Star cluster

Last updated
Messier 92, a globular cluster in the constellation of Hercules M92 arp 750pix.jpg
Messier 92, a globular cluster in the constellation of Hercules

Star clusters are large groups of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clustered groups of stars, generally containing fewer 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.


Star clusters visible to the naked eye include the Pleiades, Hyades, and 47 Tucanae.

Open cluster

The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosity Pleiades large.jpg
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, with a few rare exceptions as old as a few billion years, such as Messier 67 (the closest and most observed old open cluster) for example. [1] They form H II regions such as the Orion Nebula.

Open clusters typically have a few hundred members and are located in an area up to 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.

Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are then used as standard candles. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe (Hubble constant). Indeed, the open cluster NGC 7790 hosts three classical Cepheids which are critical for such efforts. [2] [3]

Embedded cluster

The embedded Trapezium cluster seen in X-rays which penetrate the surrounding clouds Chandra X-ray View of Orion.jpg
The embedded Trapezium cluster seen in X-rays which penetrate the surrounding clouds
Star cluster NGC 3572 and its surroundings The star cluster NGC 3572 and its dramatic surroundings.jpg
Star cluster NGC 3572 and its surroundings

Embedded clusters are groups of very young stars that are partially or fully encased in an Interstellar dust or gas which is often impervious to optical observations. Embedded clusters form in molecular clouds, when the clouds begin to collapse and form stars. There is often ongoing star formation in these clusters, so embedded clusters may be home to various types of young stellar objects including protostars and pre-main-sequence stars. An example of an embedded cluster is the Trapezium Cluster in the Orion Nebula. In ρ Ophiuchi cloud (L1688) core region there is an embedded cluster. [4]

The embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through radiation pressure, stellar winds and outflows, or supernova explosions. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in particularly dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered, often leading to the disruption of a star cluster. Most young embedded clusters disperse shortly after the end of star formation. [5]

The open clusters found in the Galaxy are former embedded clusters that were able to survive early cluster evolution. However, nearly all freely floating stars, including the Sun, [6] were originally born into embedded clusters that disintegrated. [5]

Globular cluster

The globular cluster Messier 15 photographed by HST New Hubble image of star cluster Messier 15.jpg
The globular cluster Messier 15 photographed by HST

Globular clusters are roughly spherical groupings of from 10 thousand 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 masses less than two solar masses. [7] Such stars predominate within clusters because hotter and more massive stars have exploded as supernovae, or evolved through planetary nebula phases 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 the Milky Way galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the Galactic Center, orbiting the center in highly elliptical orbits. In 1917, the astronomer Harlow Shapley made the first respectable estimate of the Sun's distance from the Galactic Center, based on the distribution of globular clusters.

Until the mid-1990s, 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, [7] some of which may have been captured cores of small galaxies stripped of stars previously in their outer margins by the tides of the Milky Way, as seems to be the case for the globular cluster M79. Some galaxies are much richer in globulars than the Milky Way: The giant elliptical galaxy M87 contains over a thousand.

A few of the brightest globular clusters are visible to the naked eye; the brightest, Omega Centauri, was observed in antiquity and catalogued as a star, before the telescopic age. The brightest globular cluster in the northern hemisphere is M13 in the constellation of Hercules.

Super star cluster

Super star clusters are very large regions of recent star formation, and are thought to be the precursors of globular clusters. Examples include Westerlund 1 in the Milky Way. [8]

Intermediate forms

Messier 68, a loose globular cluster whose constituent stars span a volume of space more than a hundred light-years across A Ten Billion Year Stellar Dance.jpg
Messier 68, a loose globular cluster whose constituent stars span a volume of space more than a hundred light-years across

In 2005, astronomers discovered a new type of star cluster in the Andromeda Galaxy, which is, in several ways, very similar to globular clusters although less dense. No such clusters (which also known as extended globular clusters) are known in the Milky Way. The three discovered in Andromeda Galaxy are M31WFS C1 [9] M31WFS C2, and M31WFS C3.

These new-found star clusters contain hundreds of thousands of stars, a similar number to 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 thus much greater. The clusters have properties intermediate between globular clusters and dwarf spheroidal galaxies. [10]

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. [10]

Another type of cluster are faint fuzzies which so far have only been found in lenticular galaxies like NGC 1023 and NGC 3384. They are characterized by their large size compared to globular clusters and a ringlike distribution around the centres of their host galaxies. As the latter they seem to be old objects. [11]

Astronomical significance

Artist's impression of an exoplanet orbiting a star in the cluster Messier 67 Artist's impression of an exoplanet orbiting a star in the cluster Messier 67.jpg
Artist's impression of an exoplanet orbiting a star in the cluster Messier 67

Star clusters are important in many areas of astronomy. The reason behind this is that almost all the stars in old clusters were born at roughly the same time. Various 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. This is primarily true for old globular clusters. In the case of young (age < 1Gyr) and intermediate-age (1 < age < 5 Gyr), factors such as age, mass, chemical compositions may also play vital roles. Based on their ages, star clusters can reveal a lot of information about their host galaxies. For example, star clusters residing in the Magellanic Clouds can provide essential information about the formation of the Magellanic Clouds dwarf galaxies. This, in turn, can help us understand many astrophysical processes happening in our own Milky Way Galaxy. These clusters, especially the young ones can explain the star formation process that might have happened in our Milky Way Galaxy.

