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Open cluster

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The Pleiades is one of the most famous open clusters.
   Enlarge
   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 centre, as well as losing cluster members
   through internal close encounters.

   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.

   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.

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.

   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 . 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.

   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.

Formation

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

   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 .

   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 .

   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 .

   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 . 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

   NGC 2158 is a rich and concentrated cluster in Gemini.
   Enlarge
   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) .

   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 .

   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.
   Enlarge
   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 . 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 .

   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.

   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 .

Stellar composition

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

   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 .

   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 .

Eventual fate

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

   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 .

   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.

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.
   Enlarge
   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.

   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.

   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.

   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 .

Open clusters and the astronomical distance scale

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

   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 .

   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 . Open clusters are also easily detected in many of the
   galaxies of the Local Group.

   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.
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