Star clusters are very large groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain 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.
Globular clusters 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 masses less than two solar masses. 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 our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the Galactic Centre, orbiting the centre in highly elliptical orbits. In 1917, the astronomer Harlow Shapley made the first reliable estimate the Sun's distance from the galactic centre 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, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79. Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand.
A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age. The brightest globular cluster in the northern hemisphere is Messier 13 in the constellation of Hercules.
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. They form from H II regions such as the Orion Nebula.
Open clusters usually contain up to a few hundred members, within a region up to about 30 light-years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as 'evaporation'.
The most prominent open clusters are the Pleiades and Hyades in Taurus. The Double 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.
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.
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.
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, were originally born into embedded clusters that disintegrated.
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 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.
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.
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 centers of their host galaxies. As the latter they seem to be old objects.
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. 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.
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 center 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 center. The first of such designations were assigned by Gosta Lynga in 1982.
The Hyades (; Greek Ὑάδες, also known as Melotte 25 or Collinder 50) is the nearest open cluster and one of the best-studied star clusters. Located about 153 light-years (47 parsecs) away from the Sun, it consists of a roughly spherical group of hundreds of stars sharing the same age, place of origin, chemical characteristics, and motion through space. From the perspective of observers on Earth, the Hyades Cluster appears in the constellation Taurus, where its brightest stars form a "V" shape along with the still-brighter Aldebaran. However, Aldebaran is unrelated to the Hyades, as it is located much closer to Earth and merely happens to lie along the same line of sight.
The five brightest member stars of the Hyades have consumed the hydrogen fuel at their cores and are now evolving into giant stars. Four of these stars, with Bayer designations Gamma, Delta 1, Epsilon, and Theta Tauri, form an asterism that is traditionally identified as the head of Taurus the Bull. The fifth of these stars is Theta1 Tauri, a tight naked-eye companion to the brighter Theta2 Tauri. Epsilon Tauri, known as Ain (the "Bull's Eye"), has a gas giant exoplanet candidate, the first planet to be found in any open cluster.
The age of the Hyades is estimated to be about 625 million years. The core of the cluster, where stars are the most densely packed, has a radius of 8.8 light-years (2.7 parsecs), and the cluster's tidal radius – where the stars become more strongly influenced by the gravity of the surrounding Milky Way galaxy – is 33 light-years (10 parsecs). However, about one-third of confirmed member stars have been observed well outside the latter boundary, in the cluster's extended halo; these stars are probably in the process of escaping from its gravitational influence.Jewel Box (star cluster)
The Jewel Box (Kappa Crucis Cluster or NGC 4755) is an open cluster in the constellation Crux, originally discovered by Nicolas Louis de Lacaille in 1751–1752. This cluster was later named the Jewel Box by John Herschel when he described its telescopic appearance as "...a superb piece of fancy jewellery". It is easily visible to the naked eye as a hazy star some 1.0° southeast of the first-magnitude star Mimosa (Beta Crucis). This hazy star was given the Bayer star designation "Kappa Crucis", from which the cluster takes one of its common names. The modern designation Kappa Crucis has been assigned to one of the stars in the base of the A-shaped asterism of the cluster
This cluster is one of the youngest known, with an estimated age of 14 million years. It has a total integrated magnitude of 4.2, is located 1.95 kpc. or 6,440 light years from Earth, and contains just over 100 stars.List of stars in Taurus
This is the list of notable stars in the constellation Taurus, sorted by decreasing brightness.NGC 110
NGC 110 is an open star cluster located in the constellation Cassiopeia. It was discovered by the English astronomer John Herschel on October 29, 1831.It is unknown if the members are physically related, or if the cluster exists at all. It is barely visible against the background sky, and the two dozen member stars seem to be at various distances. If the cluster does exist, it is at least 2,000 light years away.NGC 136
