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Messier 68 (also known as M68 or NGC 4590) is a globular cluster found in the east south-east of Hydra, away from its precisely equatorial part. It was discovered by Charles Messier in 1780. William Herschel described it as "a beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together".
The first known globular cluster, now called M 22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. [4] [5] [6] The cluster Omega Centauri, easily visible in the southern sky with the naked eye, was known to ancient astronomers like Ptolemy as a star, but was reclassified as a nebula by Edmond Halley in 1677, [7] then finally as a globular cluster in the early 19th century ...
The Messier catalogue comprises nearly all of the most spectacular examples of the five types of deep-sky object – diffuse nebulae, planetary nebulae, open clusters, globular clusters, and galaxies – visible from European latitudes. Furthermore, almost all of the Messier objects are among the closest to Earth in their respective classes ...
Many globular clusters, such as the 13-Gyr old cluster M30 (pictured), are mass segregated. In astronomy, dynamical mass segregation is the process by which heavier members of a gravitationally bound system, such as a star cluster, tend to move toward the center, while lighter members tend to move farther away from the center.
Messier 3 is the prototype for the Oosterhoff type I cluster, which is considered "metal-rich". That is, for a globular cluster, Messier 3 has a relatively high abundance of heavier elements. [15] Arcturus and Cor Caroli can be used to help locate M3
The cluster has a half-light radius of 9.1 ly (2.80 pc) [6] with a core radius of about 1.6 ly (0.5 pc) and appears not to have undergone core collapse yet. The mass density at the core is 7.9 × 10 4 M ☉ ·pc −3. [b] [2] There are 38 RR Lyrae variable stars and the cluster appears to be Oosterhoff-intermediate in terms of metallicity.
NGC 6355 is a globular cluster located in the constellation Ophiuchus. [5] It is at a distance of 28,000 light years away from Earth, and is currently part of the Galactic bulge. [3] NGC was discovered by the German-born British astronomer William Herschel on 24 May 1784. [6]
Messier 28 or M28, also known as NGC 6626, is a globular cluster of stars in the center-west of Sagittarius.It was discovered by French astronomer Charles Messier in 1764. [11] [a] He briefly described it as a "nebula containing no star... round, seen with difficulty in 3 1 ⁄ 2-foot telescope; Diam 2′."