![Pleiades large](https://upload.wikimedia.org/wikipedia/commons/4/4e/Pleiades_large.jpg)
An open cluster is a type of
star cluster
Star clusters are large groups of stars. 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 clust ...
made of up to a few thousand
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s that were formed from the same
giant molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
and have roughly the same age. More than 1,100 open clusters have been discovered within the
Milky Way
The Milky Way is the galaxy that includes our 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 ...
galaxy, and many more are thought to exist.
They are loosely bound by mutual
gravitational attraction
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
and become disrupted by close encounters with other clusters and clouds of gas as they orbit the
Galactic Center
The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact rad ...
. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters.
[ 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 cluster
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 membe ...
s of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral
In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point.
Helices
Two major definitions of "spiral" in the American Heritage Dictionary are:[irregular galaxies
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...]
, in which active is occurring.[
Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an ]H II region
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...
.[ Over time, ]radiation pressure
Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
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 of the gas away.
Open clusters are key objects in the study of stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...
. Because the cluster members are of similar age and chemical composition
A chemical composition specifies the identity, arrangement, and ratio of the elements making up a compound.
Chemical formulas can be used to describe the relative amounts of elements present in a compound. For example, the chemical formula for ...
, their properties (such as distance, age, metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
, extinction
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
, and velocity) are more easily determined than they are for isolated stars. A number of open clusters, such as the Pleiades
The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
, Hyades Hyades may refer to:
* Hyades (band)
*Hyades (mythology)
*Hyades (star cluster)
The Hyades (; Greek Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest open cluster and one of the best-studied star clusters. Loca ...
or the Alpha Persei Cluster
The Alpha Persei Cluster, also known as Melotte 20 or Collinder 39, is an open cluster of stars in the northern constellation of Perseus. To the naked eye, the cluster consists of several blue-hued spectral type B stars. The most luminous member ...
are visible with the naked eye. Some others, such as the Double Cluster
The Double Cluster (also known as Caldwell 14) consists of the open clusters NGC 869 and NGC 884 (often designated h Persei and χ (chi) Persei, respectively), which are close together in the constellation Perseus. Both visible with the ...
, are barely perceptible without instruments, while many more can be seen using binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
or telescopes
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
. The Wild Duck Cluster
The Wild Duck Cluster (also known as Messier 11, or NGC 6705) is an open cluster of stars in the constellation Scutum (the Shield). It was discovered by Gottfried Kirch in 1681. Charles Messier included it in his catalogue of diffuse objects in ...
, M11, is an example.[
]
Historical observations
The prominent open cluster the Pleiades
The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
, in the constellation Taurus, has been recognized as a group of stars since antiquity, while the Hyades (which also form part of Taurus
Taurus is Latin for 'bull' and may refer to:
* Taurus (astrology), the astrological sign
* Taurus (constellation), one of the constellations of the zodiac
* Taurus (mythology), one of two Greek mythological characters named Taurus
* '' Bos tauru ...
) is one of the oldest open clusters. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light. In his ''Almagest
The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it canoni ...
'', the Roman astronomer Ptolemy
Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of importanc ...
mentions the Praesepe
The Beehive Cluster (also known as Praesepe (Latin for "manger" or "crib"), M44, NGC 2632, or Cr 189), is an open cluster in the constellation Cancer. One of the nearest open clusters to Earth, it contains a larger population of stars than other ...
cluster, the Double Cluster
The Double Cluster (also known as Caldwell 14) consists of the open clusters NGC 869 and NGC 884 (often designated h Persei and χ (chi) Persei, respectively), which are close together in the constellation Perseus. Both visible with the ...
in Perseus
In Greek mythology, Perseus (Help:IPA/English, /ˈpɜːrsiəs, -sjuːs/; Greek language, Greek: Περσεύς, Romanization of Greek, translit. Perseús) is the legendary founder of Mycenae and of the Perseid dynasty. He was, alongside Cadmus ...
