Most stars do not form singly but in groups. The stars of an open cluster form out of the same giant molecular cloud at about the same time and are more or less gravitationally bound to each other. The clusters become disrupted by gravitational encounters, both with other members of the cluster, and with other clusters and nebulae. Eventually the stars in an open cluster are dispersed and the cluster no longer exists. As a result, open clusters are necessarily young objects, usually less than several hundred million years old.
Open clusters can be very sparse, with few stars, or very large with thousands of members. There may be a dense core of stars measuring a few light years across, surrounded by a more diffuse scattering of stars, but there is no typical shape. The cluster may still reside within the remnants of the nebula out of which it was formed.
Open clusters are usually found in the arms of spiral galaxies and scattered throughout irregular galaxies where star formation is still taking place. Because star formation has long since ceased in elliptical galaxies, open clusters are not found there. Over a thousand open clusters have been identified so far in our galaxy, the Milky Way, and many more are thought to exist.
A number of open clusters are visible to the naked eye. Below is a small selection of the brighter clusters, listed in order of descending apparent magnitude.
|Catalogue Number(s)||Popular Name||Constellation||Apparent Magnitude||Distance
|C41||Cr 50||Mel 25||Hyades||Taurus||+0.5||152||625|
|Cr 39||Mel 20||α Persei Cluster||Perseus||+1.2||562||35.5|
|M45||Cr 42||Mel 22||Pleiades||Taurus||+1.6||392||120|
|Cr 256||Mel 111||Coma Star Cluster||Coma Berenices||+1.8||283||603|
|C102||IC 2602||Cr 229||Mel 102||Southern Pleiades||Carina||+1.9||485||67.6|
|C76||NGC 6231||Cr 315||Mel 153||Northern Jewel Box||Scorpius||+2.0||?||?|
|C85||IC 2391||Cr 191||ο Velorum Cluster||Vela||+2.5||473||75.9|
|C96||NGC 2516||Cr 172||Mel 82||Carina||+3.0||1120||120|
|M7||NGC 6475||Cr 354||Mel 183||Ptolemy's Cluster||Scorpius||+3.3||882||166|
|NGC 2451||Cr 161||Puppis||+3.5||599||57.5|
|M44||NGC 2632||Cr 189||Mel 88||Beehive Cluster, Praesepe||Cancer||+3.7||592||794|
|NGC 2264||Cr 112||Mel 49||Christmas Tree Cluster||Monoceros||+3.9||?||?|
|NGC 2547||Cr 177||Mel 84||Vela||+4.0||1550||50.1|
|NGC 3114||Cr 215||Mel 98||Carina||+4.0||?||?|
|C94||NGC 4755||Cr 264||Mel 114||Jewel Box||Crux||+4.0||?||?|
|M6||NGC 6405||Cr 341||Mel 178||Butterfly Cluster||Scorpius||+4.0||?||?|
A large V-shaped open star cluster in the constellation of Taurus, the Hyades is easy to see with the naked eye. The four brightest members form an asterism that is identified as the head of Taurus the bull. However, the bright red giant star Aldebaran which forms the eye of the bull is not actually a member of this cluster but is a foreground star.
The Hyades is the nearest open cluster to Earth and probably the best-studied.
Because of its large angular size on the sky, it is better to observe the Hyades through binoculars rather than a telescope. A more detailed star identification chart may be found on the Taurus constellation page.
Since it is such an obvious naked-eye object, this cluster has been known since prehistoric times. Like the Pleiades, the Hyades star cluster was mentioned by Homer in his epic Iliad around 750 BC. It was first catalogued as a cluster in the seventeenth century.
In Greek mythology, the Hyades were five daughters of the Titan Atlas and half-sisters to the Pleiades.
The α Persei Cluster, also known as the α Persei Moving Cluster, is an open cluster in the constellation of Perseus. Its brightest member is the second-magnitude star α Persei, familiarly known as Mirfak or Algenib. Several of the stars are easily visible to the naked eye and many of them are blue, implying that they are hot, massive and very young. Even a small pair of binoculars will reveal many more cluster members.
