Open cluster

NGC 265, an open star cluster in the Small Magellanic Cloud

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist.[1] They are loosely bound to each other by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, resulting in a migration to the main body of the galaxy as well as a loss of cluster members through internal close encounters.[2] Open clusters generally survive for a few hundred million years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for many billions of years. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.[3]

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region.[4] 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 of the gas away.

Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, the effects of other stellar properties are more easily determined than they are for isolated stars.[1] A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye. Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes. The Wild Duck Cluster, M11, is an example.[5]


Historical observations

Mosaic of 30 open clusters discovered from VISTA's data. The open clusters were hidden by the dust in the Milky Way.[6] Credit ESO.

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.[7] Indeed, in 1603 Johann Bayer gave three of these clusters designations as if they were single stars.[8] In his 1610 treatise Sidereus Nuncius, based upon his observations with a telescope, Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters."[9] Possibly the first astronomer to use a telescope to find previously undiscovered open clusters was Giovanni Hodierna, who in 1654 identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451.[10] Between 1774–1781, French astronomer Charles Messier published a catalogue of celestial objects that had a nebulous appearance similar to comets. This catalogue included 26 open clusters.[8]

It was realised as early as 1767 that the stars in an open clusters were physically related,[11] when the Reverend John Michell 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.[12] In the 1790s, English astronomer William Herschel began a study of nebulous celestial objects. He discovered that many of these features could be resolved into groupings of individual stars. He conceived the idea that stars were initially scattered across space, but later became clustered together as star systems because of gravitational attraction.[13] Herschel divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars.[14]

Hundreds of open clusters were listed in the New General Catalogue, first published in 1888 by J. L. E. Dreyer, and the two supplemental Index Catalogues, published in 1896 and 1905.[8] 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.[15] The other consisted of a generally sparser population of stars in a more irregular shape that was generally found in or near the galactic plane of the Milky Way.[16][17] Astronomers dubbed the former globular clusters, 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 Robert Julius Trumpler.[18]

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.[19] However, in 1918 the Dutch-American astronomer Adriaan van Maanen was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times.[20] As astrometry became more accurate, cluster stars were found to share a common proper motion 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 stars that had a proper motion similar to the mean motion of the cluster, and were therefore likely members.[21] Spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars bound together as a group.[1]

The first color-magnitude diagrams of open clusters were published by Ejnar Hertzsprung in 1911, giving the plot for the Pleiades and Hyades star clusters. 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 clusters had different stellar populations from the Pleiades. This would subsequently be interpreted as a difference in ages of the three clusters.[22]


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

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.[23]

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.[24]

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.[24]

Another view of cluster formation is that they form rapidly out of a contracting molecular cloud core and once the massive stars begin to shine they expel the residual gas with the sound speed of the hot ionised gas. The time from the start of cloud-core contraction to gas expulsion is typically not more than one to three million years. 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 cluster which loses many and perhaps all of its stars.[25] All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. The young stars so released from their natal cluster become part of the Galactic field population. Because most if not all stars form clustered, star clusters are to be viewed 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.[26]

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.[27]

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 NGC 869 and NGC 884 (sometimes mistakenly called h and χ Persei; h refers to a neighboring star and χ to both clusters), but at least 10 more double clusters are known to exist.[28] 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.

Morphology and classification

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 stars per cubic light year.[29]

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.[30]

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).[citation needed]

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.[31] In spiral galaxies, open clusters are largely 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.[32]

In irregular galaxies, open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.[33] 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.[34]

In our galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the galactic centre, generally at substantial distances above or below the galactic plane.[35] 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.[36]

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.[citation needed]

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.[37]

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.[38]

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.[39]

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 stars can increase the velocity of a member beyond the escape velocity of the cluster. This results in the gradual 'evaporation' of cluster members.[40]

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.[40]

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.[41] 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.[42][43]

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.[44] 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.[citation needed]

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.[44] 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.[citation needed]

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.[45]

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.[45]

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.[46] Open clusters are a crucial step in this sequence.[citation needed]

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.[47][48]

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.[49]

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.[50] Open clusters are also easily detected in many of the galaxies of the Local Group.[citation needed]

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.[51]

