An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters.
Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest away.
Open clusters are very important objects in the study of stellar evolution. Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars. A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are readily visible for the naked eye. Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen in binoculars or telescopes.
Historical observations
The most prominent open clusters such as the Pleiades have been known and recognised as groups of stars since antiquity. Others were known as fuzzy patches of light, but had to wait until the invention of the telescope to be resolved into their constituent stars. Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found preferentially towards the centre of the Milky Way, and the other of which consisted of a generally sparser population of stars in a more irregular shape and found all over the sky. Astronomers dubbed the former globular clusters, and the latter open clusters. Open clusters are also occasionally referred to as galactic clusters , because they are almost exclusively found in the plane of the Milky Way, as discussed below.
It was realised early on that the stars in the open clusters were physically related. The Reverend John Michell calculated in 1767 that the probability of even just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000. As astrometry became more accurate, cluster stars were found to share a common proper motion through space, while spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars born at the same time and bound together as a group.
While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of difference in appearance between a very sparse globular cluster and a very rich open cluster. Some astronomers believe the two types of star clusters form via the same basic mechanism, with the difference being that the conditions which allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in our galaxy.
Formation
A large fraction of stars are originally formed in multiple systems because only a cloud of gas containing many times the mass of the Sun will be heavy enough to collapse under its own gravity, but such a heavy cloud cannot collapse into a single star.
The formation of an open cluster begins with the collapse of part of a giant molecular cloud, a cold dense cloud of gas containing up to many thousands of times the mass of the Sun. Many factors may trigger the collapse of a giant molecular cloud (or part of it) and a burst of star formation which will result in an open cluster, including shock waves from a nearby supernova and gravitational interactions. Once a giant molecular cloud begins to collapse, star formation proceeds via successive fragmentations of the cloud into smaller and smaller clumps, resulting eventually in the formation of up to several thousand stars. In our own galaxy, the formation rate of open clusters is estimated to be one every few thousand years.
Once star formation has begun, the hottest and most massive stars (known as OB stars) will emit copious amounts of ultraviolet radiation. This radiation rapidly ionizes the surrounding gas of the giant molecular cloud, forming an H II region. Stellar winds from the massive stars and radiation pressure begin to drive away the gases; after a few million years the cluster will experience its first supernovae, which will also expel gas from the system. After a few tens of millions of years, the cluster will be stripped of gas and no further star formation will take place. Typically, less than 10% of the gas originally in the cluster will form into stars before it is dissipated.
Another view of cluster formation is that they form rapidly out of a contracting molecular cloud core and once the massive stars begin to shine they expel the residual gas with the sound speed of the hot ionised gas. From the time of start of cloud-core contraction to gas expulsion takes typically not more than one to three million years. As only 30 to 40 per cent of the gas in the cloud core forms stars, the process of residual gas expulsion is highly damaging to the cluster which loses many and perhaps all of its stars. All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. The young stars so released from their natal cluster become part of the Galactic field population. Because most if not all stars form clustered, star clusters are to be viewed the fundamental building blocks of galaxies. The violent gas-expulsion events that shape and destroy many star clusters at birth leave their imprint in the morphological and kinematical structures of galaxies.
It is common for two or more separate open clusters to form out of the same molecular cloud. In the Large Magellanic Cloud, both Hodge 301 and R136 are forming from the gases of the Tarantula Nebula, while in our own galaxy, tracing back the motion through space of the Hyades and Praesepe, two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.
Sometimes, two clusters born at the same time will form a binary cluster. The best known example in the Milky Way is the Double Cluster of h Persei and χ Persei, but at least 10 more double clusters are known to exist. Many more are known in the Small and Large Magellanic Clouds — they are easier to detect in external systems than in our own galaxy because projection effects can cause unrelated clusters within the Milky Way to appear close to each other.
Morphology and classification
Open clusters range from very sparse clusters with only a few members to large agglomerations containing thousands of stars. They usually consist of quite a distinct dense core, surrounded by a more diffuse 'corona' of cluster members. The core is typically about 3–4 light years across, with the corona extending to about 20 light years from the cluster centre. Typical star densities in the centre of a cluster are about 1.5 stars per cubic light year (the stellar density near the sun is about 0.003 star per cubic light year).
Open clusters are often classified according to a scheme developed by Robert Trumpler in 1930. The Trumpler scheme gives a cluster a three part designation, with a Roman numeral from I-IV indicating its concentration and detachment from the surrounding star field (from strongly to weakly concentrated), an Arabic numeral from 1 to 3 indicating the range in brightness of members (from small to large range), and p , m or r to indication whether the cluster is poor, medium or rich in stars. An 'n' is appended if the cluster lies within nebulosity.
Under the Trumpler scheme, the Pleiades are classified as I3rn (strongly concentrated and richly populated with nebulosity present), while the nearby Hyades are classified as II3m (more dispersed, and with fewer members).
Numbers and distribution
There are over 1,000 known open clusters in our galaxy, but the true total may be up to ten times higher than that. In spiral galaxies, open clusters are invariably found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are strongly concentrated close to the galactic plane, with a scale height in our galaxy of about 180 light years, compared to a galactic radius of approximately 100,000 light years.
In irregular galaxies, open clusters may be found throughout the galaxy, although their con
NUS: ARI > Open Cluster > Photo Albums
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An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. [1]
Category:Open clusters - Wikimedia Commons
Media in category "Open clusters" The following 32 files are in this category, out of 32 total.