M 15 is about 35,700
light-years from
Earth,[3] and 175 light-years in diameter.[11] 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.[12]
Home to over 100,000 stars,[11] 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.[13][14] Just three others have been found in globular clusters since then.[15]
Amateur astronomy
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.[11][14] 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).[11] M15 is around 4° WNW of the brightest star of Pegasus,
Epsilon Pegasi.[14]
^Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters". Harvard College Observatory Bulletin. 849 (849): 11–14.
Bibcode:
1927BHarO.849...11S.