Star cluster M 15

Messier 15 (M15), also known as NGC 7078 and the Great Pegasus Cluster, is a bright globular cluster located in the constellation Pegasus. It was discovered by Jean-Dominique Marally in 1746 and included in the Charles Messier catalogue in 1764. It is approximately 12.5 billion years old, making it one of the oldest known globular clusters in the Milky Way.

Key Features

M15 is approximately 35,700 light-years away from Earth and has a diameter of approximately 175 light-years. With an absolute magnitude of -9.2, it has a luminosity 360,000 times that of the Sun. This is one of the most densely packed globular clusters in our galaxy. The central region of M15 has undergone a phenomenon known as "core compression", resulting in an extremely high density of stars around a possible central black hole.

Stellar Composition

The cluster contains over 100,000 stars, of which the variable stars and pulsars are particularly notable. It includes the M15-C system (a binary neutron star) and 8 pulsars, as well as 112 variable stars. M15 is also home to Pease 1, the first planetary nebula discovered inside a globular cluster in 1928.

Observing M15

With an apparent magnitude of 6.2, M15 is nearly visible to the naked eye in good seeing conditions. Through binoculars or a small telescope, it appears as a puffy star. With larger aperture telescopes (150 mm or larger), individual stars become visible, the brightest of which have a magnitude of +12.6. The cluster is about 18 arcminutes in diameter (about three-tenths of a degree). M15 is located about 4° west-northwest of Epsilon Pegasi, the brightest star in Pegasus.

X-ray Sources and a Black Hole

M15 is home to two bright X-ray sources discovered by satellites such as Uhuru and Chandra X-ray Observatory. One of them, Messier 15 X-1 (4U 2129+12), was the first X-ray source discovered in Pegasus. Additionally, M15 is the first globular cluster to contain a planetary nebula (gases ejected by a dying star). This nebula, Pease 1, was discovered in 1928 by Francis G. Pease and is one of only four planetary nebulae known in globular clusters.

M15 is also home to a rare type of black hole, an intermediate-mass black hole. These black holes are significantly smaller than the supermassive black holes that reside at the centers of galaxies, but are 4,000 times more massive than stellar black holes.

Scientific Significance

M15's unique features, including its dense stellar environment and the presence of a central black hole, make it an important target for astronomical research. Studying its formation and evolution provides valuable information about the dynamics of globular clusters and how star formation and evolution occur in such dense structures.

M15 continues to attract the attention of both amateur astronomers and professional scientists, being an outstanding example of a dense stellar array and an important object of study for astrophysics.