Open cluster

open star clusterstar clusteropen clustersgalactic clusterTrumpler classbirth clusteropen star clustersTrumpler classificationclusterclusters
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.wikipedia
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Star cluster

star clustersclusterC
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.
Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds to millions of old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young.

Pleiades

Pleiades star clusterThe PleiadesPleiades cluster
A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye. The prominent open cluster the Pleiades has been recognized as a group of stars since antiquity, while the Hyades forms part of Taurus, one of the oldest constellations.
The Pleiades, also known as the Seven Sisters and Messier 45, are an open star cluster containing middle-aged, hot B-type stars located in the constellation of Taurus.

Alpha Persei Cluster

Alpha Persei moving clusterAlpha PerseiCollinder 39
A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye.
The Alpha Persei Cluster, also known as Melotte 20 or Collinder 39, is an open cluster in the constellation of Perseus.

Hyades (star cluster)

HyadesHyades clusterHyades star cluster
A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye.
The Hyades (Greek Ὑάδες, also known as Caldwell 41, Melotte 25, or Collinder 50) is the nearest open cluster and one of the best-studied star clusters.

Double Cluster

h/χ Persei
Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes. In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus, the Coma Star Cluster, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
The Double Cluster (also known as Caldwell 14) is the common name for the open clusters NGC 869 and NGC 884 (often designated h Persei and χ Persei, respectively), which are close together in the constellation Perseus.

Wild Duck Cluster

M11Messier 116705
The Wild Duck Cluster, M11, is an example.
The Wild Duck Cluster (also known as Messier 11, or NGC 6705) is an open cluster of stars in the constellation Scutum (the Shield).

Taurus (constellation)

TaurusTaurus constellationToro
The prominent open cluster the Pleiades has been recognized as a group of stars since antiquity, while the Hyades forms part of Taurus, one of the oldest constellations.
Taurus hosts two of the nearest open clusters to Earth, the Pleiades and the Hyades, both of which are visible to the naked eye.

Messier 7

M7Ptolemy Cluster6475
In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus, the Coma Star Cluster, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
Messier 7 or M7, also designated NGC 6475 and sometimes known as the Ptolemy Cluster, is an open cluster of stars in the constellation of Scorpius.

IC 2391

Argus AssociationOmicron Velorum ClusterOmicron Velorum
In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus, the Coma Star Cluster, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
IC 2391 (also known as the Omicron Velorum Cluster or Caldwell 85) is an open cluster in the constellation Vela.

Perseus (constellation)

PerseusPerseus constellationPer
In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus, the Coma Star Cluster, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
It and many of the surrounding stars are members of an open cluster known as the Alpha Persei Cluster.

Coma Star Cluster

Collinder 256ComaComa Berenices cluster
In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus, the Coma Star Cluster, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
The Coma Star Cluster in Coma Berenices, designated Melotte 111 after its entry in the catalogue of star clusters by P. J. Melotte, is a small but nearby star cluster in our galaxy, containing about 40 brighter stars (magnitude 5 to 10) with a common proper motion.

Messier 41

M412287NGC 2287
In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451.
Messier 41 (also known as M41 or NGC 2287) is an open cluster in the constellation Canis Major.

Messier 47

M472422NGC 2422
In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451.
Messier 47 (M47 or NGC 2422) is an open cluster in the constellation Puppis.

Star formation

star-forming regionnew starsstar-forming
Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
The end product of a core collapse is an open cluster of stars.

NGC 2362

Tau Canis Majoris Cluster2362
In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451.
NGC 2362 is an open cluster in the constellation Canis Major.

NGC 2451

2451NGC 2451 A
In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451.
NGC 2451 is an open cluster in the Puppis constellation, probably discovered by Giovanni Battista Hodierna before 1654 and John Herschel in 1835.

Binoculars

binocularfield glassesfield glass
Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes.
Some open clusters, such as the bright double cluster (NGC 869 and NGC 884) in the constellation Perseus, and globular clusters, such as M13 in Hercules, are easy to spot.

Star

starsstellarmassive star
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.
Studies of the most massive open clusters suggests as an upper limit for stars in the current era of the universe.

Stellar association

OB associationassociationstellar associations
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.
A stellar association is a very loose star cluster, looser than both open clusters and globular clusters.

Beehive Cluster

PraesepeM44NGC 2632
In his Almagest, the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus, the Coma Star Cluster, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.
The Beehive Cluster (also known as Praesepe (Latin for "manger"), M44, NGC 2632, or Cr 189), is an open cluster in the constellation Cancer.

Radiation pressure

solar radiation pressurelight pressurepressure of light
Over time, radiation pressure from the cluster will disperse the molecular cloud.
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.

Milky Way

Milky Way Galaxygalaxyour galaxy
More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist.
Open clusters are also located primarily in the disk.

Proper motion

proper motionsproper-motionhigh proper motion star
As astrometry became more accurate, cluster stars were found to share a common proper motion through space.
Two or more stars, double stars or open star clusters, which are moving in similar directions, exhibit so-called shared or common proper motion (or cpm.), suggesting they may be gravitationally attached or share similar motion in space.

Blue straggler

blue stragglersblue straggler starBSS
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.
A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point for the cluster.

Ursa Major Moving Group

Ursa Major StreamCollinder 285Ursa Major association
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.
Based on the numbers of its constituent stars, the Ursa Major Moving Group is believed to have once been an open cluster, having formed from a protostellar nebula approximately 500 million years ago.