The α Persei Cluster was first catalogued in the seventeenth century.
An open star cluster in the constellation of Taurus, the Pleiades is easy to see with the naked eye. Those with good eyesight can see six stars but binoculars reveal many more. Long-exposure photographs show nebulosity surrounding the stars in the cluster. The Pleaides is a cluster of very young stars and this nebulosity is the remnants of the cloud out of which the stars formed.
A more detailed star identification chart may be found on the Taurus constellation page.
In Greek mythology, the Pleiades were the seven daughters of Atlas and Pleione, and were half-sisters of the Hyades. The brightest stars in the cluster are named for members of this family.
|η Tau||Alcyone is the brightest member of this cluster.|
Mentioned by Homer in his Iliad around 750 BC, the cluster is also referenced in three times in the Bible.
It is this cluster that gives the constellation Coma Berenices its name. According to legend, Egyptian queen Berenice sacrificed her hair to ensure the safe return of her husband from war. The Coma Star Cluster represents that hair. The word 'coma' comes from the Latin 'coma' meaning 'hair of the head' and from a similar ancient Greek word also meaning hair. (Interestingly, the word 'comet' is derived from the same words. A comet is literally a 'hairy' star!) The stars in the cluster range in apparent magnitude from 4 to 10 but no fainter stars have been identified as cluster members. It is conjectured that the low total mass of the cluster has allowed the smaller, fainter members to escape. The brighter members of the group form a V shape.
The Coma Star Cluster was first catalogued by Ptolemy in the second century.
The Southern Pleiades is an open cluster in the southern hemisphere constellation of Carina. The brightest member of the cluster, third-magnitude star θ Carina, gives this cluster its alternate name, the θ Carinae Cluster. It was first catalogued by Nicholas Louis de Lacaille in 1752 during his year-long observing run in the southern hemisphere. During this time in South Africa, Lacaille determined the positions of nearly 10,000 stars, discovered 42 'nebulous stars' (star clusters), and delineated 15 new constellations.
The Southern Pleaides is considerably less bright than the (Taurean) Pleiades. Except for the brightest star, the other members of the cluster are fifth magnitude and fainter. This is a large cluster and presents a fine site even in small binoculars.
The Northern Jewel Box is located near the star ζ Scorpii in the constellation Scorpius. It is thought to be very young, perhaps just over 3 million years old, and is approaching our solar system. It was first catalogued by Sicilian astronomer, Giovanni Batista Hodierna, in the mid-seventeenth century.
This sparse cluster was first described by Persian astronomer Al Sufi about AD 692 and formally catalogued by Nicholas Louis de Lacaille in 1752. It is located near the star ο Velorum in the constellation Vela.
Another discovery of Nicholas Louis de Lacaille, this unnamed open cluster is found in the constellation Carina. It is easily visible to the naked eye but binoculars or a small telescope yield a superior view.
Known since antiquity, this open cluster in the constellation of Scorpius was first recorded in the second century by the astronomer Ptolemy. Later, Charles Messier included it in his catalogue of 'fuzzy objects that are not comets' as the seventh object in the list. It is found near the open cluster M6 just north of the 'stinger' of the scorpion.
This object, found in the constellation of Puppis, may actually be two open clusters which just happen to lie along the same line of sight.
This sparse cluster was first catalogued by Giovanni Batista Hodierna in the mid-seventeenth century. It's an attractive binocular or telescopic object, with the brightest star being orange in hue and the surrounding stars white.
Looking nebulous to the naked eye, this open cluster in the constellation of Cancer has been known since ancient times. Galileo was the first person to observe it with a telescope.
The ecliptic runs just south of the Beehive Cluster which means that solar system objects often pass very near if not through this group of stars.
