Messier 15

Lore & Background

Messier 15 was discovered by Jean-Dominique Maraldi in 1746 and later included in Charles Messier's catalogue of comet-like objects in 1764. It lies about 35,700 light-years from Earth and spans 175 light-years in diameter. With an absolute magnitude of −9.2, its total luminosity is 360,000 times that of the Sun. The cluster is one of the most densely packed globulars known in the Milky Way, and its core has undergone a contraction known as 'core collapse,' featuring a central density cusp with an enormous number of stars surrounding what may be a central black hole.

The cluster is home to over 100,000 stars and 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, found in 1928; only three others have been found in globular clusters since then. 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. Larger telescopes (at least 6 inches aperture) reveal individual stars, the brightest of which are magnitude +12.6. The cluster appears 18 arc minutes in size and is located about 4° west-northwest of the brightest star of Pegasus, Epsilon Pegasi.

Reader's Guide

Messier 15 holds significance as one of the oldest known globular clusters, with an estimated age of 12.5±1.3 billion years, placing it among the earliest structures formed in the Milky Way. Its extreme density and core collapse make it a key object for studying stellar dynamics and the possible presence of intermediate-mass black holes. The cluster's rich population of variable stars and pulsars, including the double neutron star system M15-C, provides valuable data for stellar evolution and binary system research. The discovery of Pease 1, the first planetary nebula found in a globular cluster, marked a milestone in understanding late-stage stellar evolution within dense stellar environments. Additionally, Earth-orbiting satellites Uhuru and Chandra X-ray Observatory have detected two bright X-ray sources in the cluster—Messier 15 X-1 and Messier 15 X-2—with the former being the first astronomical X-ray source detected in the constellation Pegasus. For amateur astronomers, M15 is a rewarding target, approaching naked-eye visibility at magnitude 6.2 and revealing its dense core and individual stars through modest telescopes.

Did You Know?

• The cluster has a double neutron star system called M15-C.
• Two bright X-ray sources, Messier 15 X-1 and Messier 15 X-2, have been detected in the cluster by the Uhuru and Chandra X-ray Observatory satellites.