The Pleiades is one of the most famous open clusters.

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters. The galaxies of HCG 87, about four hundred million light-years distant. ... Image File history File linksMetadata Download high-resolution version (4877x3515, 2034 KB) The Pleiades. ... Image File history File linksMetadata Download high-resolution version (4877x3515, 2034 KB) The Pleiades. ... A shorter exposure shows less nebulosity. ... This article or section does not cite its references or sources. ... This article is about the astronomical object. ... A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... Gravity is a force of attraction that acts between bodies that have mass. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... It has been suggested that spiral nebula be merged into this article or section. ... This article or section does not adequately cite its references or sources. ... Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ...

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure 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 away. NGC 604, a giant H II region in the Triangulum Galaxy. ... Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. ...

Open clusters are very important objects in the study of stellar evolution. Because the stars are all of very similar age and chemical composition, the effects of other more subtle variables on the properties of stars are much more easily studied than they are for isolated stars. Projected timeline of the Suns life In astronomy, stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ...

Historical observations

The most prominent open clusters such as the Pleiades have been known and recognised as groups of stars since antiquity. Others were known as fuzzy patches of light, but had to wait until the invention of the telescope to be resolved into their constituent stars. Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found preferentially towards the centre of the Milky Way, and the other of which consisted of a generally sparser population of stars in a more irregular shape and found all over the sky. Astronomers dubbed the former globular clusters, and the latter open clusters. Open clusters are also occasionally referred to as galactic clusters, because they are almost exclusively found in the plane of the Milky Way, as discussed below. A shorter exposure shows less nebulosity. ... This article does not cite any references or sources. ... For other uses, see Milky Way (disambiguation). ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... The galactic plane is the plane in which the majority of a flattened galaxys mass lies. ...

It was realised early on that the stars in the open clusters were physically related. The Reverend John Michell calculated in 1767 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.^[1] As astrometry became more accurate, cluster stars were found to share a common proper motion through space, while spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars born at the same time and bound together as a group. John Michell (1724 – April 29, 1793) was an English natural philosopher and geologist, whose work was rediscovered in the 1970s. ... Illustration of the use of optical wavelength interferometry to determine precise positions of stars. ... The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real... High resolution spectrum of the Sun showing thousands of elemental absorption lines (fraunhofer lines). ... Radial velocity is the velocity of an object in the direction of the line of sight. ...

While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of difference in appearance between a very sparse globular cluster 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 which allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in our galaxy. This article or section does not cite its references or sources. ...

Formation

Infrared light reveals the dense open cluster forming at the heart of the Orion nebula.

All stars are originally formed in multiple systems, because only a cloud of gas containing many times the mass of the Sun will be heavy enough to collapse under its own gravity, but such a heavy cloud cannot collapse into a single star.^[2] Two views of the Trapezium cluster in the Orion Nebula, from the Hubble Space Telescope. ... Two views of the Trapezium cluster in the Orion Nebula, from the Hubble Space Telescope. ... For other uses, see Infrared (disambiguation). ... The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated south of Orions Belt. ... For the band of the same name, see: Binary Star (band) Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left). ... For other uses, see Mass (disambiguation). ... Sol redirects here. ... Gravity is a force of attraction that acts between bodies that have mass. ...

The formation of an open cluster begins with the collapse of part of a giant molecular cloud, a cold dense cloud of gas containing up to many thousands of times the mass of the Sun. Many factors may trigger the collapse of a giant molecular cloud (or part of it) and a burst of star formation which will result in an open cluster, including shock waves from a nearby supernova and gravitational interactions. Once a giant molecular cloud begins to collapse, star formation proceeds via successive fragmentations of the cloud into smaller and smaller clumps, resulting eventually in the formation of up to several thousand stars. In our own galaxy, the formation rate of open clusters is estimated to be one every few thousand years.^[3] A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated. ... In astronomy, the solar mass is a unit of mass used to express the mass of stars and larger objects such as galaxies. ... For other uses, see Supernova (disambiguation). ...