See also


  1. ^ a b c Frommert, Hartmut; Kronberg, Christine (August 27, 2007). "Open Star Clusters". SEDS. University of Arizona, Lunar and Planetary Lab. Retrieved 2009-01-02. 
  2. ^ Karttunen, Hannu; et al. (2003). Fundamental astronomy. Physics and Astronomy Online Library (4 ed.). Springer. p. 321. ISBN 3540001794. 
  3. ^ Payne-Gaposchkin, C. (1979). Stars and clusters. Cambridge, Mass.: Harvard University Press. Bibcode ISBN 0674834402. 
  4. ^ A good example of this is NGC 2244, in the Rosette Nebula. See also Johnson, Harold L. (November 1962). "The Galactic Cluster, NGC 2244". Astrophysical Journal 136: 1135. Bibcode 1962ApJ...136.1135J. doi:10.1086/147466. 
  5. ^ Neata, Emil. "Open Star Clusters: Information and Observations". Night Sky Info. Retrieved 2009-01-02. 
  6. ^ "VISTA Finds 96 Star Clusters Hidden Behind Dust". ESO Science Release. Retrieved 3 August 2011. 
  7. ^ Jones, Kenneth Glyn (1991). Messier's nebulae and star clusters. Practical astronomy handbook (2nd ed.). Cambridge University Press. pp. 6–7. ISBN 0521370795. 
  8. ^ a b c Kaler, James B. (2006). Cambridge Encyclopedia of Stars. Cambridge University Press. p. 167. ISBN 0-521-81803-6. 
  9. ^ D'Onofrio, Mauro; Burigana, Carlo. Mauro D'Onofrio, Carlo Burigana. ed. Questions of Modern Cosmology: Galileo's Legacy. Springer, 2009. p. 1. ISBN 3642007910. Introduction. 
  10. ^ Jones, K. G. (August 1986). "Some Notes on Hodierna's Nebulae". Journal of the History of Astronomy 17 (50): 187–188. Bibcode 1986JHA....17..187J. 
  11. ^ Chapman, A. (December 1989), "William Herschel and the Measurement of Space", Royal Astronomical Society Quarterly Journal 30 (4): 399–418, Bibcode 1989QJRAS..30..399C 
  12. ^ 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 57 (0): 234–264. Bibcode 1767RSPT...57..234M. doi:10.1098/rstl.1767.0028. 
  13. ^ Hoskin, M. (1979). "Herschel, William's Early Investigations of Nebulae - a Reassessment". Journal for the History of Astronomy 10: 165–176. Bibcode 1979JHA....10..165H. 
  14. ^ Hoskin, M. (February 1987). "Herschel's Cosmology". Journal of the History of Astronomy 18 (1): 1–34. Bibcode 1987JHA....18....1H.  See page 20.
  15. ^ Bok, Bart J.; Bok, Priscilla F. (1981). The Milky Way. Harvard books on astronomy (5th ed.). Harvard University Press. p. 136. ISBN 0674575032. 
  16. ^ Binney, James; Merrifield, Michael (1998), Galactic astronomy, Princeton series in astrophysics, Princeton University Press, p. 377, ISBN 0691025657, 
  17. ^ Basu, Baidyanath (2003). An Introduction to Astrophysics. PHI Learning Pvt. Ltd.. p. 218. ISBN 8120311213. 
  18. ^ Trumpler, R. J. (December 1925). "Spectral Types in Open Clusters". Publications of the Astronomical Society of the Pacific 37 (220): 307. Bibcode 1925PASP...37..307T. doi:10.1086/123509. 
  19. ^ Barnard, E. E., "Micrometric measures of star clusters", Publications of the Yerkes Observatory 6: 1–106, Bibcode 1931PYerO...6....1B 
  20. ^ van Maanen, Adriaan (1919), "No. 167. Investigations on proper motion. Furst paper: The motions of 85 stars in the neighborhood of Atlas and Pleione", Contributions from the Mount Wilson Observatory (Carnegie Institution of Washington) 167: 1–15, Bibcode 1919CMWCI.167....1V 
  21. ^ van Maanen, Adriaan (July 1945), "Investigations on Proper Motion. XXIV. Further Measures in the Pleiades Cluster", Astrophysical Journal 102: 26–31, Bibcode 1945ApJ...102...26V, doi:10.1086/144736 
  22. ^ Strand, K. Aa. (December 1977), "Hertzsprung's Contributions to the HR Diagram", in Philip, A. G. Davis; DeVorkin, David H., The HR Diagram, In Memory of Henry Norris Russell, IAU Symposium No. 80, held November 2, 1977, National Academy of Sciences, Washington, DC, pp. 55–59, Bibcode 1977IAUS...80S..55S 
  23. ^ Elmegreen, Bruce G.; Efremov, Yuri N. (1997). "A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas". The Astrophysical Journal 480 (1): 235–245. Bibcode 1997ApJ...480..235E. doi:10.1086/303966. 
  24. ^ a b Battinelli, P.; Capuzzo-Dolcetta, R. (1991). "Formation and evolutionary properties of the Galactic open cluster system". Monthly Notices of the Royal Astronomical Society 249: 76–83. Bibcode 1991MNRAS.249...76B. 
  25. ^ Kroupa, P.; Aarseth, S. J.; Hurley, J. (2001). "The formation of a bound star cluster: from the Orion nebula cluster to the Pleiades". Monthly Notices of the Royal Astronomical Society 321 (4): 699–712. arXiv:astro-ph/0009470. Bibcode 2001MNRAS.321..699K. doi:10.1046/j.1365-8711.2001.04050.x. 
  26. ^ Kroupa, P. (October 4–7, 2004). "The Fundamental Building Blocks of Galaxies". In C. Turon, K.S. O'Flaherty, M.A.C. Perryman. Proceedings of the Gaia Symposium "The Three-Dimensional Universe with Gaia (ESA SP-576). Observatoire de Paris-Meudon. p. 629. arXiv:astro-ph/0412069. 
