Star Clusters

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Globlular Cluster
Omega Centauri
Pleiades

Star clusters or star clouds are groups of stars. Although the stars in our night sky appear to live out their lives in isolation, many millions of stars reside in groups called open and globular clusters.

Star clusters, whether open or globular, are not static through time. Over millions of years, the clusters change physically and the stars within them age and die. However, there are major differences between the evolution of globular clusters and open clusters. An open cluster starts its life with a set of stars of similar chemical composition and age. Over hundreds of millions of years, it loses its members, either due to death of the stars or losing them to the gravitational tugs of other stars within the Milky Way. However, an open cluster can continue to manufacture stars from the original nebulous cloud from which it formed. Because of this, open clusters often contain stars of different ages at various stages of evolution.

A globular cluster is more tightly bound, and less likely to lose its stars. It also spends most of its time away from the disk of the Galaxy, avoiding interactions. In this way its structure is preserved for thousands of millions of years. Once a globular cluster has formed, the original gas and dust is ejected, and the cluster is then unable to form new stars. As the stars within a globular cluster age and die, so the cluster itself ages and dies.

Globular Clusters

A globular cluster is a tight group of hundreds of thousands stars bound together by gravity within a spherical volume. More than 150 globular clusters have been discovered in the Milky Way. Although a few are found in its central bulge, most are located in the halo. Globular clusters are made up of Population II stars, which have their own independent orbits.

Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand. A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age. The best known globular cluster in the northern hemisphere is M13 (modestly called the Great Globular Cluster in Hercules). Globular clusters are not unique to the Milky Way, and some galaxies have more globular clusters than our own.

Open Clusters

Open clusters are a more loosely clustered group of stars, generally containing less than a few hundred members, and are often very young. They are made up of sibling stars of similar age formed from the same nebulous clouds of interstellar gas and dust. Open clusters reside within the galactic plane, and are almost always found within spiral arms. Open clusters are young and often the site of new star creation.

The most prominent open clusters are the Pleiades and Hyades in Taurus. Open clusters are often dominated by hot young blue stars, because although such stars.