Messier 15 (NGC 7078) is a globular cluster in the constellation Pegasus. This bright globular was discovered by Jean-Dominique Maraldi in September of 1746, and included in Charles Messier's catalog in 1764.
With a total visual magnitude of 6.2, Messier 15 is among the more conspicuous of these great stellar swarms. It can be seen as a fuzzy star with binoculars or a small telescope. Instruments of at least 6 inches' aperture will start to reveal individual stars, the brightest of which are magnitude 12.6. This cluster is a fine, bright globular with a blazing core that thins rapidly from the center. Many arms and chains radiate from the core, out to a diameter of 18'.
Messier 15 lies approximately 33,600 light years from Earth and has a diameter of about 175 light years. Its tidal radius, beyond which member stars would escape because of the Milky Way's gravity is a bit larger: 210 light years. M 15 has an absolute magnitude of -9.2, which translates to a luminosity of 360,000 Suns. M 15 is estimated to be 13.2 billion years old, making it one of the oldest known globular clusters.
Messier 15 is one of the most densely packed globulars known in the Milky Way galaxy. The half-mass radius of M 15 is about 10 light years - half the mass of this cluster is concentrated in a sphere of that radius. M 15's core has undergone a contraction known as core collapse, which may indicate a black hole.
M 15 is the first globular cluster in which a planetary nebula, Pease 1, was identified, in 1928. To this date, Pease 1 is still one of only four planetary nebulae known in a globular cluster. Messier 15 contains over one hundred variable stars. It also contains at least 8 pulsars, including one double neutron star system.
M 15 emits X-rays; two bright X-ray sources were resolved by the Chandra X-ray observatory. These provide further evidence of a black hole at the center of M 15.