Globular Clusters

Many generations of stars have formed in the Galaxy. The oldest generation of stars is found in globular clusters. For some reason, when the Galaxy first formed, stars formed in large groups of tens or hundred millions of stars.

It turns out to be very difficult to tell what the ages of most stars are, but we can apply models for how stars evolve to determine the ages of groups of stars. The heat that causes stars to shine is produced when hydrogen fuses into heavier elements in the cores of stars. Larger stars are hotter and denser their centers, which causes the star to burn through its fuel faster. It turns out that even though a more massive star has more fuel to burn, it burns through it so fast that its life will be much shorter than a less massive one. For instance, a star 10 times more massive than the sun will have a life over 100 times shorter (100 million years, instead of 10 billion years).

Therefore, if one can find a group of stars that formed at the same time, the mass of the largest stars in the group can be used to measure the age of the star cluster. Globular clusters are just such groups of stars. The masses of the largest stars are determined by comparing the brightness and temperatures of the stars --- this works because the rate at which the fuel in a star is burned is determined by its mass. In a globular cluster, the largest stars are often half the mass of the sun, which implies that globular clusters formed shortly after the birth of the universe, over 10 billion years ago (for comparison, the sun formed only 5 billion years ago).

Globular clusters are important to study the history of star formation in a galaxy, and to study how large numbers of bodies interact gravitationally. Astrophysicists are currently studying globular clusters to understand how white dwarfs cool, what happens when stars collide, and how binary pairs of stars affect the motion of other stars in the cluster.