Once star formation has begun, the hottest and most massive stars (known as OB stars) will emit copious amounts of ultraviolet radiation. This radiation rapidly ionizes the surrounding gas of the giant molecular cloud, forming an H II region. Stellar winds from the massive stars and radiation pressure begin to drive away the gases; after a few million years the cluster will experience its first supernovae, which will also expel gas from the system. After a few tens of millions of years, the cluster will be stripped of gas and no further star formation will take place. Typically, less than 10% of the gas originally in the cluster will form into stars before it is dissipated.^[4] OB stars are hot, massive stars stars which form in loosely organized groups called OB associations. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... NGC 604, a giant H II region in the Triangulum Galaxy. ... A solar wind is a stream of particles (mostly high-energy protons ~ 500 keV) which are ejected from the upper atmosphere of a star (in the case of a star other than the Earths Sun, it may be called a stellar wind instead). ... Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. ... For other uses, see Supernova (disambiguation). ...

It is common for two or more separate open clusters to form out of the same molecular cloud. In the Large Magellanic Cloud, both Hodge 301 and R136 are forming from the gases of the Tarantula Nebula, while in our own galaxy, tracing back the motion through space of the Hyades and Praesepe, two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.^[5] The Large Magellanic Cloud (LMC) is a nearby satellite galaxy of our own galaxy, the Milky Way. ... Hodge 301 (lower right) in the Tarantula Nebula Hodge 301 is a star cluster in the Tarantula Nebula, visible from Earths Southern Hemisphere. ... The Tarantula Nebula. ... The Hyades (ÆΥάδες also known as Melotte 25 or Collinder 50 or Caldwell 41) is an open star cluster located in the constellation Taurus. ... The Beehive Cluster (also known as The Beehive, Praesepe, Open Cluster M44, Messier Object 44, Messier 44, M44, or NGC 2632) is an open cluster in the Cancer constellation. ...

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 of h Persei and χ Persei, but at least 10 more double clusters are known to exist.^[6] Many more are known in the Small and Large Magellanic Clouds — 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. The Double Cluster is the common name for NGC 884 and NGC 869, which are close together in the constellation Perseus. ... The Small Magellanic Cloud (SMC) is a dwarf galaxy[1] in orbit around the Milky Way Galaxy. ... The Large Magellanic Cloud (LMC) is a nearby satellite galaxy of our own galaxy, the Milky Way. ...

Morphology and classification

NGC 2158 is a rich and concentrated cluster in Gemini.

Open clusters range from very sparse clusters with only a few members to large agglomerations 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 star per cubic light year).^[7] Download high resolution version (891x893, 492 KB)Star cluster NGC 2158. ... Download high resolution version (891x893, 492 KB)Star cluster NGC 2158. ... NGC 2158 is an open cluster in the constellation of Gemini. ... Gemini (IPA: , Latin: , symbol , ) is one of the constellations of the zodiac known as the twins. It is part of the winter sky, lying between Taurus to the west and the dim Cancer to the east, with Auriga and the near-invisible Lynx to the north and Monoceros and Canis... In the study of human settlements, an agglomeration is an extended city or town area comprising the built-up area of a central place (usually a municipality) and any suburbs or adjacent satellite towns. ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ...

Open clusters are often classified according to a scheme developed by Robert Trumpler in 1930. The Trumpler scheme gives a cluster a three part designation, with a Roman numeral from I-IV indicating its concentration and detachment from the surrounding star field (from strongly to weakly concentrated), an Arabic numeral from 1 to 3 indicating 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 appended if the cluster lies within nebulosity.^[8] Robert Julius Trumpler (October 2, 1886 – September 10, 1956) was a Swiss-American astronomer. ... The system of Roman numerals is a numeral system originating in ancient Rome, and was adapted from Etruscan numerals. ... Arabic numerals (also called Hindu numerals or Hindu-Arabic numerals) are by far the most common form of symbolism used to represent numbers. ... The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ...

Under the Trumpler scheme, the Pleiades are classified as I3rn (strongly concentrated and richly populated with nebulosity present), while the nearby Hyades are classified as II3m (more dispersed, and with fewer members).