  27. ^ Eggen, O. J. (1960). "Stellar groups, VII. The structure of the Hyades group". Monthly Notices of the Royal Astronomical Society 120: 540–562. Bibcode 1960MNRAS.120..540E. 
  28. ^ Subramaniam, A.; Gorti, U.; Sagar, R.; Bhatt, H. C. (1995). "Probable binary open star clusters in the Galaxy". Astronomy and Astrophysics 302: 86–89. Bibcode 1995A&A...302...86S. 
  29. ^ Nilakshi, S.R.; Pandey, A.K.; Mohan, V. (2002). "A study of spatial structure of galactic open star clusters". Astronomy and Astrophysics 383 (1): 153–162. Bibcode 2002A&A...383..153N. doi:10.1051/0004-6361:20011719. 
  30. ^ Trumpler, R.J. (1930). "Preliminary results on the distances, dimensions and space distribution of open star clusters". Lick Observatory bulletin (Berkeley: University of California Press) 14 (420): 154–188. Bibcode 1930LicOB..14..154T. 
  31. ^ 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 389 (3): 871–873. arXiv:astro-ph/0203351. Bibcode 2002A&A...389..871D. doi:10.1051/0004-6361:20020668. 
  32. ^ Janes, K.A.; Phelps, R.L. (1980). "The galactic system of old star clusters: The development of the galactic disk". The Astronomical Journal 108: 1773–1785. Bibcode 1994AJ....108.1773J. doi:10.1086/117192. 
  33. ^ Hunter, D. (1997). "Star Formation in Irregular Galaxies: A Review of Several Key Questions". Publications of the Astronomical Society of the Pacific 109: 937–950. Bibcode 1997PASP..109..937H. doi:10.1086/133965. 
  34. ^ Binney; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 9780691025650. OCLC 39108765. 
  35. ^ Friel, Eileen D. (1995). "The Old Open Clusters Of The Milky Way". Annual Reviews of Astronomy & Astrophysics 33: 381–414. Bibcode 1995ARA&A..33..381F. doi:10.1146/annurev.aa.33.090195.002121. 
  36. ^ van den Bergh, S.; McClure, R.D. (1980). "Galactic distribution of the oldest open clusters". Astronomy & Astrophysics 88: 360. Bibcode 1980A&A....88..360V. 
  37. ^ Andronov, N.; Pinsonneault, M.; Terndrup, D. (2003). "Formation of Blue Stragglers in Open Clusters". Bulletin of the American Astronomical Society 35: 1343. Bibcode 2003AAS...203.8504A. 
  38. ^ 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 595 (1): L53–L56. arXiv:astro-ph/0308261. Bibcode 2003ApJ...595L..53F. doi:10.1086/379005. 
  39. ^ Hills, J. G. (February 1, 1980). "The effect of mass loss on the dynamical evolution of a stellar system - Analytic approximations". Astrophysical Journal 235 (1): 986–991. Bibcode 1980ApJ...235..986H. doi:10.1086/157703. 
  40. ^ a b de La Fuente, M.R. (1998). "Dynamical Evolution of Open Star Clusters". Publications of the Astronomical Society of the Pacific 110 (751): 1117–1117. Bibcode 1998PASP..110.1117D. doi:10.1086/316220. 
  41. ^ Soderblom, David R.; Mayor, Michel (1993). "Stellar kinematic groups. I - The Ursa Major group". Astronomical Journal 105 (1): 226–249. Bibcode 1993AJ....105..226S. doi:10.1086/116422. ISSN 0004-6256. 
  42. ^ Majewski, S. R.; Hawley, S. L.; Munn, J. A. (1996). "Moving Groups, Stellar Streams and Phase Space Substructure in the Galactic Halo". ASP Conference Series 92: 119. Bibcode 1996ASPC...92..119M. 
  43. ^ Sick, Jonathan; de Jong, R. S. (2006). "A New Method for Detecting Stellar Streams in the Halos of Galaxies". Bulletin of the American Astronomical Society 38: 1191. Bibcode 2006AAS...20921105S. 
  44. ^ a b "Diagrammi degli ammassi ed evoluzione stellare" (in italian). O.R.S.A. - Organizzazione Ricerche e Studi di Astronomia. Retrieved 2009-01-06. 
  45. ^ a b 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 116 (825): 997–1011. Bibcode 2004PASP..116..997V. doi:10.1086/426340. 
  46. ^ Keel, Bill. "The Extragalactic Distance Scale". Department of Physics and Astronomy - University of Alabama. Retrieved 2009-01-09. 
  47. ^ Brown, A.G.A. (2001). "Open clusters and OB associations: a review". Revista Mexicana de Astronomía y Astrofísica 11: 89–96. Bibcode 2001RMxAC..11...89B. 
  48. ^ Percival, S. M.; Salaris, M.; Kilkenny, D. (2003). "The open cluster distance scale - A new empirical approach". Astronomy & Astrophysics 400 (2): 541–552. arXiv:astro-ph/0301219. Bibcode 2003A&A...400..541P. doi:10.1051/0004-6361:20030092. 
  49. ^ Hanson, R.B. (1975). "A study of the motion, membership, and distance of the Hyades cluster". Astronomical Journal 80: 379–401. Bibcode 1975AJ.....80..379H. doi:10.1086/111753. 
  50. ^ 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 429 (3): 881–886. arXiv:astro-ph/0409046. Bibcode 2005A&A...429..881B. doi:10.1051/0004-6361:20041049. 
  51. ^ Rowan-Robinson, Michael (March 1988). "The extragalactic distance scale". Space Science Reviews 48 (1–2): 1–71. Bibcode 1988SSRv...48....1R. doi:10.1007/BF00183129. ISSN 0038-6308. 

