globular cluster

A roughly spherical, densely packed cluster of hundreds of thousands or even millions of stars. The brightest globular cluster in the sky is Omega Centauri (w Cen), which has a diameter of 620 light years. It is also one of the oldest globular clusters known, believed to be 15 billion years old. The globular clusters in our Galaxy contain some of its oldest stars. They are distributed within a spherical halo around the Galaxy, in contrast with open clusters, which are found only in the disc, and seem to move in highly elliptical orbits around the centre of the Galaxy. The stars in globular clusters contain low abundances of the elements heavier than helium. This is consistent with their having been formed from the original material of the Galaxy, before the composition of the interstellar medium had been enriched by elements made only inside stars. Globular clusters have also been recognized in other galaxies.