Numbers and distribution

NGC 346, an open cluster in the Small Magellanic Cloud.

There are over 1,000 known open clusters in our galaxy, but the true total may be up to ten times higher than that.^[9] In spiral galaxies, open clusters are invariably found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are strongly concentrated close to the galactic plane, with a scale height in our galaxy of about 180 light years, compared to a galactic radius of approximately 100,000 light years.^[10] Download high resolution version (749x800, 116 KB)Hubble Space Telescope image of a cluster of stars forming in the Small Magellanic Cloud. ... Download high resolution version (749x800, 116 KB)Hubble Space Telescope image of a cluster of stars forming in the Small Magellanic Cloud. ... The Small Magellanic Cloud (SMC) is a dwarf galaxy[1] in orbit around the Milky Way Galaxy. ... It has been suggested that spiral nebula be merged into this article or section. ... A scale height is a term often used in scientific contexts for a distance over which a quantity decreases by a factor of e. ...

In irregular galaxies, 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: star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed. This article or section does not adequately cite its references or sources. ... The giant elliptical galaxy ESO 325-G004. ...

In our galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the galactic centre. Tidal forces 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.^[11] The Galactic Center is the rotational center of the Milky Way galaxy. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

Stellar composition

A cluster of stars a few million years old at the lower right illuminates the Tarantula Nebula in the Large Magellanic Cloud.

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. Download high resolution version (750x756, 143 KB)Part of the Tarantula Nebula, a giant HII region in the LMC. The star cluster at the lower right is Hodge 301. ... Download high resolution version (750x756, 143 KB)Part of the Tarantula Nebula, a giant HII region in the LMC. The star cluster at the lower right is Hodge 301. ... The Tarantula Nebula. ... The Large Magellanic Cloud (LMC) is a nearby satellite galaxy of our own galaxy, the Milky Way. ...

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These blue stragglers 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.^[12] Blue stragglers are stars in open or globular clusters that are hotter and bluer than other cluster stars having the same luminosity. ...

Once they have exhausted their supply of hydrogen through nuclear fusion, medium to low mass stars shed their outer layers to form a planetary nebula and evolve into white dwarfs. 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 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, enough to eject it from the cluster.^[13] This article is about the chemistry of hydrogen. ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ... NGC 6543, The Cats Eye Nebula NGC 6853, The Dumbbell Nebula A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. ... This article or section does not adequately cite its references or sources. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ... kilometre per second is an SI derived unit of both speed (scalar) and velocity (vector), signified by the symbol km/s or km s-1. ...

Eventual fate

NGC 604 in the Triangulum Galaxy is a very massive open cluster surrounded by an H II region.

Many open clusters are inherently unstable, with a small enough mass that the escape velocity of the system is lower than the average velocity 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. Download high resolution version (1127x1201, 2479 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (1127x1201, 2479 KB) Wikipedia does not have an article with this exact name. ... Categories: Astronomy stubs | Nebulae | NGC objects ... The Triangulum Galaxy (also known as Messier 33 or NGC 598) is a spiral galaxy about 3. ... NGC 604, a giant H II region in the Triangulum Galaxy. ... Space Shuttle Atlantis launches on mission STS-71. ... This article is about velocity in physics. ...

Clusters which 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 members of the cluster will often result in the velocity of one being increased to beyond the escape velocity of the cluster, which 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 forces 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, after which half the original cluster members will have been lost, range from 150–800 million years, depending on the original density.^[14] Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ... This article is about the computer game. ...

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. Several of the brightest stars in the 'Plough' of Ursa Major 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 unrelated stars. A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. ... This article is about the constellation. ... The Ursa Major Moving Group is the closest moving group to Earth, with its core being located roughly 80 light years away. ...

Studying stellar evolution

Hertzsprung-Russell diagrams for two open clusters. NGC 188 is older, and shows a lower turn off from the main sequence than that seen in M67.