, the Coma Star Cluster
The Coma Star Cluster (also known as Melotte 111 or Collinder 256) is a small but nearby open cluster located in the constellation Coma Berenices. The cluster contains about 40 brighter stars (between magnitudes 5 and 10) with a common proper mo ...
, and the Ptolemy Cluster
Messier 7 or M7, also designated NGC 6475 and sometimes known as the Ptolemy Cluster, is an open cluster of stars in the constellation of Scorpius. The cluster is easily detectable with the naked eye, close to the "stinger" of Scorpius. With a d ...
, while the Persian astronomer Al-Sufi
ʿAbd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی; December 7, 903 – May 25, 986) was an iranianRobert Harry van Gent. Biography of al-Sūfī'. "The Persian astronomer Abū al-Husayn ‘Abd al-Rahmān ibn ‘Umar al-Sūfī was born in ...
wrote of the Omicron Velorum cluster
IC 2391 (also known as the Omicron Velorum Cluster or Caldwell 85) is an open cluster in the constellation Vela. The Persian astronomer Al Sufi may have first described it about 964. It was found by Abbe Lacaille and cataloged as Lac II 5.
T ...
.[ However, it would require the invention of the ]telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
to resolve these "nebulae" into their constituent stars.[ Indeed, in 1603 ]Johann Bayer
Johann Bayer (1572 – 7 March 1625) was a German lawyer and uranographer (celestial cartographer). He was born in Rain, Lower Bavaria, in 1572. At twenty, in 1592 he began his study of philosophy and law at the University of Ingolstadt, a ...
gave three of these clusters designations as if they were single stars.[
]
The first person to use a telescope to observe the night sky and record his observations was the Italian scientist Galileo Galilei
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
in 1609. When he turned the telescope toward some of the nebulous patches recorded by Ptolemy, he found they were not a single star, but groupings of many stars. For Praesepe, he found more than 40 stars. Where previously observers had noted only 6–7 stars in the Pleiades, he found almost 50.[ In his 1610 treatise '']Sidereus Nuncius
''Sidereus Nuncius'' (usually ''Sidereal Messenger'', also ''Starry Messenger'' or ''Sidereal Message'') is a short Astronomy, astronomical treatise (or ''pamphlet'') published in New Latin by Galileo Galilei on March 13, 1610. It was the first ...
'', Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters."[ Influenced by Galileo's work, the Sicilian astronomer Giovanni Hodierna became possibly the first astronomer to use a telescope to find previously undiscovered open clusters.][ In 1654, he identified the objects now designated ]Messier 41
Messier 41 (also known as M41 or NGC 2287) is an open cluster in the constellation Canis Major, sometimes referred to as The Little Beehive Cluster. It was discovered by Giovanni Batista Hodierna before 1654 and was perhaps known to Aristotle abou ...
, Messier 47
Messier 47 (M47 or NGC 2422) is an open cluster in the mildly southern constellation of Puppis. It was discovered by Giovanni Batista Hodierna before 1654 and in his then keynote work re-discovered by Charles Messier on 1771. It was also independe ...
, NGC 2362
NGC 2362, also known as Caldwell 64, is an open cluster of stars in the southern constellation of Canis Major. It was discovered by the Italian court astronomer Giovanni Batista Hodierna, who published his finding in 1654. William Herschel called ...
and NGC 2451.[
It was realized as early as 1767 that the stars in a cluster were physically related,][ when the English naturalist Reverend ]John Michell
John Michell (; 25 December 1724 – 21 April 1793) was an English people, English natural philosophy, natural philosopher and clergyman who provided pioneering insights into a wide range of scientific fields including astronomy, geology, opti ...
calculated 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.[ Between 1774–1781, French astronomer ]Charles Messier
Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects''. Messier's purpose f ...