NGC 136 is an open cluster in the constellation Cassiopeia. It was discovered by William Herschel on November 26, 1788.NGC 1893
NGC 1893 is an open cluster in the constellation Auriga. It is about 12,400 light years away. The star cluster is embedded in the HII region IC 410.Images of the star cluster by the Chandra X-ray Observatory suggest that it contains approximately 4600 young stellar objects.NGC 1907
NGC 1907 is an open star cluster around 4,500 light years from Earth. It contains around 30 stars and is over 500 million years old. With a magnitude of 8.2 it is visible as part of the constellation Auriga.NGC 206
NGC 206 is a bright star cloud in the Andromeda Galaxy. It is notable for being the brightest star cloud in Andromeda as viewed from Earth.NGC 265
NGC 265 is an open cluster in the Small Magellanic Cloud, which is located in the constellation Tucana.NGC 272
NGC 272 is an open cluster (rather an 'L' shaped asterism) located in the constellation Andromeda. It was discovered on August 2, 1864 by Heinrich d'Arrest.NGC 306
NGC 306 is an open cluster in the Small Magellanic Cloud. It is located in the constellation Tucana. It was discovered on October 4, 1836 by John Herschel.NGC 330
NGC 330 is a star cluster in the Small Magellanic Cloud. It is located in the constellation Tucana. It was discovered on August 1, 1826 by James Dunlop. It was described by Dreyer as "a globular cluster, very bright, small, a little extended, stars from 13th to 15th magnitude."NGC 433
NGC 433 is an open cluster located in the constellation Cassiopeia. It was discovered on September 29, 1829 by John Herschel. It was described by Dreyer as "cluster, small, a little compressed."NGC 436
NGC 436 is an open cluster located in the constellation Cassiopeia. It was discovered on November 3, 1787 by William Herschel. It was described by Dreyer as a "cluster, small, irregular figure, pretty compressed."NGC 465
NGC 465 is an open cluster in the Magellanic Clouds. Being part of the Tucana constellation, it was discovered by Scottish astronomer James Dunlop in 1826.NGC 6866
NGC 6866 is an open cluster in the constellation Cygnus. It was discovered by Caroline Herschel on 23 July 1783.NGC 7686
NGC 7686 is a moderate-sized open cluster in the constellation Andromeda, containing about 80 stars. At magnitude 5.6, it is an easy target for binoculars and small telescopes.According to Johnson et al. (1961), the "color-magnitude diagram shows merely a uniform scatter with no significant tendency to show a cluster main sequence". They conclude that this is not actually a star cluster.Pleiades
The Pleiades (), also known as the Seven Sisters and Messier 45, are 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.
The cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from the formation of the cluster, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing.Computer simulations have shown that the Pleiades were probably formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.Super star cluster
A super star cluster (SSC) is a very massive young open cluster that is thought to be the precursor of a globular cluster. These clusters are referred to as "super" due to the fact that they are relatively more luminous and contain more mass than other young star clusters. The SSC, however, does not have to physically be larger than other clusters of lower mass and luminosity. They typically contain a very large number of young, massive stars that ionize a surrounding HII region or a so-called "Ultra dense HII regions (UDHIIs)" in the Milky Way Galaxy as well as in other galaxies (however, SSCs do not always have to be inside an HII region). An SSC's HII region is in turn surrounded by a cocoon of dust. In many cases, the stars and the HII regions will be invisible to observations in certain wavelengths of light, such as the visible spectrum, due to high levels of extinction. As a result, the youngest SSCs are best observed and photographed in radio and infrared. SSCs, such as Westerlund 1 (Wd1), have been found in the Milky Way Galaxy. However, most have been observed in farther regions of the universe. In the galaxy M82 alone, 197 young SSCs have been observed and identified using the Hubble Space Telescope.
Generally, SSCs have been seen to form in the interactions between galaxies and in regions of high amounts of star formation with high enough pressures to satisfy the properties needed for the formation of a star cluster. These regions can include newer galaxies with a lot of new star formation, dwarf starburst galaxies, arms of a spiral galaxy that have a high star formation rate, and in the merging of galaxies. In an Astronomical Journal published in 1996, using pictures taken in the ultraviolet (UV) spectrum by the Hubble Space Telescope of star-forming rings in five different barred galaxies, numerous star clusters were found in clumps within the rings which had high rates of star formation. It was found that these clusters had masses of about M☉ to M☉, ages of about 100 Myr, and radii of about 5 pc, and are thought to evolve into globular clusters later in their lifetimes. These properties match those found in SSCs.