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.

Nearly all stars in the Galactic field, including the Sun, were initially born in regions with embedded clusters that disintegrated. This means that properties of stars and planetary systems may have been affected by early clustered environments. This appears to be the case for our own Solar System, in which chemical abundances point to the effects of a supernova from a nearby star early in our Solar System's history.

Star cloud

Scutum Star Cloud with open cluster Messier 11 at lower left Scutum Star Cloud.jpg
Scutum Star Cloud with open cluster Messier 11 at lower left

Technically not star clusters, star clouds are large groups of many stars within a galaxy, spread over very many light-years of space. Often they contain star clusters within them. The stars appear closely packed, but are not usually part of any structure. [13] Within the Milky Way, star clouds show through gaps between dust clouds of the Great Rift, allowing deeper views along our particular line of sight. [14] Star clouds have also been identified in other nearby galaxies. [15] Examples of star clouds include the Large Sagittarius Star Cloud, Small Sagittarius Star Cloud, Scutum Star Cloud, Cygnus Star Cloud, Norma Star Cloud, and NGC 206 in the Andromeda Galaxy.


In 1979, the International Astronomical Union's 17th general assembly recommended that newly discovered star clusters, open or globular, within the Galaxy have designations following the convention "Chhmm±ddd", always beginning with the prefix C, where h, m, and d represent the approximate coordinates of the cluster centre in hours and minutes of right ascension, and degrees of declination, respectively, with leading zeros. The designation, once assigned, is not to change, even if subsequent measurements improve on the location of the cluster centre. [16] The first of such designations were assigned by Gosta Lynga in 1982. [17] [18]

See also

Related Research Articles

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Globular cluster</span> Spherical collection of stars

A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".

<span class="mw-page-title-main">Open cluster</span> Large group of stars less bound than globular clusters

An open cluster is a type of star cluster made of tens 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. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

<span class="mw-page-title-main">Star formation</span> Process by which dense regions of molecular clouds in interstellar space collapse to form stars

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

<span class="mw-page-title-main">Andromeda Galaxy</span> Barred spiral galaxy in the Local Group

The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about 46.56 kiloparsecs approximately 765 kpc from Earth and the nearest large galaxy to the Milky Way. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

<span class="mw-page-title-main">Cepheid variable</span> Type of variable star that pulsates radially

A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.

<span class="mw-page-title-main">Triangulum Galaxy</span> Spiral galaxy in the constellation Triangulum

The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (New General Catalogue) 598. With the D25 isophotal diameter of 18.74 kiloparsecs (61,100 light-years), the Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way.

<span class="mw-page-title-main">Spiral galaxy</span> Class of galaxy that has spiral structures extending from their cores.

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

<span class="mw-page-title-main">RR Lyrae variable</span> Type of variable star

RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and brightest example, RR Lyrae.

<span class="mw-page-title-main">Sagittarius Dwarf Spheroidal Galaxy</span> Dwarf galaxy

The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy, is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in its main body, with the brightest of them – NGC 6715 (M54) – being known well before the discovery of the galaxy itself in 1994. Sgr dSph is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth, travelling in a polar orbit at a distance of about 50,000 light-years from the core of the Milky Way. In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past. In 2018 the Gaia project of the European Space Agency showed that Sgr dSph had caused perturbations in a set of stars near the Milky Way's core, causing unexpected rippling movements of the stars triggered when it moved past the Milky Way between 300 and 900 million years ago.

<span class="mw-page-title-main">Cosmic distance ladder</span> Succession of methods by which astronomers determine the distances to celestial objects

The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of galaxies comprise the halo:

<span class="mw-page-title-main">Omega Centauri</span> Globular cluster in the constellation Centaurus

Omega Centauri is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 17,090 light-years, it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way.

<span class="mw-page-title-main">Dwarf galaxy</span> Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

<span class="mw-page-title-main">Antennae Galaxies</span> Interacting galaxy in the constellation Corvus

The Antennae Galaxies are a pair of interacting galaxies in the constellation Corvus. They are currently going through a starburst phase, in which the collision of clouds of gas and dust, with entangled magnetic fields, causes rapid star formation. They were discovered by William Herschel in 1785.

The Canis Major Overdensity or Canis Major Dwarf Galaxy is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.

<span class="mw-page-title-main">NGC 6822</span> Barred Irregular galaxy in the constellation Sagittarius

NGC 6822 is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1884, with a six-inch refractor telescope. It is the closest non-satellite galaxy to the Milky Way, but lies just outside its virial radius. It is similar in structure and composition to the Small Magellanic Cloud. It is about 7,000 light-years in diameter.