published a catalogue of celestial objects that had a nebulous appearance similar to comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
s. This catalogue included 26 open clusters.[ In the 1790s, English astronomer ]William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
began an extensive study of nebulous celestial objects. He discovered that many of these features could be resolved into groupings of individual stars. Herschel conceived the idea that stars were initially scattered across space, but later became clustered together as star systems because of gravitational attraction.[ He divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars.][
]
The number of clusters known continued to increase under the efforts of astronomers. Hundreds of open clusters were listed in the New General Catalogue
The ''New General Catalogue of Nebulae and Clusters of Stars'' (abbreviated NGC) is an astronomical catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. The NGC contains 7,840 objects, including galaxies, star clusters and ...
, first published in 1888 by the Danish-Irish astronomer J. L. E. Dreyer, and the two supplemental Index Catalogue
The ''New General Catalogue of Nebulae and Clusters of Stars'' (abbreviated NGC) is an astronomical catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. The NGC contains 7,840 objects, including galaxies, star clusters an ...
s, published in 1896 and 1905.[ Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found all across the sky but preferentially towards the centre of the ]Milky Way
The Milky Way is the galaxy that includes our 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 other type consisted of a generally sparser population of stars in a more irregular shape. These were generally found in or near the ]galactic plane
The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usual ...
of the Milky Way.[ Astronomers dubbed the former ]globular cluster
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 membe ...
s, and the latter open clusters. Because of their location, open clusters are occasionally referred to as ''galactic clusters'', a term that was introduced in 1925 by the Swiss-American astronomer Robert Julius Trumpler
Robert Julius Trumpler (until 1915 Robert Trümpler, born October 2, 1886 in Zürich, Switzerland; died September 10, 1956 in Berkeley, United States) was a Swiss-American astronomer.
Career
After initial schooling, Trumpler entered the Univers ...
.[
Micrometer measurements of the positions of stars in clusters were made as early as 1877 by the German astronomer E. Schönfeld and further pursued by the American astronomer E. E. Barnard prior to his death in 1923. No indication of stellar motion was detected by these efforts.][ However, in 1918 the Dutch-American astronomer ]Adriaan van Maanen
Adriaan van Maanen (March 31, 1884 – January 26, 1946) was a Dutch–American astronomer.
Van Maanen, born into a well-to-do family in Friesland, studied astronomy at the University of Utrecht (earning his Ph.D. in 1911) and worked briefl ...
was able to measure the proper motion of stars in part of the Pleiades
The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
cluster by comparing photographic plates taken at different times.[ As ]astrometry
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
His ...
became more accurate, cluster stars were found to share a common proper motion
Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...
through space. By comparing the photographic plates of the Pleiades cluster taken in 1918 with images taken in 1943, van Maanen was able to identify those stars that had a proper motion
Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...
similar to the mean motion of the cluster, and were therefore more likely to be members.[ ]Spectroscopic
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
measurements revealed common radial velocities
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection o ...
, thus showing that the clusters consist of stars bound together as a group.[
The first color-magnitude diagrams of open clusters were published by ]Ejnar Hertzsprung
Ejnar Hertzsprung (; Copenhagen, 8 October 1873 – 21 October 1967, Roskilde) was a Danish chemist and astronomer.
Career
Hertzsprung was born in Frederiksberg, Denmark, the son of Severin and Henriette. He studied chemical engineering at Cop ...
in 1911, giving the plot for the Pleiades
The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance of ...
and Hyades Hyades may refer to:
* Hyades (band)
*Hyades (mythology)
*Hyades (star cluster)
The Hyades (; Greek Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest open cluster and one of the best-studied star clusters. Loca ...
star cluster
Star clusters are large groups of stars. 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 clust ...
