# Astronomy:Messier 15

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)
ClassIV[1]
ConstellationPegasus
Right ascension 21h 29m 58.33s[2]
Declination+12° 10′ 01.2″[2]
Distance33 kly (10 kpc)[3]
Apparent magnitude (V)+6.2
Apparent dimensions (V)18′.0
Physical characteristics
Mass5.6×105[4] M
VHB15.83
Metallicity$\displaystyle{ \begin{smallmatrix}\left[\ce{Fe}/\ce{H}\right]\end{smallmatrix} }$ = –2.37[6] dex
Estimated age12.0 Gyr[7]
Notable featuressteep central cusp
Other designationsNGC 7078, GCl 120[8]

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.

## Characteristics

M 15 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. It also contains Pease 1, the first planetary nebula discovered within a globular cluster in 1928.[11] Just three others have been found in globular clusters since then.[12]

## 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.[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 sources

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.

## References

1. Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters". Harvard College Observatory Bulletin 849 (849): 11–14. Bibcode1927BHarO.849...11S.
2. 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. doi:10.1088/0004-6256/140/6/1830. Bibcode2010AJ....140.1830G.
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. doi:10.1086/521780. Bibcode2007ApJ...670..363H.
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. doi:10.1111/j.1365-2966.2010.16813.x. Bibcode2010MNRAS.406.2000M.  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. doi:10.1088/0004-637X/742/1/51. Bibcode2011ApJ...742...51B.
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. doi:10.1111/j.1365-2966.2008.12908.x. Bibcode2008MNRAS.385.1998K.
8. "Evidence for an intermediate-mass black hole in the globular cluster M 15. II. Kinematic analysis and dynamical modeling". Astronomical Journal. Hubble Space Telescope 125 (1): 376–377. 2003. doi:10.1086/345574. Bibcode2003AJ....125..376G.
9. Cohen, J.G.; Gillett, F.C. (1989). "The peculiar planetary nebula in M 22". Astrophysical Journal 346: 803–807. doi:10.1086/168061. Bibcode1989ApJ...346..803C.
10. 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. doi:10.1086/190561. Bibcode1978ApJS...38..357F.
11. 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. doi:10.1086/324561. Bibcode2001ApJ...561L.101W.