Wednesday, June 29, 2011

Globular Cluster Photo Series (Part 3): M13

Today we're taking a look at another of the biggest and brightest globular clusters in the sky, Messier 13, also known as the Great Globular Cluster in Hercules. (It's located around the area of his kidney. We can but hope it's not a Herculean kidney stone!) It is nearly the same size as Omega Centauri, about 170 light years across. However, it has only a 3rd as many stars, about 300,000.

Messier 13, the Great Globular Cluster in Hercules. Click for larger image.
Although it's as large as Omega Centauri, M13 appears 2/3 as large on the sky because it is almost twice as far away; 25,000 light years compared to Omega Centauri's ~16,000. (Its apparent diameter is 20 arcminutes compared to Omega Centauri's 36.3, so M13 is about 2/3 the size of the full Moon on the sky.)

Other than that, I don't have too much to say about M13. It, Omega Centauri, and another cluster called 47 Tucanae are generally considered the best and most beautiful globular clusters. 47 Tucanae is just below the horizon from Hawai‘i, so I won't be showing you any pictures of it. In terms of visual impressiveness it will probably go down from here since most globular clusters are either smaller or further away (or both) than these three, leading them to appear smaller on the sky.

Of course, one reason I started this project was to catalog the differences between the many Milky Way globular clusters. On the face of it they're not particularly interesting, big balls of stars held together by self-gravitation. Just as all stars are big balls of gas held together by self-gravitation. And yet, just as there is almost infinite variation among stars, I'm already starting to see differences just between the three globular clusters I've shown here. Some are larger, some smaller, some very tightly packed, some less so. My goal for this series is to get a sense for that same variability found in globular clusters.

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