Messier 15

M 15 is about 35,700 light-years from Earth,^[3] and 175 light-years in diameter. 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.^[11]

Home to over 100,000 stars,the cluster 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 in 1928.^[12][13] Just three others have been found in globular clusters since then.^[14]

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.^[13] Telescopes with a larger aperture (at least 6 in. (150 mm)) will start to reveal individual stars, the brightest of which are of magnitude +12.6. The cluster appears 18 arc minutes in size (three tenths of a degree across). M15 is around 4° WNW of the brightest star of Pegasus, Epsilon Pegasi.^[13]

Earth-orbiting satellites Uhuru and Chandra X-ray Observatory have detected two bright X-ray sources in this cluster: Messier 15 X-1 (4U 2129+12) and Messier 15 X-2.^[15][16] The former appears to be the first astronomical X-ray source detected in Pegasus.

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  The central square arcminute of M15 imaged using the lucky imaging technique
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  Map showing the location of M15
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  M15 photographed by HST. The planetary nebula Pease 1 can be seen as a small blue object to the upper left of the core of the cluster.

• List of Messier objects
• X-ray astronomy