<span class="mw-page-title-main">Milky Way</span> Galaxy containing the Solar System

The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικὸς κύκλος, meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.

<span class="mw-page-title-main">Gaia Sausage</span> Remains galaxy merger in the Milky Way

The Gaia Sausage or Gaia Enceladus is the remains of a dwarf galaxy that merged with the Milky Way about 8–11 billion years ago. At least eight globular clusters were added to the Milky Way along with 50 billion solar masses of stars, gas and dark matter. It represents the last major merger of the Milky Way.


  1. Brent A. Archinal; Steven J. Hynes (2003). Star Clusters. Willmann-Bell. ISBN   978-0-943396-80-4.
  2. Sandage, Allan (1958). "Cepheids in Galactic Clusters. I. CF Cass in NGC 7790". The Astrophysical Journal. 128: 150. Bibcode:1958ApJ...128..150S. doi:10.1086/146532.
  3. Majaess, D.; Carraro, G.; Moni Bidin, C.; Bonatto, C.; Berdnikov, L.; Balam, D.; Moyano, M.; Gallo, L.; Turner, D.; Lane, D.; Gieren, W.; Borissova, J.; Kovtyukh, V.; Beletsky, Y. (2013). "Anchors for the cosmic distance scale: The Cepheids U Sagittarii, CF Cassiopeiae, and CEab Cassiopeiae". Astronomy & Astrophysics. 560: A22. arXiv: 1311.0865 . Bibcode:2013A&A...560A..22M. doi:10.1051/0004-6361/201322670. S2CID   55934597.
  4. Greene, Thomas P; Meyer, Michael R (1995). "An Infrared Spectroscopic Survey of the rho Ophiuchi Young Stellar Cluster: Masses and Ages from the H-R Diagram". Astrophysical Journal. 450: 233. Bibcode:1995ApJ...450..233G. doi:10.1086/176134.
  5. 1 2 Lada, Charles J.; Lada, Elizabeth A. (2003). "Embedded Clusters in Molecular Clouds". Annual Review of Astronomy and Astrophysics. 41 (1): 57–115. arXiv: astro-ph/0301540 . Bibcode:2003ARA&A..41...57L. doi:10.1146/annurev.astro.41.011802.094844. ISSN   0066-4146. S2CID   16752089.
  6. Gounelle, M.; Meynet, G. (2012-08-27). "Solar system genealogy revealed by extinct short-lived radionuclides in meteorites". Astronomy & Astrophysics. EDP Sciences. 545: A4. arXiv: 1208.5879 . Bibcode:2012A&A...545A...4G. doi:10.1051/0004-6361/201219031. ISSN   0004-6361. S2CID   54970631.
  7. 1 2 Dinwiddie, Robert; Gater, Will; Sparrow, Giles; Stott, Carole (2012). Stars and Planets. Nature Guide. DK. pp. 14, 134–137. ISBN   978-0-7566-9040-3.
  8. "Young and Exotic Stellar Zoo: ESO's Telescopes Uncover Super Star Cluster in the Milky Way". ESO. 2005-03-22. Archived from the original on 2017-12-01. Retrieved 2017-11-27.
  9. "@1592523". Retrieved 28 April 2018.
  10. 1 2 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 (3): 993–1006. arXiv: astro-ph/0412223 . Bibcode:2005MNRAS.360.1007H. doi:10.1111/j.1365-2966.2005.09086.x. S2CID   6215035.
  11. A. Burkert; J. Brodie; S. Larsen 3 (2005). "Faint Fuzzies and the Formation of Lenticular Galaxies". The Astrophysical Journal. 628 (1): 231–235. arXiv: astro-ph/0504064 . Bibcode:2005ApJ...628..231B. doi:10.1086/430698. S2CID   11466131.
  12. "First Planet Found Around Solar Twin in Star Cluster". ESO Press Release. Archived from the original on 2017-12-01. Retrieved 2017-11-27.
  13. Patrick Moore (2005). The Observer's Year: 366 Nights in the Universe. Springer. p. 199. ISBN   1-85233-884-9.
  14. Bob King (2016-07-13). "Paddle the Milky Way's Dark River". Retrieved 2020-09-29.
  15. Bob King (2016-10-05). "Resolving Andromeda - How to See Stars 2.5 Million Light-Years Away". Retrieved 2020-09-20.
  16. XVIIth General Assembly (PDF) (14–23 August 1979). Montreal, Canada: International Astronomical Union. Summer 1979. p. 13. Archived (PDF) from the original on 18 January 2015. Retrieved 18 December 2014.
  17. Lynga, G. (October 1982). "IAU numbers for some recently discovered clusters". Bulletin d'Information du Centre de Données Stellaires. 23: 89. Bibcode:1982BICDS..23...89L.
  18. "Dictionary of Nomenclature of Celestial Objects". Simbad. Centre de données astronomiques de Strasbourg. 1 December 2014. Archived from the original on 8 October 2014. Retrieved 21 December 2014.