Further reading

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

External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • open cluster — Astron. a comparatively young, irregularly shaped group of stars, often numbering up to several hundred, and held together by mutual gravitation; usually found along the central plane of the Milky Way and other galaxies. Also, galactic cluster.… …   Universalium

  • open cluster — noun : a cluster of stars in which all the individual members may be discerned with an optical aid and which is much less compact and has fewer members than a globular cluster; often : a galactic cluster …   Useful english dictionary

  • open cluster — noun a more or less irregular star cluster containing tens to thousands of stars …   Wiktionary

  • open cluster — /oʊpən ˈklʌstə/ (say ohpuhn klustuh) noun a group of stars having a common motion through space in which the stars are not densely packed (as in a globular cluster) but are interspersed with gas and dust clouds …   Australian English dictionary

  • open cluster — noun Astronomy a loose grouping of stars …   English new terms dictionary

  • Open Cluster Framework — (OCF) is a set of standards for computer clustering. The project started as a working group of the Free Standards Group, now part of the Linux Foundation. Original supporters included several computing companies and groups, including Compaq,… …   Wikipedia

  • Open cluster remnant — In astronomy, an open cluster remnant (OCR) is the final stage in the evolution of an open star cluster. Contents 1 Theory 2 Observations 3 Simulations 4 References …   Wikipedia

  • Open cluster family — In astronomy, an open cluster family is a group of approximately coeval (age range ∼30 Myr) young open star clusters located in a relatively small region of the Galactic disk (radius ∼250 pc). Introduction Open star clusters do not form in… …   Wikipedia

  • cluster — clusteringly, adv. clustery, adj. /klus teuhr/, n. 1. a number of things of the same kind, growing or held together; a bunch: a cluster of grapes. 2. a group of things or persons close together: There was a cluster of tourists at the gate. 3. U.S …   Universalium

  • Cluster — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Sur les autres projets Wikimedia : « Cluster », sur le Wiktionnaire (dictionnaire universel) Sommaire …   Wikipédia en Français

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.