Sunday, August 7, 2011

Globular Cluster Photo Series (Part 9): M15

Today I have a picture of the globular cluster Messier 15 for you to enjoy. M15 is a large cluster at approximately 170 light years across. It appears about two-thirds the size of the full Moon on the sky (18.0 arcminutes), and would appear larger were it not for the fact that it is nearly 33,600 light years away (or about as far from us as we are from the galactic core).

Messier 15 in Pegasus.
M15 is one of the most densely packed globular clusters in the Milky Way, due to its core having undergone something called "core collapse". This is a simple process whereby kinetic energy is transferred from the stars in the core to the stars on the periphery, causing the outer layers to "puff up" while the stars in the center contract and pull in closer to each other. M15 has over a hundred thousand stars, though, so there are plenty to go around.

In fact, that enormous number of stars includes some interesting things, including 112 variable stars, 8 pulsars, and a planetary nebula. This was the first planetary nebula (which have nothing to do with planets besides appearance in small instruments) found in a globular cluster, and one of only four located in globular clusters found to this point. It is also thought that there may be a massive black hole in the center of this cluster. So all in all, a bit more interesting than some of the clusters I've focused on recently.

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