between star, globular cluster and galaxy formation?
Just trying to understand some details. Thanks.
If the Sagittarius Dwarf Elliptical Galaxy is near perpendicular to the plane of the Milky Way and is passing through it, did the Earth pass through the plane of the Sagittarius Dwarf Elliptical Galaxy? If so, does the timing corrolate with mass extinctions?
Last I heard, globular clusters should collapse quickly, so there should be no old globular clusters -- but there they are. What has been discovered, since then, that allows them to be old? Or is this one of those things it's not polite to discuss?
#8, "not polite to discuss?"
I suppose that you are right; that is a frustration.
I too would like to know what "allows them to be old?" Or know the answer is not this or that; and that the inconsistency to be understood is here or there.
Few are comfortable explaining what they don't know; it's like being on the couch.