s. He continued this work on open clusters for the next twenty years. From spectroscopic data, he was able to determine the upper limit of internal motions for open clusters, and could estimate that the total mass of these objects did not exceed several hundred times the mass of the Sun. He demonstrated a relationship between the star colors and their magnitudes, and in 1929 noticed that the Hyades and Praesepe
The Beehive Cluster (also known as Praesepe (Latin for "manger" or "crib"), M44, NGC 2632, or Cr 189), is an open cluster in the constellation Cancer. One of the nearest open clusters to Earth, it contains a larger population of stars than other ...
clusters had different stellar populations than the Pleiades. This would subsequently be interpreted as a difference in ages of the three clusters.[
]
Formation
The formation of an open cluster begins with the collapse of part of a giant molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
, a cold dense cloud of gas and dust containing up to many thousands of times the mass of the Sun
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
. These clouds have densities that vary from 102 to 106 molecules of neutral hydrogen
The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...
per cm3, with star formation occurring in regions with densities above 104 molecules per cm3. Typically, only 1–10% of the cloud by volume is above the latter density.[ Prior to collapse, these clouds maintain their mechanical equilibrium through magnetic fields, turbulence, and rotation.][
Many factors may disrupt the equilibrium of a giant molecular cloud, triggering a collapse and initiating the burst of star formation that can result in an open cluster. These include shock waves from a nearby ]supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
, collisions with other clouds, or gravitational interactions. Even without external triggers, regions of the cloud can reach conditions where they become unstable against collapse.[ The collapsing cloud region will undergo hierarchical fragmentation into ever smaller clumps, including a particularly dense form known as infrared dark clouds, eventually leading to the formation of up to several thousand stars. This star formation begins enshrouded in the collapsing cloud, blocking the protostars from sight but allowing infrared observation.][ In the Milky Way galaxy, the formation rate of open clusters is estimated to be one every few thousand years.][
]
The hottest and most massive of the newly formed stars (known as OB star
OB stars are hot, massive stars of spectral types O or early-type B that form in loosely organized groups called OB associations. They are short lived, and thus do not move very far from where they formed within their life. During their lifet ...
s) will emit intense ultraviolet radiation
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
, which steadily ionizes the surrounding gas of the giant molecular cloud, forming an H II region
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...
. Stellar wind
A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.
D ...
s and radiation pressure
Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
from the massive stars begins to drive away the hot ionized gas at a velocity matching the speed of sound in the gas. After a few million years the cluster will experience its first core-collapse supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
e, which will also expel gas from the vicinity. In most cases these processes will strip the cluster of gas within ten million years and no further star formation will take place. Still, about half of the resulting protostellar objects will be left surrounded by circumstellar disk
A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the ...
s, many of which form accretion disks.[
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 star formation process. All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. At this point, the formation of an open cluster will depend on whether the newly formed stars are gravitationally bound to each other; otherwise an unbound stellar association will result. Even when a cluster such as the Pleiades does form, it may only hold on to a third of the original stars, with the remainder becoming unbound once the gas is expelled.][ The young stars so released from their natal cluster become part of the Galactic field population.
Because most if not all stars form in clusters, ]star cluster
Star clusters are large groups of stars. 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 clust ...
s are to be viewed as 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.[ Most open clusters form with at least 100 stars and a mass of 50 or more ]solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
es. The largest clusters can have over 104 solar masses, with the massive cluster Westerlund 1
Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young super star cluster about 3.8 kpc (12,000 ly) away from Earth. It is thought to be the most massive young star cluster in the Milky Way, and was discovered by Ben ...
being estimated at 5 × 104 solar masses and R136
R136 (formerly known as RMC 136 from the Radcliffe Observatory Magellanic Clouds catalogue) is the central concentration of stars in the NGC 2070 star cluster, which lies at the centre of the Tarantula Nebula in the Large Magellanic Cloud. When ...
at almost 5 x 105, typical of globular clusters.[ While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of intrinsic difference between a very sparse globular cluster such as ]Palomar 12
Palomar 12 is a globular cluster in the constellation Capricornus, and is a member of the Palomar Globular Clusters group.
