| Messier 15 | |
| Epoch: | J2000 |
| Class: | IV |
| Constellation: | Pegasus |
| Appmag V: | 6.2[1] |
| Size V: | 18′.0 |
| Radius Ly: | ~88 ly[2] |
| Metal Fe: | –2.37 |
| V Hb: | 15.83 |
| Age: | 12.0 Gyr |
| Notes: | steep central cusp |
| Names: | NGC 7078, GCl 120 |
Messier 15 or M15 (also designated NGC 7078 and sometimes known as the Great Pegasus Cluster) is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764. At an estimated billion years old, it is one of the oldest known globular clusters.
M 15 is about 35,700 light-years from Earth, and 175 light-years in diameter. It has an absolute magnitude of -9.2, which translates to a total luminosity of 360,000 times that of the Sun. Messier 15 is one of the most densely packed globulars known in the Milky Way galaxy. Its core has undergone a contraction known as "core collapse" and it has a central density cusp with an enormous number of stars surrounding what may be a central black hole.[3]
Home to over 100,000 stars,[4] the cluster is notable for containing a large number of variable stars (112) and pulsars (8), including one double neutron star system, M15-C. It also contains Pease 1, the first planetary nebula discovered within a globular cluster in 1928.[5] Just three others have been found in globular clusters since then.[6]
At magnitude 6.2, M15 approaches naked eye visibility under good conditions and can be observed with binoculars or a small telescope, appearing as a fuzzy star. Telescopes with a larger aperture (at least 6 in. (150 mm)) will start to reveal individual stars, the brightest of which are of magnitude +12.6. The cluster appears 18 arc minutes in size (three tenths of a degree across). M15 is around 4° WNW of the brightest star of Pegasus, Epsilon Pegasi.
Earth-orbiting satellites Uhuru and Chandra X-ray Observatory have detected two bright X-ray sources in this cluster: Messier 15 X-1 (4U 2129+12) and Messier 15 X-2.[7] [8] The former appears to be the first astronomical X-ray source detected in Pegasus.