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.
contrast, globular clusters are very tightly bound by gravity.
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
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
Open clusters are very important objects in the study of stellar
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.
The most prominent open clusters such as
the Pleiades have been
known and recognized 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
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
It was realized 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.
reveals the dense open cluster forming at the heart of the Orion
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
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
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
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.
Another view to 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
ionized gas. From the time of start of cloud-core contraction to gas
expulsion takes 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 .
All clusters thus suffer significant
infant weight loss, while a large fraction undergoes 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
It is common for two or more separate open clusters to form out of
the same molecular cloud. In the Large Magellanic Cloud, both
and R136 are forming from the gases of the
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
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
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).
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
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.
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
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
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 kilometers per
second, enough to eject it from the cluster.
NGC 604 in the
Triangulum Galaxy is a very massive open cluster surrounded by an H
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
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
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
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 center of the
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.
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
Transactions, v. 57, p. 234–264
Mathieu, R. D. (1994). "Pre-Main-Sequence
Binary Stars". Annual
Reviews of Astronomy and Astrophysics
A.P. (1998), The Jeans Mass
Constraint and the Fragmentation of
Molecular Cloud Cores, Astrophysical
Journal Letters v.501, p.L77
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
see Battinelli P as previous reference.
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, v. 321, 699-712
P. (2005), "The Fundamental Building
Blocks of Galaxies", in Proceedings of
the Gaia Symposium "The
Three-Dimensional Universe with Gaia" (ESA
SP-576). Held at the Observatoire de
Paris-Meudon, 4-7 October 2004. Editors:
C. Turon, K.S. O'Flaherty, M.A.C.
O. J. (1960), Stellar groups, VII.
The structure of the Hyades group,
Monthly Notices of the Royal
Astronomical Society, v. 120, p.540
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
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.
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
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
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.
den Bergh S., McClure R.D. (1980),
Galactic distribution of the oldest open
clusters, Astronomy & Astrophysics,
Andronov N., Pinsonneault M., Terndrup
D. (2003), Formation of Blue
Stragglers in Open Clusters,
American Astronomical Society Meeting
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.
Fuente M.R. (1998), Dynamical
Evolution of Open Star Clusters,
Publications of the Astronomical Society
of the Pacific, v. 110, pp. 1117–1117
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
A.G.A. (2001), Open clusters and OB
associations: a review, Revista
Mexicana de Astronomía y Astrofísica, v.
R.B. (1975), A study of the motion,
membership, and distance of the Hyades
cluster, Astronomical Journal, v.
80, p. 379–401
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