A collection of stars formed together from the same gas cloud and bound together by mutual gravitational attraction. There are two main types of star cluster. Globular clusters contain from tens of thousands to millions of old stars packed into a near-spherical ball, and are found in the haloes of galaxies. Open clusters are much more sparse and irregular in shape, consisting of no more than a few hundred relatively young stars in the spiral arms of galaxies. A group looser than an open cluster is known as a stellar association.
Subjects: Astronomy and Astrophysics — Physics.