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.
- Main article: Globular cluster
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).
- Main article: Open cluster
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.
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. The new-found 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.
- Main article: Stellar association
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. The 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.
Astronomical significance of clusters
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 diagram is plotted for a cluster whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated. This process is known as main-sequence fitting. Reddening and stellar populations must be accounted for when using this method.
- ↑ 1.0 1.1 A.P. Huxor, N.R. Tanvir, M.J. Irwin, R. Ibata (2005). "A new population of extended, luminous, star clusters in the halo of M31". Monthly Notices of the Royal Astronomical Society 360: 993-1006.
- Star Clusters, SEDS Messier pages