Messier 15

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.
Observation data (J2000 epoch)
Class	IV
Constellation	Pegasus
Right ascension	21h 29m 58.33s
Declination	+12° 10′ 01.2″
Distance	33 kly (10 kpc)
Apparent magnitude (V)	+6.2
Apparent dimensions (V)	18′.0
Physical characteristics
Mass	7005560000000000000♠5.6×105 M☉
Radius	~88 ly
VHB	15.83
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –2.37dex
Estimated age	12.0 Gyr
Notable features	steep central cusp
Other designations	NGC 7078, GCl 120
See also: Globular cluster, List of globular clusters

Messier 15 or M15 (also designated NGC 7078) is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764. At an estimated 12.0 billion years old, it is one of the oldest known globular clusters.

M15 is about 33,600 light-years from Earth, 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.

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. M15 also contains Pease 1, the first planetary nebula discovered within a globular cluster in 1928. Just 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. Telescopes with a larger aperture (at least 6 in./150 mm diameter) will start to reveal individual stars, the brightest of which are of magnitude +12.6. The cluster appears 18 arc minutes in size.