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Details of Star cluster

Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Globular clusters
Globular clusters, (GC) are roughly spherical groups of anything between 10,000 and several million stars in a region about 10 to 30 light years across. They generally consist of very old Population II stars, just a few hundred million years younger than the universe itself. The constituent stars tend to be yellow and red, and weigh less than about two solar masses. This is because the hotter, more massive stars have either exploded as supernovae or passed through a planetary nebula phase to become white dwarfs. However, some anomalous blue stars are found in globulars, and are believed to have been formed by stellar mergers in the dense inner regions of the cluster. These stars are known as blue stragglers.

In our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the galactic centre, orbiting the centre in highly elliptical orbits. In 1917, the astronomer Harlow Shapley was able to estimate the Sun's distance from the galactic centre based on the distribution of globular clusters; previously the Sun's location within the Milky Way was by no means well established.

Until recently, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the Hipparcos satellite and increasingly accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less.

Super star clusters, such as Westerlund 1 in the Milky Way, may be the precursors of globular clusters.

Our galaxy has about 1500 globular clusters, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79. 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).

Intermediate forms
In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy, which are, in several ways, very similar to globular clusters (although less dense). Currently, there are not any intermediate clusters (also known as extended globular clusters) discovered in the Milky Way. The three discovered in Andromeda Galaxy are M31WFS C1 , M31WFS C2, & M31WFS C3.

These new-found star clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. The clusters also share other characteristics with globular clusters, e.g. the stellar populations and metallicity. What distinguishes them from the globular clusters is that they are much larger - several hundred light years across - and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters. Parametrically, these clusters lie somewhere between a (low dark-matter) globular cluster and a (dark matter-dominated) dwarf spheroidal galaxy.

How these clusters are formed is not yet known, but their formation might well be related to that of globular clusters. Why M31 has such clusters, while the Milky Way has not, is not yet known. It is also unknown if any other galaxy contains this kind of clusters, but it would be very unlikely that M31 is the sole galaxy with extended clusters.