Star cluster

Star clusters are massive groups of stars inside galaxies. There are two classes of star clusters: globular clusters are very large, dense clusters containing between a few hundred and a million stars and measuring a few hundred light years across, and open clusters, are smaller groups, usually containing a few dozen stars, and measuring no more than a hundred light-years across. Open clusters also have a subtype known as moving groups which are clusters which move through the galaxy along with their constituent stars.

Specific examples
Star clusters visible to the naked eye from Earth include the Pleiades, Hyades, and 47 Tucanae. The two globular clusters of highest absolute luminosity are in the Southern Hemisphere in the constellations Centaurus and Tucana. Omega Centauri, with an (integrated) absolute visual magnitude of −10.26, is the richest cluster in variables, with nearly 200 known in the early 21st century. From this large group, three types of RR Lyrae stars were first distinguished in 1902. Omega Centauri is relatively nearby, at a distance of 17,000 light-years, and it lacks a sharp nucleus.

Nomenclature
In 1979, the International Astronomical Union's 17th general assembly recommended that newly discovered star clusters, open or globular, within the Galaxy have designations following the convention "Chhmm±ddd", always beginning with the prefix C, where h, m, and d represent the approximate coordinates of the cluster centre in hours and minutes of right ascension, and degrees of declination, respectively, with leading zeros.