A star cluster is exactly what you think it is; a bunch of stars held together by their mutual gravity. However,there are two major types of clusters. Open clusters are loose groups of stars that occur when many stars form in the same area, like in a nebula. These clusters often break up as they age, and usually consist of hot young stars. Globular Clusters, on the other hand, are tightly bound spheres of old metal deficient stars. These clusters are huge, and often contain thousands of stars. These glowing orbs are often golden yellow, and nearly 150 of them orbit the milky way in our halo, many of them nearly 10 billion years old.