Messier 15 or M15 (also designated NGC 7078) 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 13.2 billion years old, it is one of the oldest known globular clusters.
M15 is about 33,600 light-years from Earth. 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.
Messier 15 contains 112 variable stars, a rather high number. It also contains at least 8 pulsars, including one double neutron star system, M15 C. Moreover, M15 houses Pease 1, one of only four planetary nebulae known to reside within a globular cluster, which was discovered in 1928.