First discovered on the National Geographic Society – Palomar Observatory Sky Survey plates by Robert George Harri ...
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 that allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in the Milky Way.[
It is common for two or more separate open clusters to form out of the same molecular cloud. In the ]Large Magellanic Cloud
The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...
, both Hodge 301
Hodge 301 is a star cluster in the Tarantula Nebula, visible from Earth's Southern Hemisphere. The cluster and nebula lie about 168,000 light years away, in one of the Milky Way's orbiting satellite galaxies, the Large Magellanic Cloud.
Ho ...
and R136
R136 (formerly known as RMC 136 from the Radcliffe Observatory Magellanic Clouds catalogue) is the central concentration of stars in the NGC 2070 star cluster, which lies at the centre of the Tarantula Nebula in the Large Magellanic Cloud. When ...
have formed from the gases of the Tarantula Nebula
The Tarantula Nebula (also known as 30 Doradus) is a large H II region in the Large Magellanic Cloud (LMC), forming its south-east corner (from Earth's perspective).
Discovery
The Tarantula Nebula was observed by Nicolas-Louis de Lacaille durin ...
, while in our own galaxy, tracing back the motion through space of the Hyades Hyades may refer to:
* Hyades (band)
*Hyades (mythology)
*Hyades (star cluster)
The Hyades (; Greek Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest open cluster and one of the best-studied star clusters. Loca ...
and Praesepe
The Beehive Cluster (also known as Praesepe (Latin for "manger" or "crib"), M44, NGC 2632, or Cr 189), is an open cluster in the constellation Cancer. One of the nearest open clusters to Earth, it contains a larger population of stars than other ...
, 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
The Double Cluster (also known as Caldwell 14) consists of the open clusters NGC 869 and NGC 884 (often designated h Persei and χ (chi) Persei, respectively), which are close together in the constellation Perseus. Both visible with the ...
of NGC 869 and NGC 884 (also known as h and χ Persei), but at least 10 more double clusters are known to exist.[ Many more are known in the ]Small
Small may refer to:
Science and technology
* SMALL, an ALGOL-like programming language
* Small (anatomy), the lumbar region of the back
* ''Small'' (journal), a nano-science publication
* <small>, an HTML element that defines smaller text ...
and Large Magellanic Cloud
The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...
s—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 agglomeration
Agglomeration may refer to:
* Urban agglomeration, in standard English
* Megalopolis, in Chinese English, as defined in China's ''Standard for basic terminology of urban planning'' (GB/T 50280—98). Also known as " city cluster".
* Economies of ag ...
s 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 stars per cubic light year.[
Open clusters are often classified according to a scheme developed by ]Robert Trumpler
Robert Julius Trumpler (until 1915 Robert Trümpler, born October 2, 1886 in Zürich, Switzerland; died September 10, 1956 in Berkeley, California, Berkeley, United States) was a Switzerland, Swiss-United States, American astronomer.
Career
After ...
in 1930. The Trumpler scheme gives a cluster a three part designation, with a Roman numeral
Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, eac ...
from I-IV for little to very disparate, an Arabic numeral
Arabic numerals are the ten numerical digits: , , , , , , , , and . They are the most commonly used symbols to write decimal numbers. They are also used for writing numbers in other systems such as octal, and for writing identifiers such as ...
from 1 to 3 for 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 further appended if the cluster lies within nebulosity
A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...
.[
Under the Trumpler scheme, the Pleiades are classified as I3rn, the nearby Hyades are classified as II3m.
]
Numbers and distribution
There are over 1,100 known open clusters in our galaxy, but the true total may be up to ten times higher than that.[ In ]spiral galaxies
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''[scale height
In atmospheric, earth, and planetary sciences, a scale height, usually denoted by the capital letter ''H'', is a distance ( vertical or radial) over which a physical quantity decreases by a factor of e (the base of natural logarithms, approxima ...]
in our galaxy of about 180 light years, compared to a galactic radius of approximately 50,000 light years.[
In ]irregular galaxies
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...
