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Encyclopedia > Star cluster
Messier 92 in the Hercules constellation.
Messier 92 in the Hercules constellation.

Star clusters are groups of stars which are gravitationally bound. Two distint 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. Download high resolution version (750x786, 93 KB)M92 globular cluster. ... Download high resolution version (750x786, 93 KB)M92 globular cluster. ... Globular Cluster M92 (also known as Messier Object 92, Messier 92, M92, or NGC 6341) is a globular cluster in the Hercules constellation. ... Named after the Roman name (Hercules) of the Greek mythological hero Herakles, Hercules is the fifth largest of the 88 modern constellations. ... This article is about the astronomical object. ... Gravity is a force of attraction that acts between bodies that have mass. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... The Pleiades is one of the most famous open clusters. ... Gravity redirects here. ... A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light-years in diameter and approximately 60 million light-years distant. ... A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. ...

Contents

Globular clusters

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. The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... A light-year or lightyear, symbol ly, is a unit of length. ... Stars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... Multiwavelength X-ray image of the remnant of Keplers Supernova, SN 1604. ... NGC 6543, the Cats Eye Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... Image of Sirius A and Sirius B taken by the Hubble Space Telescope. ... Blue stragglers are stars in open or globular clusters that are hotter and bluer than other cluster stars having the same luminosity. ...


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. The galactic halo is a region of space surrounding spiral galaxies, including our galaxy, the Milky Way. ... The Galactic Center is the rotational center of the Milky Way galaxy. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Year 1917 (MCMXVII) was a common year starting on Monday of the Gregorian calendar (see link for calendar) or a common year starting on Tuesday of the 13-day slower Julian calendar (see: 1917 Julian calendar). ... An astronomer or astrophysicist is a person whose area of interest is astronomy or astrophysics. ... Harlow Shapley in his earlier years. ... The Sun is the star at the center of the Solar System. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy of the Local Group. ...


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. A giant Hubble mosaic of the Crab Nebula, a supernova remnant. ... In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime; the hundreds of thousands, millions or billions of years during which it emits light and heat. ... Hipparcos (for High Precision Parallax Collecting Satellite) was an astrometry mission of the European Space Agency (ESA) dedicated to the measurement of stellar parallax and the proper motions of stars. ... Hubbles law is the statement in astronomy that the redshift in light coming from distant galaxies is proportional to their distance. ... Robert Boyles self-flowing flask fills itself in this diagram, but perpetual motion machines cannot exist. ...


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. M79 refers to: Messier 79 a Messier object and a globular cluster in the Lepus constellation. ... An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties: The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies. ... The jet emitted by M87 in this image is thought to be caused by a supermassive black hole at the galaxys center. ...


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 naked eye is a figure of speech referring to human visual perception that is unaided by enhancing equipment, such as a telescope or binoculars. ... A small region at the heart of Omega Centauri, containing some 50,000 stars (NASA/STScI) Omega Centauri or NGC 5139 is a globular cluster of stars orbiting our galaxy, the Milky Way. ... Messier Object 13, the Great Globular Cluster in Hercules; one of the most prominent and best known globular clusters of the Northern celestial hemisphere. ... Messier Object 13, the Great Globular Cluster in Hercules; one of the most prominent and best known globular clusters of the Northern celestial hemisphere. ...


Open clusters

Main article: Open cluster
The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosity
The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosity

Open clusters 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. The Pleiades is one of the most famous open clusters. ... The Pleiades. ... The Pleiades. ... The galactic plane is the plane in which the majority of a flattened galaxys mass lies. ... A spiral galaxy presents a face-on view of its spiral arms. ... NGC 604, a giant H II region in the Triangulum Galaxy. ... The entire Orion Nebula in visible light Optical images reveal clouds of gas and dust in the Orion Nebula; an infrared image (right) reveals the new stars shining within. ...


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'. A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ...


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. The Pleiades are an open cluster dominated by hot blue stars surrounded by reflection nebulosity A shorter exposure shows less nebulosity. ... The Hyades (‘Υάδες) are an open star cluster located in the constellation Taurus. ... For other uses of the word Taurus see Taurus. ... The Double Cluster is the common name for NGC 884 and NGC 869, which are close together in the constellation Perseus. ... Open Cluster NGC 869 is an open cluster in the Perseus constellation. ... Open Cluster NGC 884, also known as Chi Persei, is an open cluster in the Perseus constellation. ...


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. 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.[1] M31 in a small telescope The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Andromeda Nebula) is a spiral galaxy approximately 2. ... Dwarf spheroidal galaxy (dSph) is a term in astronomy applied to the nine low luminosity dwarf elliptical galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31. ...


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.[1]


Stellar associations

Main article: Stellar association
The Christmas Tree Cluster will eventually break apart.
The Christmas Tree Cluster will eventually break apart.

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. A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. ... Image File history File linksMetadata Download high-resolution version (2310x3897, 8023 KB) Newborn stars, hidden behind thick dust, are revealed in this image of a section of the Christmas Tree Cluster from NASAs Spitzer Space Telescope. ... Image File history File linksMetadata Download high-resolution version (2310x3897, 8023 KB) Newborn stars, hidden behind thick dust, are revealed in this image of a section of the Christmas Tree Cluster from NASAs Spitzer Space Telescope. ... Big Dipper map The seven brightest stars of the constellation Ursa Major, the Great Bear, form a well-known asterism that has been recognized as a distinct grouping in many cultures from time immemorial. ... The Ursa Major Moving Group is the closest moving group to Earth, with its core being located roughly 80 light years away. ... The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real... Alpha Coronae Borealis (α CrB / α Coronae Borealis) is a binary star in the constellation Corona Borealis. ... Zeta Trianguli Australis (ζ TrA / ζ Trianguli Australis) is a binary star in the constellation Triangulum Australe. ... The Sun is the star at the center of the Solar System. ...


Another stellar association is that surrounding MirfakPersei), 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. Mirfak (also known as Algenib, and designated α Persei) is the brightest star in the constellation of Perseus. ... Perseus is a northern constellation, named after the Greek hero who slew the monster Medusa. ... Porro-prism binoculars with central focusing Binocular telescopes, or binoculars, (also known as field glasses) are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, one to be viewed through each of the users eyes to present the viewer...


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. The cosmic distance ladder refers to the methods by which astronomers determine the distances to objects. ... This does not cite its references or sources. ... The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars. ... Luminosity has different meanings in several different fields of science. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... Extinction is a term used in astronomy to describe the absorption of light from astronomical objects by matter between them and the observer. ... Stars observed in our galaxy appear to group into two general types called Population I and Population II. (A hypothetical third group, Population III, does not occur in our galaxy. ...


References

  1. ^ a b 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.

See also

Protostar is a period after clouds of hydrogen, helium and dust begin to contract and before the star reaches the main sequence. ... Hodge 301 (lower right) in the Tarantula Nebula Hodge 301 is a star cluster in the Tarantula Nebula, visible from Earths Southern Hemisphere. ...

External links

  • Star Clusters, SEDS Messier pages

  Results from FactBites:
 
star cluster: Definition and Much More from Answers.com (2212 words)
Star clusters are important because, within each cluster, member stars probably are at the same distance from the Sun, and have the same age and the same initial chemical composition.
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.
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.
Sea and Sky's Cosmic Wonders: Star Clusters (775 words)
Star clusters are groupings of stars held together by a common gravitational bond.
The stars are usually packed into a spherical arrangement with the highest density of stars occurs in the center of the cluster.
Star cluster are among the easiest objects to observe in the night sky.
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