Star clusters
Globular clusters
Globular star clusters
A spherical aggregate of stars made up of thousands to a few million stars which is an orbiting satellite of a galaxy. There are over 150 globular clusters orbiting our galaxy. Globular clusters are gravitationally bound systems, highly concentrated to the center (up to a few 10^3 stars per cubic light-years), with a volume ranging from a few dozen up to more than 300 light-years in diameter. They are generally old and metal-poor and are among the first objects to be formed in a galaxy. There is also strong evidence that they form in major galaxy interactions and mergers. The stars in a globular cluster are thought to have a common origin and thus a single age and chemical abundance; with some exceptions such as Omega Centauri and NGC 2808, which exhibit multiple populations. The presence of various sub-populations within a globular cluster is interpreted as indicating distinct epochs of mass accretion and/or major star formation. The Milky Way hosts about 200 globular clusters. They are spherically distributed about the Galactic Center up to a radius of 350 light-years, with a maximum concentration toward the Galactic center. All but the smallest dwarf galaxies possess globular clusters. Some galaxies, e.g. M87, contain several thousands of them. There are, however, important differences. While all the globular clusters in our Galaxy and in M31 are old (ages of about 10 billion years, at least), there are galaxies, such as the two Magellanic Clouds and M33, that host much younger globular clusters (ages of a few billion years, or less).