Messier 15

 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.^{[citation needed]} At an estimated 12.5±1.3 billion years old, it is one of the oldest known globular clusters.^[10]

Messier 15
Deep Broadband (RGB) image of M15
Observation data (J2000 epoch)
Class	IV[1]
Constellation	Pegasus
Right ascension	21h 29m 58.33s[2]
Declination	+12° 10′ 01.2″[2]
Distance	35.69 ± 0.43 kly (10.944 ± 0.131 kpc)[3]
Apparent magnitude (V)	6.2[4]
Apparent dimensions (V)	18′.0
Physical characteristics
Mass	5.6×105[5] M☉
Radius	~88 ly[6]
VHB	15.83
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –2.37[7] dex
Estimated age	12.0 Gyr[8]
Notable features	steep central cusp
Other designations	NGC 7078, GCl 120[9]
See also: Globular cluster, List of globular clusters

CharacteristicsEdit

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] Just three others have been found in globular clusters since then.^[14]

Amateur astronomyEdit

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] 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 4.2° WNW of the brightest star of Pegasus, Epsilon Pegasi.

X-ray sourcesEdit

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.

GalleryEdit

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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.