When a Hertzsprung-Russell diagram is plotted for an open cluster, most stars lie on the main sequence. The most massive stars have begun to evolve away from the main sequence and are becoming red giants; the position of the turn-off from the main sequence can be used to estimate the age of the cluster. HR diagrams for two open clusters, showing the main sequence turn-off at different ages. ... HR diagrams for two open clusters, showing the main sequence turn-off at different ages. ... The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. ... NGC 188 is an open cluster in the Cepheus constellation. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... Open Cluster M67 (also known as Messier Object 67, Messier 67, M67, or NGC 2682) is an open cluster in the Cancer constellation. ... The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude diagram, or CMD) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. ...

Because the stars in an open cluster are all at roughly the same distance from Earth, and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members is 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. This article is about Earth as a planet. ...

The study of the abundances of lithium and beryllium in open cluster stars can give important clues about the evolution of stars and their interior structures. While hydrogen nuclei cannot fuse to form helium 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. This article is about the chemical element named Lithium. ... General Name, symbol, number beryllium, Be, 4 Chemical series alkaline earth metals Group, period, block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... This article is about the chemistry of hydrogen. ... For other uses, see Helium (disambiguation). ... For other uses, see Kelvin (disambiguation). ...

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 in stellar interiors can 'overshoot' into regions where radiation is normally the dominant mode of energy transport.^[15] Convection in the most general terms refers to the movement of currents within fluids (i. ... Radiant energy is the energy of electromagnetic waves. ...

Open clusters and the astronomical distance scale

M11, the Wild Duck Cluster is a very rich cluster located towards the center of the Milky Way.

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 Wild Duck Cluster M11. ... The Wild Duck Cluster M11. ... The Wild Duck Cluster (also known as Open Cluster M11, Messier Object 11, Messier 11, M11, or NGC 6705) is an open cluster in the Scutum constellation. ... For other uses, see Milky Way (disambiguation). ...

The closest open clusters can have their distance measured directly by one of two methods. First, the parallax (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 position-measuring satellite yielded accurate distances for several clusters.^[16] For other uses, see Parallax (disambiguation). ... Hipparcos (for High Precision Parallax Collecting Satellite) was an astrometry mission of the European Space Agency (ESA) dedicated to the measurement of stellar parallax and the proper motions of stars. ...

The other direct method is the so-called moving cluster method. 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. The radial velocity of cluster members can be determined from Doppler shift 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 will reveal the distance to the cluster. The Hyades are the best known application of this method, which reveals their distance to be 46.3 parsecs.^[17] 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. ... For other uses, see Vanishing point (disambiguation). ... The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... Legend γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultraviolet Visible light NIR = Near infrared MIR = Moderate infrared FIR = Far infrared Radio waves EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultra high frequency VHF = Very high frequency HF = High... Wikibooks has a book on the topic of Trigonometry All of the trigonometric functions of an angle θ can be constructed geometrically in terms of a unit circle centered at O. Trigonometry (from Greek trigōnon triangle + metron measure[1]), informally called trig, is a branch of mathematics that deals with... The Hyades (ÆΥάδες also known as Melotte 25 or Collinder 50 or Caldwell 41) is an open star cluster located in the constellation Taurus. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ...

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 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: 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.^[18] Open clusters are also easily detected in many of the galaxies of the Local Group. Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... The Hyades (ÆΥάδες also known as Melotte 25 or Collinder 50 or Caldwell 41) is an open star cluster located in the constellation Taurus. ... The Ursa Major Moving Group is the closest moving group to Earth, with its core being located roughly 80 light years away. ... The Hyades (ÆΥάδες also known as Melotte 25 or Collinder 50 or Caldwell 41) is an open star cluster located in the constellation Taurus. ... A member of the Local Group of galaxies, irregular galaxy Sextans A is 4. ...

Accurate knowledge of open cluster distances is vital for calibrating the period-luminosity relationship shown by variable stars such as cepheid and RR Lyrae stars, which allows them to be used as standard candles. 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. This article or section contains a plot summary that is overly long or excessively detailed. ... A Cepheid variable is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute stellar luminosity. ... RR Lyrae is a variable star in the Lyra constellation. ... A standard candle is an astronomical object that has a known luminosity. ...

