What Is #27: A Globular Cluster?

It has been several weeks since I've added to this "What Is" series, so I thought it would be worthwhile to get back to it as there are definitely more than twenty six astronomical things/objects that are important to know. For the twenty-seventh post in this series, I take a look at globular clusters. Any amateur astronomer who has looked at the night sky is likely familiar with globular clusters, also referred to as closed clusters, but what are they?

Globular Cluster M3 - The sky image is obtained by Sloan Digital Sky Survey, DR14 with SciServer.

Globular clusters are clusters of stars in which all stars are gravitationally bound to each other. These are typically older stars that formed as the Milky Way Galaxy was in its early stages, approximately 9 to 10 billion years ago and possibly longer. They exist in the halo of the galaxy, the spherical region of our galaxy that surrounds the disk and center.

Since these stars are old, they are smaller, redder, and dimmer. The larger, bluer, and brighter stars died off long ago. Globular clusters can have up to a million stars packed into a region only a few tens to a couple of hundred light years across. That is a lot of stars in a small space! For reference, the closest star to us (not in a globular cluster) is 4 light years away. In a globular cluster it is a guarantee there'd be stars closer than four light years. Your night sky, however, would not necessarily be filled with more stars than we see in our current sky. The stars we see in the night sky are larger and brighter stars. There'd be stars in the sky, no doubt, but because all of the stars in a globular cluster are dimmer, only the closest would be visible to the naked eye. A dim star only 4 light years away is barely visible to the naked eye in our own region.

There are around 150 known globular clusters in our galaxy, but many more in other galaxies. They are great stars to study if you are looking at studying old stars. If globular clusters are also called closed clusters, then what are open clusters? These will be discussed in the next post in this series.