, open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.[ Open clusters are not seen in ]elliptical galaxies
An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...
: star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed.[
In the Milky Way galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the ]Galactic Center
The Galactic Center or Galactic Centre is the rotational center, the barycenter, of the Milky Way galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact rad ...
, generally at substantial distances above or below the galactic plane
The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usual ...
.[ ]Tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s are stronger nearer the centre of the galaxy, increasing the rate of disruption of clusters, and also the giant molecular clouds which cause the disruption of clusters are concentrated towards the inner regions of the galaxy, so clusters in the inner regions of the galaxy tend to get dispersed at a younger age than their counterparts in the outer regions.[
]
Stellar composition
Because open clusters tend to be dispersed before most of their stars reach the end of their lives, the light from them tends to be dominated by the young, hot blue stars. These stars are the most massive, and have the shortest lives of a few tens of millions of years. The older open clusters tend to contain more yellow stars.
The frequency of stars within binary systems
Binary may refer to:
Science and technology Mathematics
* Binary number, a representation of numbers using only two digits (0 and 1)
* Binary function, a function that takes two arguments
* Binary operation, a mathematical operation that ...
has been observed to be higher within open clusters compared to outside of open clusters. This is seen as evidence that single stars get ejected from open clusters due to dynamical interactions.
Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These blue straggler
A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while perform ...
s are also observed in globular clusters, and in the very dense cores of globulars they are believed to arise when stars collide, forming a much hotter, more massive star. However, the stellar density in open clusters is much lower than that in globular clusters, and stellar collisions cannot explain the numbers of blue stragglers observed. Instead, it is thought that most of them probably originate when dynamical interactions with other stars cause a binary system to coalesce into one star.[
Once they have exhausted their supply of ]hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
through nuclear fusion
Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manifest ...
, medium- to low-mass stars shed their outer layers to form a planetary nebula
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelated to ...
and evolve into white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
s. While most clusters become dispersed before a large proportion of their members have reached the white dwarf stage, the number of white dwarfs in open clusters is still generally much lower than would be expected, given the age of the cluster and the expected initial mass distribution of the stars. One possible explanation for the lack of white dwarfs is that when a red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
expels its outer layers to become a planetary nebula, a slight asymmetry in the loss of material could give the star a 'kick' of a few kilometres per second
The metre per second is the unit of both speed (a scalar quantity) and velocity (a vector quantity, which has direction and magnitude) in the International System of Units (SI), equal to the speed of a body covering a distance of one metre in a ...
, enough to eject it from the cluster.[
Because of their high density, close encounters between stars in an open cluster are common. For a typical cluster with 1,000 stars with a 0.5 parsec half-mass radius, on average a star will have an encounter with another member every 10 million years. The rate is even higher in denser clusters. These encounters can have a significant impact on the extended circumstellar disks of material that surround many young stars. Tidal perturbations of large disks may result in the formation of massive planets and ]brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most ...
s, producing companions at distances of 100 AU or more from the host star.[
]
Eventual fate
Many open clusters are inherently unstable, with a small enough mass that the escape velocity
In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically ...
of the system is lower than the average velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...
of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.[
Clusters that have enough mass to be gravitationally bound once the surrounding nebula has evaporated can remain distinct for many tens of millions of years, but over time internal and external processes tend also to disperse them. Internally, close encounters between stars can increase the velocity of a member beyond the escape velocity of the cluster. This results in the gradual 'evaporation' of cluster members.][
Externally, about every half-billion years or so an open cluster tends to be disturbed by external factors such as passing close to or through a molecular cloud. The gravitational ]tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s generated by such an encounter tend to disrupt the cluster. Eventually, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in similar directions at similar speeds. The timescale over which a cluster disrupts depends on its initial stellar density, with more tightly packed clusters persisting for longer. Estimated cluster half lives
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
, after which half the original cluster members will have been lost, range from 150–800 million years, depending on the original density.[
After a cluster has become gravitationally unbound, many of its constituent stars will still be moving through space on similar trajectories, in what is known as a stellar association, moving cluster, or ]moving group
In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.
Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well a ...
. Several of the brightest stars in the 'Plough
A plough or plow ( US; both ) is a farm tool for loosening or turning the soil before sowing seed or planting. Ploughs were traditionally drawn by oxen and horses, but in modern farms are drawn by tractors. A plough may have a wooden, iron or ...
' of Ursa Major
Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...
are former members of an open cluster which now form such an association, in this case, the Ursa Major Moving Group.[ Eventually their slightly different relative velocities will see them scattered throughout the galaxy. A larger cluster is then known as a stream, if we discover the similar velocities and ages of otherwise well separated stars.][
]
Studying stellar evolution
When a Hertzsprung-Russell diagram is plotted for an open cluster, most stars lie on the main sequence
In astronomy, 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 Her ...
.[ The most massive stars have begun to evolve away from the main sequence and are becoming ]red giant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...
s; the position of the turn-off from the main sequence can be used to estimate the age of the cluster.
Because the stars in an open cluster are all at roughly the same distance from Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members are due only to their mass.[ This makes open clusters very useful in the study of stellar evolution, because when comparing one star to another, many of the variable parameters are fixed.][
The study of the abundances of ]lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...
and beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
in open cluster stars can give important clues about the evolution of stars and their interior structures. While hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
nuclei cannot fuse to form helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
until the temperature reaches about 10 million K, lithium and beryllium are destroyed at temperatures of 2.5 million K and 3.5 million K respectively. This means that their abundances depend strongly on how much mixing occurs in stellar interiors. By studying their abundances in open cluster stars, variables such as age and chemical composition are fixed.[
Studies have shown that the abundances of these light elements are much lower than models of stellar evolution predict. While the reason for this underabundance is not yet fully understood, one possibility is that ]convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
in stellar interiors can 'overshoot' into regions where radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
is normally the dominant mode of energy transport.[
]
Astronomical distance scale
Determining the distances to astronomical objects is crucial to understanding them, but the vast majority of objects are too far away for their distances to be directly determined. Calibration of the astronomical distance scale relies on a sequence of indirect and sometimes uncertain measurements relating the closest objects, for which distances can be directly measured, to increasingly distant objects.[ Open clusters are a crucial step in this sequence.
The closest open clusters can have their distance measured directly by one of two methods. First, the ]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. Due to foreshortening, nearby objects ...
(the small change in apparent position over the course of a year caused by the Earth moving from one side of its orbit around the Sun to the other) of stars in close open clusters can be measured, like other individual stars. Clusters such as the Pleiades, Hyades and a few others within about 500 light years are close enough for this method to be viable, and results from the Hipparcos
''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...
position-measuring satellite yielded accurate distances for several clusters.[
The other direct method is the so-called ]moving cluster method
In astrometry, the moving-cluster method and the closely related convergent point method are means, primarily of historical interest, for determining the distance to star clusters. They were used on several nearby clusters in the first half of th ...
. This relies on the fact that the stars of a cluster share a common motion through space. Measuring the proper motions of cluster members and plotting their apparent motions across the sky will reveal that they converge on a vanishing point
A vanishing point is a point on the image plane of a perspective drawing where the two-dimensional perspective projections of mutually parallel lines in three-dimensional space appear to converge. When the set of parallel lines is perpendicul ...