References

1. ^ Michell J. (1767), An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation, Philosophical Transactions, v. 57, p. 234–264
2. ^ Boss A.P. (1998), The Jeans Mass Constraint and the Fragmentation of Molecular Cloud Cores, Astrophysical Journal Letters v.501, p.L77
3. ^ Battinelli P., Capuzzo-Dolcetta R. (1991), Formation and evolutionary properties of the Galactic open cluster system, Monthly Notices of the Royal Astronomical Society, v. 249, p. 76–83
4. ^ also see Battinelli P as previous reference.
5. ^ Eggen O. J. (1960), Stellar groups, VII. The structure of the Hyades group, Monthly Notices of the Royal Astronomical Society, v. 120, p.540
6. ^ Subramaniam A., Gorti U., Sagar R., Bhatt H. C. (1995), Probable binary open star clusters in the Galaxy, Astronomy and Astrophysics, v.302, p.86
7. ^ Nilakshi S.R., Pandey A.K., Mohan V. (2002), A study of spatial structure of galactic open star clusters, Astronomy and Astrophysics, v. 383, p. 153–162
8. ^ Trumpler R.J. (1930), Preliminary results on the distances, dimensions and space distribution of open star clusters, Lick Observatory bulletin no. 420, Berkeley : University of California Press, p. 154–188
9. ^ Dias W.S., Alessi B.S., Moitinho A., Lépine J.R.D. (2002), New catalogue of optically visible open clusters and candidates, Astronomy and Astrophysics, v. 389, p. 871–873
10. ^ Janes K.A., Phelps R.L. (1980), The galactic system of old star clusters: The development of the galactic disk, The Astronomical Journal, v. 108, p. 1773–1785
11. ^ van den Bergh S., McClure R.D. (1980), Galactic distribution of the oldest open clusters, Astronomy & Astrophysics, v.88, p.360
12. ^ Andronov N., Pinsonneault M., Terndrup D. (2003), Formation of Blue Stragglers in Open Clusters, American Astronomical Society Meeting 203
13. ^ Fellhauer M., Lin D.N.C., Bolte M., Aarseth S.J., Williams K.A. (2003), The White Dwarf Deficit in Open Clusters: Dynamical Processes, The Astrophysical Journal, v. 595, pp. L53-L56
14. ^ de La Fuente M.R. (1998), Dynamical Evolution of Open Star Clusters, Publications of the Astronomical Society of the Pacific, v. 110, pp. 1117–1117
15. ^ VandenBerg, D.A., Stetson P.B. (2004), On the Old Open Clusters M67 and NGC 188: Convective Core Overshooting, Color-Temperature Relations, Distances, and Ages, Publications of the Astronomical Society of the Pacific, v. 116, pp. 997–1011
16. ^ Brown A.G.A. (2001), Open clusters and OB associations: a review, Revista Mexicana de Astronomía y Astrofísica, v. 11, p89–96
17. ^ Hanson R.B. (1975), A study of the motion, membership, and distance of the Hyades cluster, Astronomical Journal, v. 80, p. 379–401
18. ^ Bragaglia A., Held E.V., Tosi M. (2005), Radial velocities and membership of stars in the old, distant open cluster Berkeley 29, Astronomy and Astrophysics, v. 429, p. 881–886

Further reading

• W.J. Kaufmann (1994). Universe. W H Freeman. ISBN 0-7167-2379-4. 
• E.V.P. Smith, K.C. Jacobs, M. Zeilik, S.A. Gregory (1997). Introductory Astronomy and Astrophysics. Thomson Learning. ISBN 0-03-006228-4. 

External links

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• The Jewel Box (also known as NGC 4755 or Kappa Crucis Cluster) - open cluster in the Crux constellation @ SKY-MAP.ORG
• Open Star Clusters @ SEDS Messier pages
• A general overview of open clusters
• Open and globular clusters overview
• The moving cluster method
• Open Clusters - Information and amateur observations
• Clickable table of Messier objects including open clusters

Image File history File links En-Open_cluster-article. ... Image File history File links Sound-icon. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 197th day of the year (198th in leap years) in the Gregorian calendar. ... Image File history File links Sound-icon. ...