Star clusters represent distinct steps in the evolution of the Universe. On the one hand, globular clusters are among the oldest stellar systems in galaxies and can preserve information about the early history of their hosts. Some unusually complex globular clusters may even be the surviving nuclei of dwarf galaxies that were disrupted and absorbed by larger galaxies. Open clusters, on the other hand, are generally younger groups whose constituent stars formed at roughly the same time from the same molecular cloud.
By keeping the above in mind when looking at the types of star clusters on this list, the items presented here may become more than just pretty pictures; instead, they may come to be seen as important records of how stars and galaxies form and evolve.
Omega Centauri (NGC 5139)
- Constellation: Centaurus
- Class: VIII
- Coordinates: RA 13h 26m 47.28s | Dec. −47° 28′ 46.1″
- Distance: About 15,800 light-years
- Mass: About 4 million solar masses
- Radius: 86 light-years
- Magnitude: 3.9
- Estimated age: About 12 billion years
- Other designations: NGC 5139, GCl 24, Caldwell 80
Under really dark skies, Omega Centauri appears almost as big as the full Moon, which is not surprising given that this, the biggest globular cluster orbiting the Milky Way, spans more than 150 light-years and contains millions of stars. In fact, the cluster is so distinctively different from other globular clusters that some investigators believe it may be the surviving nucleus of a dwarf galaxy that was assimilated into the Milky Way.
One of the most distinguishing features of this cluster is its complex population of old, evolved stars. Its stars show a wide distribution of metallicities and ages, which is unusual for a globular cluster and supports the idea that Omega Centauri experienced a more complicated formation history than most such systems. Investigations are continuing.
47 Tucanae (NGC 104)
- Constellation: Tucana
- Class: III
- Coordinates: RA 00h 24m 05.22s | Dec. −72° 04′ 57.9″
- Distance: 16,700 light-years
- Mass: About 1 million solar masses
- Radius: About 60 light-years
- Magnitude: 4.95
- Estimated age: About 13 billion years
- Other designations: NGC 104, GCl 1, Caldwell 106
Apart from the fact that 47 Tucanae is one of the biggest and brightest globular clusters around the Milky Way, and also contains millions of stars, it includes many stars of scientific interest. For instance, the cluster contains blue stragglers, which are stars that appear younger and more massive than other stars in the same population, most likely as a result of stellar mergers or mass transfer. 47 Tucanae also contains numerous X-ray sources.
Furthermore, the cluster houses a large population of millisecond pulsars. Searches have also been conducted for an intermediate-mass black hole in its center, although its existence remains uncertain. Despite extensive searches, no planets have yet been confirmed in the cluster.
Messier 2 (NGC 7089)
- Constellation: Aquarius
- Class: II
- Coordinates: RA 21h 33m 27.02s | Dec. −00° 49′ 23.7″
- Distance: 37,500 light-years
- Mass: About 900,000 solar masses
- Radius: About 87 light-years
- Magnitude: 6.3
- Estimated age: About 13 billion years
- Other designations: NGC 7089
Discovered in 1746 by Jean-Dominique Maraldi, Messier 2 was at first thought to be nothing more than a cloud of nebulosity, until William Herschel was able to resolve individual stars in 1783. Spanning about 175 light-years, this large cluster is just visible to the naked eye under exceptionally dark skies, while a small telescope will begin to resolve some of the brighter stars in its outer regions. M2 contains around 150,000 stars, including a number of variable stars. The brightest members of the cluster include red and yellow giant stars.
NGC 1049 (in Fornax Dwarf Galaxy)
- Constellation: Fornax
- Class: V
- Coordinates: RA 02h 39m 52.5s | Dec. −34° 16′ 08″
- Distance: About 460,000 light-years
- Radius: About 12 light-years
- Magnitude: 12.9
- Other designation: Hodge 3
Located hundreds of thousands of light-years away in the Fornax Dwarf Spheroidal Galaxy, this compact globular cluster is an unusual observing target. Its parent galaxy has extremely low surface brightness and is much harder to see than NGC 1049 itself, which means that observers can detect the cluster without necessarily seeing the galaxy it belongs to.
Mayall II (Andromeda’s Cluster)
- Constellation: Andromeda
- Coordinates: RA 00h 32m 46.51s | Dec. +39° 34′ 39.7″
- Distance: About 2.5 million light-years
- Mass: Several million solar masses
- Radius: About 21 light-years
- Magnitude: 13.8
- Estimated age: About 12 billion years
- Other designations: G1, NGC-224-G1, Andromeda’s Cluster
Mayall II, commonly known as G1, is one of the most massive globular clusters in the Local Group and orbits far from the center of the Andromeda Galaxy. Estimates of its mass vary, but it is substantially more massive than a typical globular cluster. Evidence has also been reported for an intermediate-mass black hole in its core, although the interpretation remains debated.
