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 103 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).
Etymology (EN): Globular, from → globule + -ar,
variant of → -al; → cluster.
Etymology (PE): Xušé, → cluster; guysân “shaped like a globe,” from
guy, → globe + -sân “manner, semblance”
(variant sun, Mid.Pers. sân “manner, kind,”
Sogdian šôné “career”).
