Tuesday, June 28, 2011

Globular Cluster Photo Series (Part 2): Omega Centauri

First off, an apology. If you read my post from yesterday, it contained a grave error -- namely, that the object in the photo was not, after all, Messier 4, but the object I intend to show you today, Omega Centauri.

I'm still scratching my head trying to figure out how it happened, unless I had a critical failure in reading the file names. However, the situation has now been rectified, and a picture of M4 has been reduced and added to the post in question.

Today's globular cluster is Omega Centauri, commonly called the most beautiful globular cluster in the night sky. You may have already seen it if you read my post yesterday before I fixed it, but I've re-reduced the picture and I like this version better. Omega Centauri is the brightest and largest cluster found orbiting the Milky Way, and the second largest in the Local Group of Galaxies (one of the clusters orbiting the Andromeda galaxy is larger). It is located a mere 15,800 light years from us (give a take a thousand light years or so), making it just about twice as far away as M4, which we looked at yesterday. However, if you compare the two pictures, you'll see that Omega Centauri looks a bit bigger; this is partly because it is bigger, being around 170 light years across compared to M4's 70. It has about the same apparent diameter on the sky, too, being about the size of the full moon (36.3 arcminutes). It is just bright enough to be faintly visible with the naked eye (I've seen it from Mauna Kea).

The fact that it appears star-like to the naked eye is actually responsible for its name. Back in 1603 a German astronomer by the name of Johann Bayer created a sky catalog called Uranometria in which he assigned Greek letters to the prominent stars of each constellation. Because of its star-like appearance Omega Centauri received a Greek letter like any other star. Though the fact that the letter it received comes at the end of the Greek alphabet shows that it wasn't considered very important, which makes sense, since it doesn't appear very bright in the sky.

Omega Centauri in Centaurus. Click for larger image.
At first glance, you can tell that Omega Centauri is a lot brighter and more compact than M4. Some estimates place it at a million stars, on the high end of the several-hundred-thousand-star range common to globular clusters. In fact, the high number of stars combined with some other unusual aspects of Omega Centauri lead some to suspect that it is in fact the core of a smaller galaxy whose outer stars were absorbed into the Milky Way. Kapteyn's Star, a red dwarf a mere 13 light years from Earth, may even be one such star.

Another interesting fact about Omega Centauri (and other globular clusters in general) is its high stellar density. Near the center stars are estimated to be packed so tightly that the mean distance between them is a mere 0.1 light year, or only around 6,300 Astronomical Units (the average distance between the Earth and the Sun, abbreviated AU). Even with that helpful comparison, that number may not mean much to you (it didn't even for an astronomy student like me), so I took the liberty of creating this helpful picture:

In the top half of the picture we see the Sun and its nearest neighbors Proxima Centauri and Alpha Centauri A and B (interestingly our nearest neighbors lie very close in the sky to Omega Centauri). In this picture, one light year is 100 pixels, so the closest star to our Sun, Proxima Centauri, is 424 pixels (4.24 light years) away, while Alpha and Beta are a little further at 4.37 light years. Note that the sizes of stars in the picture are vastly over-exaggerated for visibility; one pixel is 0.01 light years, or about 632 AU. For comparison, the orbit of Neptune is almost exactly 60 AU in diameter.

In the bottom half we have a representation of the stellar density of Omega Centauri. The stars arranged in the regular grid at the bottom left are 0.1 light year apart, but since I rapidly tired of placing them in regular grid rows 0.1 light year apart (an unrealistic arrangement in nature), the rest are placed at random and slightly less dense positions. Since the density decreases as you go out from the center, this gives a semi-realistic view of the center of the cluster.

Of course, at the scale represented in the picture Omega Centauri would be approximately 26 times as wide as the picture, so imagine a giant sphere of stars about 10 feet in diameter packed about as closely shown in the picture. That's a lot of stars.

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