Open cluster
Open cluster | |
---|---|
Characteristics | |
Type | Loose cluster of stars |
Size range | < 30 ly in diameter |
Density | ~ 1.5 stars / cubic ly |
External links | |
Media category | |
Q11387 | |
Additional Information |
An open cluster is a type of
Young open clusters may 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, their properties (such as distance, age, metallicity, extinction, and velocity) are more easily determined than they are for isolated stars.[1] A number of open clusters, such as the Pleiades, the Hyades and 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
The prominent open cluster the
The first person to use a telescope to observe the night sky and record his observations was the Italian scientist
It was realized as early as 1767 that the stars in a cluster were physically related,[15] when the English naturalist 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.[16] Between 1774 and 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.[9] In the 1790s, English astronomer William Herschel 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.[17] He divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars.[18]
The number of clusters known continued to increase under the efforts of astronomers. Hundreds of open clusters were listed in the
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.[23] 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.[24] 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 those stars that had a proper motion similar to the mean motion of the cluster, and were therefore more likely to be members.[25] Spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars bound together as a group.[1]
The first
Formation
The formation of an open cluster begins with the collapse of part of a
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, collisions with other clouds and gravitational interactions. Even without external triggers, regions of the cloud can reach conditions where they become unstable against collapse.[28] 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.[27] In the Milky Way galaxy, the formation rate of open clusters is estimated to be one every few thousand years.[29]
The hottest and most massive of the newly formed stars (known as
As only 30 to 40 percent 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 hold on to only a third of the original stars, with the remainder becoming unbound once the gas is expelled.[30] 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 clusters 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.[31] Most open clusters form with at least 100 stars and a mass of 50 or more solar masses. The largest clusters can have over 104 solar masses, with the massive cluster Westerlund 1 being estimated at 5 × 104 solar masses and R136 at almost 5 x 105, typical of globular clusters.[27] 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 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.[32]
It is common for two or more separate open clusters to form out of the same molecular cloud. In the
Morphology and classification
Open clusters range from very sparse clusters with only a few members to large
Open clusters are often classified according to a scheme developed by
Under the Trumpler scheme, the Pleiades are classified as I3rn, and 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.[38] 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 with a galactic radius of approximately 50,000 light years.[39]
In irregular galaxies, open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.[40] 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.[41]
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, generally at substantial distances above or below the galactic plane.[42] Tidal forces are stronger nearer the center 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.[43]
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, a few tens of millions of years. The older open clusters tend to contain more yellow stars.[citation needed]
The frequency of binary star systems has been observed to be higher within open clusters than outside open clusters. This is seen as evidence that single stars get ejected from open clusters due to dynamical interactions.[44]
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.[45]
Once they have exhausted their supply of
Because of their high density, close encounters between stars in an open cluster are common.[
Eventual fate
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.[48]
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.[49]
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
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
Studying stellar evolution
When a Hertzsprung–Russell diagram is plotted for an open cluster, most stars lie on the main sequence.[53] 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.[54]
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 are due only to their mass.[53] This makes open clusters very useful in the study of stellar evolution, because when comparing one star with another, many of the variable parameters are fixed.[54]
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. Through study of their abundances in open-cluster stars, variables such as age and chemical composition can be fixed.[55]
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.[55]
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.[56] 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 (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.[57][58]
The other direct method is the so-called
Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching the
Accurate knowledge of open cluster distances is vital for calibrating the period–luminosity relationship shown by
Planets
The stars in open clusters can host exoplanets, just like stars outside open clusters. For example, the open cluster NGC 6811 contains two known planetary systems, Kepler-66 and Kepler-67. Additionally, several hot Jupiters are known to exist in the Beehive Cluster.[64]
See also
References
- ^ a b c Frommert, Hartmut; Kronberg, Christine (August 27, 2007). "Open Star Clusters". SEDS. University of Arizona, Lunar and Planetary Lab. Archived from the original on December 22, 2008. Retrieved 2009-01-02.
- ISBN 3-540-00179-4.
- ISBN 0-674-83440-2.
- doi:10.1086/147466.
- ^ Neata, Emil. "Open Star Clusters: Information and Observations". Night Sky Info. Retrieved 2009-01-02.
- ^ "VISTA Finds 96 Star Clusters Hidden Behind Dust". ESO Science Release. Retrieved 3 August 2011.
- ISBN 978-0-521-89935-2
- ISBN 0-521-37079-5.
- ^ ISBN 0-521-81803-6.
- ^ "A Star Cluster in the Wake of Carina". ESO Press Release. Retrieved 27 May 2014.
- ISBN 978-1-933771-59-5
- ISBN 978-3-642-00791-0.
- S2CID 118328541
- S2CID 117590918.
- Bibcode:1989QJRAS..30..399C
- .
- S2CID 125219390.
- S2CID 125888787.
- ISBN 0-674-57503-2.
- ISBN 0-691-02565-7
- ISBN 81-203-1121-3.
- S2CID 122892308.
- Bibcode:1931PYerO...6....1B
- Bibcode:1919CMWCI.167....1V
- doi:10.1086/144736
- Bibcode:1977IAUS...80S..55S)
{{citation}}
: CS1 maint: location missing publisher (link - ^ S2CID 20180097
- ^
- .
- S2CID 11660522
- Bibcode:2005ESASP.576..629K.
- doi:10.1086/303966.
- .
- Bibcode:1995A&A...302...86S.
- ^ "Buried in the Heart of a Giant". Retrieved 1 July 2015.
- .
- .
- S2CID 18502004.
- doi:10.1086/117192.
- doi:10.1086/133965.
- OCLC 39108765.
- .
- Bibcode:1980A&A....88..360V.
- S2CID 236171384.
- Bibcode:2003AAS...203.8504A.
- S2CID 15439614.
- S2CID 3438729
- doi:10.1086/157703.
- ^ doi:10.1086/316220.
- ISSN 0004-6256.
- Bibcode:1996ASPC...92..119M.
- Bibcode:2006AAS...20921105S.
- ^ a b "Diagrammi degli ammassi ed evoluzione stellare" (in Italian). O.R.S.A. – Organizzazione Ricerche e Studi di Astronomia. Retrieved 2009-01-06.
- ^ ISBN 978-1-108-42216-1.
- ^ doi:10.1086/426340.
- ^ Keel, Bill. "The Extragalactic Distance Scale". Department of Physics and Astronomy – University of Alabama. Retrieved 2009-01-09.
- Bibcode:2001RMxAC..11...89B.
- S2CID 10544370.
- doi:10.1086/111753.
- S2CID 16669438.
- S2CID 121736592.
- ^ 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
- S2CID 118825401.
Further reading
- Kaufmann, W. J. (1994). Universe. W H Freeman. ISBN 0-7167-2379-4.
- ISBN 0-03-006228-4.