I learned something very interesting the other day. I knew that a large portion of matter in spiral galaxies is contained in the galactic halo, the spherical mass (of mostly dark matter) that surrounds the galactic disk. I thought that all the stars were in the galactic disk and the galactic halo was just dark matter. I was surprised to learn that there are a fair number of stars in the galactic halo as well. They orbit the center of the galaxy just like the stars in the disk, but their plane of orbit doesn't line up with the disk, so they spend most of their time in the halo.
Apparently, this is where astronomical feature globular clusters exist. They're large groups of stars held together by their own gravitational force, not large enough to be a galaxy. They orbit the center of a galaxy through its halo, the entire cluster moving as one. Most of the stars in the galactic halo are much older than stars in the disk, because the halo doesn't contain the interstellar dust and gas that the disk has. These areas usually create "star nurseries" where new stars are born, and thus the stars in the disk are much younger than those in the halo.
The difference between old stars in the halo and young stars in the disk is mostly characterized by the metallicity of the stars. Metallicity is the amount, by mass, of elements heaver than hydrogen or helium in the star's make-up. Younger stars tend to have higher metallicities because the dust clouds they were formed out of were at least partly created by supernova explosions of dying stars, which create the violent environments necessary for heavy metals to be created.
I knew of globular clusters in the past, but I always thought they were odd groupings of stars outside of galaxies. I never knew that stars could be part of a galactic gravity system without being part of the disk where most of the stars are.
Globular clusters are also cool because they can be used to set a lower limit on the age of the universe. Large stars are much much brighter than small stars, and so they burn through all their hydrogen long before small stars do. We can predict the amount of time stars take to burn through their hydrogen, based on their mass. We can assume that all the stars in the globular cluster formed at roughly the same time. Then, the globular cluster is roughly as old as the lifetime of the most massive stars that are still burning hydrogen (i.e. they are just running out of hydrogen to burn). It turns out the ages of globular clusters in the Milky Way are about 12 billion years - almost as old as the Universe! For a while, when we weren't as good at age-dating globular clusters, we measured the age to be 15-16 billion years, which was very confusing because by other methods we have measured the age of the Universe as 13.6 billion years. We should not get globular clusters older than the Universe :P
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