Like Omega Centauri, Mayall II has a complex stellar population and a spread in metallicity, which has led to suggestions that it could be the surviving nucleus of a dwarf galaxy that was tidally stripped by the Andromeda Galaxy. Note that the two bright objects in the frame are unrelated foreground stars.
Southern Pleiades (IC 2602)
- Constellation: Carina
- Coordinates: RA 10h 42m 57.5s | Dec. −64° 23′ 39″
- Distance: About 480 light-years
- Apparent dimensions: About 50′ × 50′
- Magnitude: 1.9
- Estimated age: About 50 million years
- Other designations: Theta Carinae Cluster, Melotte 102, Collinder 229
Like its namesake in the Northern Hemisphere, the Southern Pleiades is a conspicuous naked-eye object. It has fewer prominent members than the Pleiades in Taurus, with its brightest star, Theta Carinae, shining at magnitude +2.74. The cluster is young, with estimates placing its age at roughly 50 million years.
Jewel Box (NGC 4755)
- Constellation: Crux
- Coordinates: RA 12h 53m 42s | Dec. −60° 22′
- Distance: About 6,400 light-years
- Magnitude: 4.2
- Estimated age: About 14 million years
- Other designations: NGC 4755, Herschel’s Jewel Box, Kappa Crucis Cluster, Caldwell 94
Located close to the Southern Cross, the aptly named Jewel Box is a very young open star cluster. Clearly visible without optical aid under suitable skies, it contains a striking mixture of brightly colored stars. John Herschel famously compared the view through a telescope to a piece of fancy jewelry because of the contrasting colors of its constituent stars.
The most luminous stars in this little cluster include massive supergiants and rank among the most luminous types of stars in the Milky Way, making the Jewel Box particularly impressive through binoculars and small telescopes.
Pearl Cluster (NGC 3766)
- Constellation: Centaurus
- Coordinates: RA 11h 36.1m | Dec. −61° 37′
- Distance: About 5,500 light-years
- Apparent diameter: About 12 arcminutes
- Magnitude: 5.3
- Estimated age: About 15–20 million years
- Other designations: NGC 3766, Lacaille III.7, Dunlop 289, Melotte 107, Collinder 248, Caldwell 97
One of the most attractive aspects of this young open cluster is the contrast between its numerous hot blue-white stars and several evolved red stars. The cluster contains hundreds of stellar members and is particularly interesting because it includes an unusually large population of variable stars.
Its young stars share a common origin and motion through space, having formed from the same broad cloud of interstellar material. With an integrated magnitude of about 5.3, the Pearl Cluster can be glimpsed without optical aid under good southern skies and becomes a much richer sight through binoculars or a small telescope.
Hodge 301 (in Large Magellanic Cloud)
- Constellation: Dorado
- Coordinates: RA 05h 38m 27s | Dec. −69° 04′ 26″
- Distance: About 160,000 light-years
- Magnitude: About 11
- Estimated age: About 20–25 million years
Unlike the next item on this list, Hodge 301 is an older open cluster located in the Tarantula Nebula within the Large Magellanic Cloud. Many stellar bodies in this cluster have already had time to evolve away from the main sequence, and numerous massive stars are thought to have exploded as supernovae. These events have helped shape the gas surrounding the cluster and contributed to the complex structure of the Tarantula Nebula.
The presence of major star clusters of different ages within the nebula has enabled astronomers to study how supernova explosions and powerful stellar winds interact with dense concentrations of gas and dust.
R136 (in Large Magellanic Cloud)
- Constellation: Dorado
- Coordinates: RA 05h 38m 42.4s | Dec. −69° 06′ 03.4″
- Distance: About 160,000 light-years
- Mass: About 450,000 solar masses
- Magnitude: About 9.5
- Estimated age: About 1–2 million years
While open clusters within larger stellar associations are not exactly rare, it is not often that a concentration of stars is energetic enough to dominate an enormous star-forming region. R136 lies at the heart of the Tarantula Nebula in the Large Magellanic Cloud and contains an extraordinary concentration of massive young stars.
Previously known as RMC 136, this tightly packed knot of stars forms the central concentration of the larger NGC 2070 association. It contains some of the most massive and luminous stars known, including numerous very hot O-type stars and Wolf-Rayet stars.
R136 is only about 1–2 million years old, so its stellar population is dominated by extremely young, massive stars. Their intense ultraviolet radiation and powerful stellar winds play a major role in shaping the surrounding Tarantula Nebula.