Globular Cluster M22 in Hubble’s Eye

Globular Clusters are tightly packed collections of hundreds of thousands of very old stars, spherically distributed around the Milky Way Galaxy.  They undergo little change, have nearly no gas, and have a stellar density way higher than the rest of the Galaxy.  The first one discovered, in 1665, is messier 22, one of the most well studied, easily visible, and interesting globular clusters.

Globular Cluster M22, 70 light years across. Image credit: ESA / Hubble / NASA

Based on observations, this globular cluster contains at least two black holes. It is also one of only three globular clusters ever found to host a planetary nebula, a gaseous shell emitted by a dying star with a low mass similar to the Sun.

Globular clusters are beautiful in binoculars or a small telescope, and often appear as a fuzzy round blob.  It’s very clear that they are brightest near the centre, where the most stars are tightly packed together.

It’s pretty amazing when you think of them as the remnants of ancient galaxies that collided and fed the formation of the Milky Way.  This is one theory of where they came from, and my favourite idea about them.  It would explain why they only have old stars and why they are so densely packed.  If they collided to form larger galaxies, any stars that weren’t near their centre of mass would have been stripped away, along with any gas that was near the centre.  All that would be left is a densely packed region of very old red stars, exactly what we see.

Do you agree with this idea? Where else could globular clusters have come from?

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