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 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.
Our galaxy has about 150 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).
 |
The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosity |
Open clusters are very different to globular clusters. Unlike the spherically-distributed globulars, they are confined to the
galactic plane, and are almost always found within
spiral arms. They are generally young objects, up to a few tens of millions of years old. They form from
H II regions such as the
Orion Nebula.
Open clusters usually contain up to a few hundred members, within a region up to about 30 light years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of
giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as 'evaporation'.
The most prominent open clusters are the
Pleiades and
Hyades in
Taurus. The
Double Cluster of h+Chi Persei can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.
Once an open cluster has become gravitationally unbound, the constituent stars will continue to move on similar paths through space. The group is then known as a stellar association, or a moving group. Most of the stars in the
Big Dipper are members of a former open cluster, the
Ursa Major Moving Group, and have similar
proper motions. Other stars across the sky, including
Alphecca and
Zeta Trianguli Australis, are related to this group. Our
Sun lies at the edge of this stream of stars at the moment, but isn't a member as is shown by its different galactic orbit, age, and chemical composition.
Another stellar association is that surrounding
Mirfak (α
Persei), which is very prominent in
binoculars. Distant moving clusters can't readily be detected since the proper motions of the stars need to be known.
The study of star clusters is very important in many areas of astronomy. Because the stars were all born at roughly the same time, the different properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters.
Clusters are also a crucial step in determining the distance scale of the universe. A few of the nearest clusters are close enough for their distances to be measured using
parallax. A
Hertzsprung-Russell Diagram can be plotted for these clusters which has absolute values known on the luminosity axis. Then, when similar diagrams are plotted for clusters whose distance is not known, the position of the
main sequence can be compared to that of the first cluster and the distance estimated.
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Protostar* Star cluster
Hodge 301*
Star Clusters, SEDS Messier pages
