- 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, or GC, are roughly spherical groupings of from 10,000 to several million stars packed into regions of from 10 to 30
light years across. They commonly consist of very old Population II stars -- just a few hundred million years younger than the universe itself -- which are mostly yellow and red, weighing a bit less than two solar masses. Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebulaphases to end as white dwarfs. Yet a few rare blue stars exist in globulars, thought to be formed by stellar mergers in their dense inner regions; 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 Shapleywas 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 Waywas by no means well established.
Until recently, globular clusters were the cause of a great mystery in
astronomy, as theories of stellar evolutiongave 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 Hipparcossatellite and increasingly accurate measurements of the Hubble constantresolved 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 1in the Milky Way, may be the precursors of globular clusters. [cite news | date= 2005-03-22| title=ESO | publisher=Young and Exotic Stellar Zoo: ESO's Telescopes Uncover Super Star Cluster in the Milky Way | url=http://www.eso.org/outreach/press-rel/pr-2005/pr-08-05.html | accessdate=2007-03-20 ]
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 galaxyM87 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).
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 Galaxyare M31WFS C1 [http://simbad.u-strasbg.fr/simbad/sim-id?Ident=%401592523&M31WFS%20C1] , 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.cite journal | author=A.P. Huxor, N.R. Tanvir, M.J. Irwin, R. Ibata | title=A new population of extended, luminous, star clusters in the halo of M31 | journal=Monthly Notices of the Royal Astronomical Society | year=2005 | volume=360 | pages=993–1006 | url=http://arxiv.org/abs/astro-ph/0412223 | doi = 10.1111/j.1365-2966.2005.09086.x ]
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.
Open clusters, (OC) are very different from 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 Clusterof 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.
uper star cluster
Super star cluster, (SSC) is a very large region of star formation thought to be the precursor of a globular cluster.
Embedded clusters, (EC) are stellar clusters that are partially or fully incased in an Interstellar dust or gas. The most famous example of an embedded cluster is the
Trapezium cluster. In ρ Ophiuchi cloud (L1688) core region has an embedded cluster. [http://www-astro.phast.umass.edu/preprints/starform/kspec/node1.html]
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 Dipperare 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 Sunlies 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 Diagramcan be plotted for these clusters which has absolute values known on the luminosityaxis. Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the main sequencecan 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.
* Star cluster
Robust Associations of Massive Baryonic Objects (RAMBOs)
* [http://www.sky-map.org/?ra=7.6357&de=38.882&zoom=11&img_source=IMG_all NGC 2419] -Globular Claster on SKY-MAP.ORG
* [http://www.seds.org/messier/cluster.html Star Clusters] , SEDS Messier pages
* [http://www2.cfa.harvard.edu/rg/star_and_planet_formation/embedded_clusters.html RG Research: Embedded Clusters]
* [http://www.britannica.com/eb/article-9110473/star-cluster Encyclopaedia Britannica, Star cluster - full article]
* [http://www.universetoday.com/am/publish/super_star_cluster.html Super Star Cluster Discovered in Our Own Milky Way]
* [http://de.arxiv.org/abs/astro-ph/0506507 Probing the Birth of Super Star Clusters: Implications for Massive Star Formation] , Kelsey E. Johnson, 2005
* [http://arxiv.org/abs/astro-ph/0412223 A new population of extended, luminous star clusters in the halo of M31] , A.P. Huxor et al., 2004
* [http://arxiv.org/abs/astro-ph/0306377 HST/NICMOS Observations of the Embedded Cluster in NGC 2024: Constraints on the IMF and Binary Fraction] , Wilson M. Liu et al., 2003
* [http://arxiv.org/abs/astro-ph/0002530 The Discovery of an Embedded Cluster of High-Mass Stars Near SGR 1900+14] , Frederick J. Vrba et al., 2000
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Look at other dictionaries:
star cluster — noun : a relatively compact group of stars forming a gravitating unit and containing either not more than a few hundred stars or tens of thousands of stars * * * Astron. a number of stars of common origin held together as a group by gravitational … Useful english dictionary
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star cluster — Astron. a number of stars of common origin held together as a group by gravitational attraction. Cf. globular cluster, open cluster, stellar association. * * * ▪ astronomy Introduction either of two general types of stellar assemblages held… … Universalium
star cluster — noun A group of gravitationally bound stars and other material (such as nebulae and planets), conventionally with one generation of similarly aged stars from the same pre stellar nebula. Syn: cluster of stars, Cl, ,Cl … Wiktionary
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Coma Star Cluster — Melotte 111, taken from a rural location in Derbyshire, England, in March 2003. Observation data (J2000 epoch) Constellation … Wikipedia
Super star cluster — A super star cluster (SSC) is a very large region of star formation thought to be the precursor of a globular cluster.cite journal author = Gallagher Grebel year = 2002 title = Extragalactic Star Clusters: Speculations on the Future journal =… … Wikipedia