The alternate name, Praesepe, is Latin for manger. The ancient Greeks and Romans saw it as the manger from which two donkeys, represented by two nearby stars, ate.
The Christmas Tree Cluster and associated Cone Nebula were both discovered by British astronomer William Herschel. This bright cluster is found within the constellation of Monoceros although filters are required to reveal the surrounding nebulosity.
Another discovery of Nicholas Louis de Lacaille, this large cluster in Vela reveals dozens of stars in binoculars.
Barely visible to the naked eye, this unnamed open cluster in the constellation Carina is better viewed through a telescope.
Possibly the best open cluster discovered by Nicholas Louis de Lacaille, the Jewel Box was named by British astronomer Sir John Herschel because of its variously coloured stars when viewed through a telescope. This cluster is easy to find, located just south of the star β Crucis in the constellation Crux.
This Messier object in the constellation of Scorpius is another discovery of the Sicilian astronomer, Giovanni Batista Hodierna, who catalogued it in the mid-seventeenth century. Although not as visually impressive as its neighbour, Ptolemy's Cluster, it is visible to the naked eye. Magnification is necessary to reveal the fainter stars which give the cluster the appearance of a butterfly.
Unlike the young, irregularly-shaped open clusters of stars, globular clusters are nearly-spherical groups of old stars. Indeed, observations have shown that globular clusters belonging to the Milky Way are 10 billion years old or even older, making the stars within these clusters some of the oldest stars in our galaxy. Whereas open clusters are young objects found in star-forming regions of the spiral arms, globular clusters are found in the galactic halo, a spherical region encompassing the whole of the galaxy.
The Milky Way has at least 150 globular clusters and these spherical objects have been detected around other galaxies as well. Whilst most globular clusters are very old objects, our neighbouring galaxy, the Large Magellanic Cloud, contains a globular cluster which seems to be very young. These clusters typically contain hundreds of thousands of stars and are free of gas and dust.
There are eight globular clusters which are visible to the naked eye, most of them in the southern hemisphere. All are fine binocular objects.
|Catalogue Number(s)||Popular Name||Constellation||Apparent Magnitude|
|C80||NGC 5139||Mel 118||ω Centauri||Centaurus||+3.7|
|C106||NGC 104||Mel 1||47 Tucanae||Tucana||+4.0|
|M22||NGC 6656||Mel 208||Sagittarius||+5.1|
|C93||NGC 6752||Mel 218||Pavo||+5.4|
|M4||NGC 6121||Mel 144||Scorpius||+5.6|
|M5||NGC 5904||Mel 133||Serpens||+5.7|
|C86||NGC 6397||Mel 176||Ara||+5.7|
|M13||NGC 6205||Mel 150||Great Globular Cluster||Hercules||+5.8|
ω Centauri is the largest globular cluster in the Milky Way and is so distinctive from other globulars that it is thought that is might actually be the core of a disrupted dwarf galaxy rather than a true globular. 47 Tucanae is one of the most massive globular clusters in the Milky Way. M22 is more elliptical than spherical in shape and is one one of the very few globular clusters to contain planetary nebulae. C93 is one of the closer globular clusters but not as close as C86 which, along with M4, is the closest globular cluster to Earth. M4 has the further distinction of being the first globular cluster in which individual stars were resolved. M5 is one of the largest globular clusters so far identified. In 1974, a radio message was beamed from the Arecibo radio telescope to the Great Globular Cluster. The message will take 25,000 years to reach its destination.
|IC||Index Catalogue of Nebulae and Clusters of Stars|
|NGC||New General Catalogue of Nebulae and Clusters of Stars|
|Cr||Collinder's Catalogue of Open Star Clusters|
|Mel||Melotte Catalogue of Star Clusters|
Open cluster distances and ages are obtained from the (PDF) paper Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue, F. van Leeuwen, Astronomy & Astrophysics, 497, 1, 209-242. Cluster magnitudes and other information are derived from BinocularSky and SEDS.