“The man’s a born straggler… another lucky exception to the rules of natural selection. A million years ago he would’ve been an easy snack for a saber-toothed tiger.” -Carl Hiaasen
Welcome to the latest Messier Monday, where each week we take a look at one of Charles Messier’s original catalogue of 110 deep-sky objects that comet-hunters might easily confuse with those transient passers-by in our Solar System.
Quite to the contrary, each of the 110 objects in the Messier catalogue are (semi-)permanent fixtures in our night sky, and all lie well beyond our Solar System’s reach. This week, I’d like to take a look at Messier 30, a globular cluster in the Southern Hemisphere that’s still visible just after sunset for those of you (like me) in the north. Here’s how to get there.
Depending upon your latitude, the prominent star Fomalhaut — the 18th brightest in the entire sky — will be visible above the horizon in the southern part of the sky just after sunset. For those of you down by the tropics or in the southern hemisphere, Fomalhaut will be easy to spot, but for those of you at high northern latitudes like me (I’m at ~46 degrees north), you’ll have to look low for a chance to spot it.
But if you can, look higher in the sky and farther to the west; you’ll find a collection of naked-eye stars in the constellation Capricorn.
Just a little bit back towards Fomalhaut, below the “line” of bright stars in Capricorn, you’ll find a really ancient globular cluster, Messier 30, whose oldest stars are 12.93 billion years old, or 94% the age of the Universe!
Through a small telescope or large binoculars, it’s clearly a compact, star-filled object.
But you don’t really get a sense of scale for this object unless you take a closer, more powerful look. Either a modest (6″) telescope with an excellent camera,
or a large (12.5″) telescope with just a simple camera,
reveals the truth: that this is a cluster with a very dense core, and a population of uncharacteristically bright, luminous, blue stars. At nearly 30,000 light-years distant with a mass of about 160,000 suns, Messier 30 is only about 93 light years in diameter, but has a very dense core and, like I said, bright blue stars sprinkled throughout it, but bluer at the core.
Which, as you may know, should not be the case for an object that’s practically 13 billion years old!
Why not? Here’s the thing: stars come in a wide variety of colors, magnitudes and sizes. When a star cluster — whether an open cluster within our galaxy or a globular cluster like M30 here — is created, it produces seven different classes of stars, ranging from the brightest, bluest, hottest and most massive type (O-stars) to the dimmest, reddest, coolest and least massive types (M-stars).
The brightest stars burn through their fuel the fastest, and therefore live the shortest. By time even one billion years goes by, there are no O-or-B stars left; by time 13 billion years have passed, only K-and-M stars will remain.
The remaining stars, when they run out of hydrogen in their core, undergo a variety of well-understood processes, and when we measure the color and magnitudes of the stars in a cluster, the shape that a complete graph of all such stars in the cluster makes gives us an idea of how old that cluster is.
By measuring this “turn-off” temperature, we can determine the age of any star cluster. And that’s how we know the age of this particular one, Messier 30. Only, that doesn’t explain the blue stars in there.
In fact, these blue stars appear to mess up the Hertzsprung-Russell diagram, because they exist in a place where — according to the standard picture I just painted for you — they ought to not exist! And yet, there they are.
So what’s going on here? Quite clearly, there are bright, blue stars — these so-called Blue Stragglers — that exist in these clusters. A close-up look at the Hubble image from above reveals this without a doubt.
And when I told you it was an even more severe effect in the core, I wasn’t lying to you; a visual inspection allows you to verify this for yourself! (At least, it’s clearly less yellow, even if you can’t easily tell it’s more blue.)
What’s gone on over here? While a burst of star-formation could have, in theory, been responsible, this is one of the densest globular clusters in the core region ever discovered, with a central density of over 30,000 solar masses per cubic light year! This means the cluster has undergone core collapse, a gravitational process where the center of such a cluster becomes incredibly dense and just keeps increasing the closer you get to the center.
And when this happens, there are two processes that can turn these old, low-mass, doddering stars into bright, short-lived blue ones!
Either, as shown at the top, two low-mass stars can merge, creating a heavier, bluer, hotter star that will burn through its fuel faster. This explanation makes a lot of sense, particularly when the stellar density is very high! Or, as shown at the bottom, a star in a binary system can steal mass from its companion, changing its color and luminosity in the process.
There is some evidence that the collision idea is dominant, but that both processes take place. And that’s why, after nearly 13 billion years, Messier 30, one of the oldest globular clusters in the entire Universe, contains more than its share of young-looking, hot blue stars: blue stragglers! So take a look inside; you won’t regret it!
- M1, The Crab Nebula: October 22, 2012
- M8, The Lagoon Nebula: November 5, 2012
- M15, An Ancient Globular Cluster: November 12, 2012
- M30, A Straggling Globular Cluster: November 26, 2012
- M45, The Pleiades: October 29, 2012
- M81, Bode’s Galaxy: November 19, 2012
Which one will be next? Join me next Monday for our seventh entry, and let me know what you’d like to see next! There’ll be a new Messier object — and a new window into the night sky — waiting for you to discover!