# 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. At an estimated 12.0 billion years old, it is one of the oldest known globular clusters.

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[1]
Constellation Pegasus
Right ascension 21h 29m 58.33s[2]
Declination +12° 10′ 01.2″[2]
Distance 33 kly (10 kpc)[3]
Apparent magnitude (V) +6.2
Apparent dimensions (V) 18′.0
Physical characteristics
Mass 5.6×105[4] M
VHB 15.83
Metallicity ${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –2.37[6] dex
Estimated age 12.0 Gyr[7]
Notable features steep central cusp
Other designations NGC 7078, GCl 120[8]

## CharacteristicsEdit

M15 is about 33,600 light-years from Earth, and 175 light years in diameter.[9] 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.[10]

Home to over 100,000 stars,[9] 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[11] in 1928. Just three others have been found in globular clusters since then.[12]

## 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.[9] 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.[9]

## 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.[13][14] The former appears to be the first astronomical X-ray source detected in Pegasus.

## ReferencesEdit

1. ^ Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters". Harvard College Observatory Bulletin. 849 (849): 11–14. Bibcode:1927BHarO.849...11S.
2. ^ a b Goldsbury, Ryan; et al. (December 2010). "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters". The Astronomical Journal. 140 (6): 1830–1837. Bibcode:2010AJ....140.1830G. arXiv:. doi:10.1088/0004-6256/140/6/1830.
3. ^ Hessels, J. W. T.; et al. (November 2007). "A 1.4 GHz Arecibo Survey for Pulsars in Globular Clusters". The Astrophysical Journal. 670 (1): 363–378. Bibcode:2007ApJ...670..363H. arXiv:. doi:10.1086/521780.
4. ^ Marks, Michael; Kroupa, Pavel (August 2010). "Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way". Monthly Notices of the Royal Astronomical Society. 406 (3): 2000–2012. Bibcode:2010MNRAS.406.2000M. arXiv:. doi:10.1111/j.1365-2966.2010.16813.x. Mass is from MPD on Table 1.
5. ^ distance × sin( diameter_angle / 2 ) = 88 ly radius
6. ^ Boyles, J.; et al. (November 2011). "Young Radio Pulsars in Galactic Globular Clusters". The Astrophysical Journal. 742 (1): 51. Bibcode:2011ApJ...742...51B. arXiv:. doi:10.1088/0004-637X/742/1/51.
7. ^ Koleva, M.; et al. (April 2008). "Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting". Monthly Notices of the Royal Astronomical Society. 385 (4): 1998–2010. Bibcode:2008MNRAS.385.1998K. arXiv:. doi:10.1111/j.1365-2966.2008.12908.x
8. ^ "SIMBAD Astronomical Database". Results for NGC 7078. Retrieved 2006-11-16.
9. ^ a b c d http://www.astropix.com/HTML/SHOW_DIG/M15_Pease1.HTM
10. ^ Gerssen, J; van der Marel, R P; Gebhardt, K; Guhathakurta, P; Peterson, R C; Pryor, C (2003). "Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling" (PDF). Astronomical Journal. 125 (1): 376–377. Bibcode:2003AJ....125..376G. arXiv:. doi:10.1086/345574.
11. ^ Cohen, J. G.; Gillett, F. C. (1989). "The peculiar planetary nebula in M22". Astrophysical Journal. 346: 803–807. Bibcode:1989ApJ...346..803C. doi:10.1086/168061.
12. ^ http://messier.seds.org/more/m015_h2.html
13. ^ Forman W; Jones C; Cominsky L; Julien P; Murray S; Peters G (1978). "The fourth Uhuru catalog of X-ray sources". Astrophysical Journal Supplement Series. 38: 357. Bibcode:1978ApJS...38..357F. doi:10.1086/190561.
14. ^ White NE; Angelini L (2001). "The discovery of a second luminous low-mass X-ray binary in the globular cluster M15". Astrophysical Journal Letters. 561 (1): L101–5. Bibcode:2001ApJ...561L.101W. arXiv:. doi:10.1086/324561.