Star cluster

Star cluster

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Star clusters or star clouds are groups of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s. Two types of star clusters can be distinguished: globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

s are tight groups of hundreds of thousands of very old stars which are gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

ally bound, while open cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

s, more loosely clustered groups of stars, 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
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, 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.

Star clusters visible to the naked eye include Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...

, Hyades
Hyades (star cluster)
The Hyades is the nearest open cluster to the Solar System and one of the best-studied of all star clusters. The Hipparcos satellite, the Hubble Space Telescope, and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster's center of ~153 ly...

 and the Beehive Cluster
Beehive Cluster
The Beehive Cluster, also known as Praesepe , M44, NGC 2632, or Cr 189, is an open cluster in the constellation Cancer. It is one of the nearest open clusters to the Solar System, and it contains a larger star population than most other nearby clusters...

.

Globular cluster



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, with mass just less than two solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es. Such stars predominate within clusters because hotter and more massive stars have exploded as supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e, or evolved through planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...

 phases to end as white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

s. 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 straggler
Blue straggler
Blue stragglers are main sequence stars in open or globular clusters that are more luminous and bluer than stars at the main sequence turn-off point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3...

s.

In our galaxy, globular clusters are distributed roughly spherically in the galactic halo
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...

, around the galactic centre, orbiting the centre in highly elliptical orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

s. In 1917, the astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

 Harlow Shapley
Harlow Shapley
Harlow Shapley was an American astronomer.-Career:He was born on a farm in Nashville, Missouri, and dropped out of school with only the equivalent of a fifth-grade education...

 was able to estimate the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's distance from the galactic centre based on the distribution of globular clusters; previously the Sun's location within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 was by no means well established.
Until recently, globular clusters were the cause of a great mystery in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, as theories of stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

 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
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

 satellite and increasingly accurate measurements of the Hubble constant resolved the paradox
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...

, 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 1
Westerlund 1
Westerlund 1 is a compact young star cluster in the Milky Way galaxy, about 3.5–5 kpc away from Earth. In fact, it is the most massive compact young star cluster known in the entire Local Group of galaxies...

 in the Milky Way, may be the precursors of globular clusters.

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
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

 M87 contains over a thousand.

A few of the brightest globular clusters are visible to the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...

, with the brightest, Omega Centauri
Omega Centauri
Omega Centauri or NGC 5139 is a globular clusterin the constellation of Centaurus, discovered by Edmond Halley in 1677 who listed it as a nebula. Omega Centauri had been listed in Ptolemy's catalog 2000 years ago as a star. Lacaille included it in his catalog as number I.5...

, 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).

Intermediate forms


In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy, which is, 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 Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

 are M31WFS C1 http://simbad.u-strasbg.fr/simbad/sim-id?Ident=%401592523&M31WFS%20C1, M31WFS C2, and 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
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity 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.

Open clusters


Open clusters (OC) are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane
Galactic plane
The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...

, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67
Messier 67
Messier 67 is an open cluster in the constellation of Cancer. M67's Trumpler class is variously given as II 2 r, II 2 m, or II 3 r. It was discovered by Johann Gottfried Koehler in 1779. Age estimates for the cluster range between 3.2 and 5 billion years, with the most recent estimate implying...

 (the closest and most observed old open cluster) for example. They form from H II region
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...

s such as the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...

.

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
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...

 and Hyades
Hyades (star cluster)
The Hyades is the nearest open cluster to the Solar System and one of the best-studied of all star clusters. The Hipparcos satellite, the Hubble Space Telescope, and infrared color-magnitude diagram fitting have been used to establish a distance to the cluster's center of ~153 ly...

 in Taurus
Taurus (constellation)
Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...

. The Double Cluster
Double Cluster
The Double Cluster is the common name for the naked-eye open clusters NGC 884 and NGC 869 , which are close together in the constellation Perseus. NGC 884 and NGC 869 are at distances of 7600 and 6800 light-years away, respectively, so they are also close to one another in space...

 of h
NGC 869
NGC 869 is an open cluster located 7600 light years away in the constellation of Perseus. The cluster is most likely around 13 million years old. It is the westernmost of the Double Cluster with NGC 884. Located in the Perseus OB1 association both clusters are located physically close to one...

+Chi Persei
NGC 884
NGC 884 is an open cluster located 7600 light years away in the constellation of Perseus. The cluster is most likely around 12.5 million years old. It is the easternmost of the Double Cluster with NGC 869. Located in the Perseus OB1 association both clusters are located physically close to one...

 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.

Super star cluster


Super star cluster (SSC) is a very large region of star formation thought to be the precursor of a globular cluster.

Embedded cluster


Embedded clusters (EC) are stellar clusters that are partially or fully encased in an Interstellar dust or gas
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

. 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

Stellar associations



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
Big Dipper
The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...

 are members of a former open cluster, the Ursa Major Moving Group
Ursa Major Moving Group
The Ursa Major Moving Group, also known as Collinder 285 or Ursa Major association, is a nearby stellar moving group, a set of stars with common velocities in space and thought to have a common origin. Its core is located roughly 80 light years away...

, and have similar proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

s. Other stars across the sky, including Alphecca and Zeta Trianguli Australis
Zeta Trianguli Australis
Zeta Trianguli Australis is a spectroscopic binary in the constellation Triangulum Australe. It is approximately 39.5 light years from Earth....

, are related to this group. The Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 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
Perseus (constellation)
Perseus is a constellation in the northern sky, named after the Greek hero Perseus. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations defined by the International Astronomical Union...

), which is very prominent in binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

. Distant moving clusters cannot readily be detected since the proper motions of the stars need to be known.

Astronomical significance of clusters


Stellar clusters are 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
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

. A Hertzsprung–Russell Diagram
Hertzsprung–Russell diagram
The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...

 can be plotted for these clusters which has absolute values known on the luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

 axis. Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 can be compared to that of the first cluster and the distance estimated. This process is known as main-sequence fitting. Reddening
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...

 and stellar populations must be accounted for when using this method.

See also


  • Protostar
    Protostar
    A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...

  • Robust Associations of Massive Baryonic Objects (RAMBOs)
  • Hypercompact stellar system
    Hypercompact stellar system
    A hypercompact stellar system is a dense cluster of stars around a supermassive black hole that has been ejected from the centre of its host galaxy...

  • Open cluster
    Open cluster
    An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

     and globular cluster
    Globular cluster
    A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...


External links