“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.
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.
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.
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!
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.
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.
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:
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?
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?
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!
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!