M15, also known as the Great Pegasus Cluster, is a globular cluster located in the northern constellation Pegasus. Its other designations are NGC 7078, GCl 120, C 2127+119, MWSC 3518. The cluster has an apparent magnitude of 6.2 and lies at a distance of 33,600 light-years from Earth at right ascension 21h 29m 58.33s, declination +12°10’01.2”. Messier 15 is one of the oldest known globular clusters in our galaxy. It has an estimated age of 12 billion years and only 1 per cent of the Sun’s iron content. The cluster has an absolute magnitude of -9.2, which makes it about 360,000 times more luminous than the Sun. It has an apparent diameter of about 175 light-years. The tidal radius of M15 spans about 210 light-years from the cluster’s centre. Messier 15 has a density classification IV and is one of the most densely concentrated clusters of its kind. The cluster is notable for its steep central cusp, with an exceptionally large number of stars orbiting what is likely a central black hole. The central density cusp is a result of the cluster has undergone a core-collapse, a contraction of its core region. This is a common occurrence in globular clusters as they evolve: Messier 30 and Messier 70 also contain a collapsed core. A total of 21 of the 157 known globular clusters in the Milky Way, and possibly 8 more, including Messier 62 and Messier 79, have undergone a core-collapse. Its core is very small – about 1.4 light-years – compared to the cluster’s size. Half the cluster’s mass is concentrated within the central 10 light-years. Scientists have theorized that either the cluster contains a supermassive black hole at its core or the concentration of mass is a result of the gravitational interaction of the stars in this area. The black hole is 4,000 times the mass of the sun.
A survey of the cluster’s inner 22 light-years alone revealed about 30,000 stars. The cluster contains more than 100,000 stars, including a considerable number of variables (112) and pulsars (8), neutron stars formed in supernova explosions that occurred when M15 and the universe itself were still young. The only other globular clusters with more known variables are Messier 3 and Omega Centauri. The brightest stars in M15 are of magnitude 12.6. They have an absolute magnitude of -2.8, which makes them about 1,000 times more luminous than the Sun. M15 is approaching us at 107 km/s. Messier 15 is home to a double neutron star system, designated M15 C, and the first-ever planetary nebula discovered in a globular cluster. The nebula, Pease 1 (K 648), was first observed in 1928. It is one of only four nebulae discovered within globular clusters. It was named after Francis Gladheim Pease, the American astronomer who discovered it. The nebula occupies an area 3 arc seconds in size and has a visual magnitude of 15.5. Two bright sources of X-ray emissions have been detected in M15 by the space satellites Chandra X-ray Observatory and Uhuru. The sources are designated Messier 15 X-1 and Messier 15 X-2. The first one, also catalogued as 4U 2129+12, is the first astronomical X-ray source found in Pegasus constellation.
Messier 15 was discovered by the Italian astronomer Jean-Dominique Maraldi on September 7, 1746. Maraldi stumbled upon the cluster while searching for a comet. Charles Messier included the object in his catalogue on June 3, 1764. Johann Elert Bode, who observed the cluster a decade later, could not make out any stars in it either and also described M15 as a “small nebula.“ It was William Herschel who finally resolved the cluster into individual stars in 1783.
Messier 15 is located 3 ½ degrees west and 2 ¼ degrees north of Enif, Epsilon Pegasi. As easy way to find the cluster is to first identify the Great Square of Pegasus, an asterism formed by the constellation’s brightest stars, and then to find the brightest star, Markab, Alpha Pegasi, which marks the southwestern corner. Credits: Messier Objects, NASA, Universe Today, Wikipedia.