. The radial velocity of cluster members can be determined from Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
measurements of their spectra, and once the radial velocity, proper motion and angular distance from the cluster to its vanishing point are known, simple trigonometry
Trigonometry () is a branch of mathematics that studies relationships between side lengths and angles of triangles. The field emerged in the Hellenistic world during the 3rd century BC from applications of geometry to astronomical studies. T ...
will reveal the distance to the cluster. The Hyades Hyades may refer to:
* Hyades (band)
*Hyades (mythology)
*Hyades (star cluster)
The Hyades (; Greek Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest open cluster and one of the best-studied star clusters. Loca ...
are the best known application of this method, which reveals their distance to be 46.3 parsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
s.[
Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching the ]main sequence
In astronomy, 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 Her ...
on the Hertzsprung-Russell diagram for a cluster at a known distance with that of a more distant cluster, the distance to the more distant cluster can be estimated. The nearest open cluster is the Hyades Hyades may refer to:
* Hyades (band)
*Hyades (mythology)
*Hyades (star cluster)
The Hyades (; Greek Ὑάδες, also known as Caldwell 41, Collinder 50, or Melotte 25) is the nearest open cluster and one of the best-studied star clusters. Loca ...
: the stellar association consisting of most of the Plough stars is at about half the distance of the Hyades, but is a stellar association rather than an open cluster as the stars are not gravitationally bound to each other. The most distant known open cluster in our galaxy is Berkeley 29, at a distance of about 15,000 parsecs.[ Open clusters, especially super star clusters, are also easily detected in many of the galaxies of the ]Local Group
The Local Group is the galaxy group that includes the Milky Way.
It has a total diameter of roughly , and a total mass of the order of .
It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...
and nearby: e.g., NGC 346 and the SSCs R136
R136 (formerly known as RMC 136 from the Radcliffe Observatory Magellanic Clouds catalogue) is the central concentration of stars in the NGC 2070 star cluster, which lies at the centre of the Tarantula Nebula in the Large Magellanic Cloud. When ...
and NGC 1569 A and B.
Accurate knowledge of open cluster distances is vital for calibrating the period-luminosity relationship shown by variable star
A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...
s such as cepheid
A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude.
A strong direct relationship between a Cepheid varia ...
stars, which allows them to be used as standard candle
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
s. These luminous stars can be detected at great distances, and are then used to extend the distance scale to nearby galaxies in the Local Group.[ Indeed, the open cluster designated NGC 7790 hosts three ]classical Cepheids
Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cepheid variables) are a type of Cepheid variable star. They are population I variable stars that exhibit regular radial pulsations with periods of a few days to a ...
.[Sandage, Allan (1958)]
''Cepheids in Galactic Clusters. I. CF Cass in NGC 7790.''
AJ, 128[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''
A&A, 260 RR Lyrae variables
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 ...
are too old to be associated with open clusters, and are instead found in globular clusters
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 ...
.
Planets
The stars in open clusters can host exoplanets, just like stars outside of open clusters. For example, the open cluster NGC 6811 contains two known planetary systems, Kepler-66 and Kepler-67. Additionally, several hot Jupiter
Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s are known to exist in the Beehive Cluster
The Beehive Cluster (also known as Praesepe (Latin for "manger" or "crib"), M44, NGC 2632, or Cr 189), is an open cluster in the constellation Cancer. One of the nearest open clusters to Earth, it contains a larger population of stars than oth ...
.
See also
* List of open clusters
This is a list of open clusters located in the Milky Way. An open cluster is a gravitationally bound association of up to a few thousand stars that all formed from the same giant molecular cloud. There are over 1,000 known open clusters in the M ...
* Moving groups
In astronomy, stellar kinematics is the observational study or measurement of the kinematics or motions of stars through space.
Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way and its satellites as well as t ...
* Open cluster family
* Open cluster remnant
References
Further reading
*
*
External links
The Jewel Box (also known as NGC 4755 or Kappa Crucis Cluster) – open cluster in the Crux constellation @ SKY-MAP.ORG
Open Clusters – Information and amateur observations
{{DEFAULTSORT:Open Cluster
Open clusters
Star clusters