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Globular clusters

M13

 Globular star clusters are massive groups of stars  bound together by their gravitational pull.  These clusters contain old stars around 10 billion years old and can contain hundreds of thousands to millions of stars. they are spread out from about 20 to about 200 light years in diameter. They are mostly found around the halo at the center of the galaxy. This is because these clusters of stars formed early on in the history of the Galaxy, before the majority of the material had settled into a disk.

The  study of globular clusters forms an important and major part of the research interest of the international astronomical community. They have long been used to study some of the most fundamental problems in astronomy.

Radial velocity measurements have revealed that most globulars are moving in highly eccentric elliptical orbits that take them far outside the Milky Way; they form a halo of roughly spherical shape which is highly concentrated to the Galactic Center, but reaches out to a distance of several 100,000 light years, much more than the dimension of the Galaxy's disk. As they don't participate in the Galaxy's disk rotation, they can have high relative velocities of several 100 km/sec with respect to our solar system; this is what shows up in the radial velocity measurements. 

Spectroscopic study of globular clusters shows that they are much lower in heavy element abundance than stars such as the Sun that form in the disks of galaxies. Thus, globular clusters are believed to be very old and consisted from an earlier generation of stars (Population II), which have formed from the more primordial matter present in the young galaxy just after (or even before) its
formation. The disk stars, by contrast, have evolved through many cycles of star birth and supernovae, which enrich the heavy element concentration in star-forming clouds and may also trigger their collapse. 

These clusters of stars are significant not only as valuable test beds for theories of stellar structure and evolution but also because they are among the few objects in the Galaxy for which relatively precise ages can be measured. Because of their extreme long life they provide a very useful lower limit to the age of the Universe . The distribution of their ages, and correlations between cluster ages and metal abundances makes these systems an important probe into the processes of galaxy formation.

The stars of any one globular cluster share a common history and differ one from the other only in their mass. They thus form ideal candidates for study. Here at Leeds we use color-magnitude diagrams to determine properties of the cluster under study.