SkyEye

Star Clusters

Open Clusters

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
(ly)
Age
(mega years)
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 ? ?

Hyades

Hyades Open Cluster 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.

Alpha Persei Cluster

alpha Persei Open Cluster 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.

Pleiades

Pleiades Open Cluster 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 TauAlcyone is the brightest member of this cluster.
16 TauCelaeno
17 TauElectra
19 TauTaygeta
20 TauMaia
21 TauAsterope
23 TauMerope
27 TauAtlas
28 TauPleione

Mentioned by Homer in his Iliad around 750 BC, the cluster is also referenced in three times in the Bible.

Coma Star Cluster

Coma Star Cluster 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.

Southern Pleiades

Southern Pleiades 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.

Northern Jewel Box

Northern Jewel Box Open Cluster 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.

Omicron Velorum Cluster

omicron Velorum Open Cluster 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.

NGC 2516

NGC 2516 Open Cluster 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.

Ptolemy's Cluster

Ptolemy's Open Cluster 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.

NGC 2451

NGC 2451 Open Cluster 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.

Beehive Cluster or Praesepe

Beehive Open Cluster 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.

Christmas Tree Cluster

Christmas Tree Open Cluster 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.

NGC 2547

NGC 2547 Open Cluster Another discovery of Nicholas Louis de Lacaille, this large cluster in Vela reveals dozens of stars in binoculars.

NGC 3114

NGC 3114 Open Cluster Barely visible to the naked eye, this unnamed open cluster in the constellation Carina is better viewed through a telescope.

Jewel Box

Jewel Box Open Cluster 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.

Butterfly Cluster

Butterfly Open Cluster 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.

Globular Clusters

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.

Notes

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

Sources

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.