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)
Class	IV[1]
Constellation	Pegasus
Right ascension	21^h 29^m 58.33^s[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	5.6×10^5[4] M☉
Radius	~88 ly[5]
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]

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

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 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.

Images

• 
  The central square arcminute of M15 imaged using the lucky imaging technique
• 
  Messier 15 with amateur telescope
• 
  Map showing the location of M15
• 
  Deep Broadband (RGB) image of M15 from the Mount Lemmon SkyCenter

See also

• X-ray astronomy

References

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. 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. arXiv:1008.2755. Bibcode:2010AJ....140.1830G. 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. arXiv:0707.1602. Bibcode:2007ApJ...670..363H. 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. arXiv:1004.2255. Bibcode:2010MNRAS.406.2000M. doi:10.1111/j.1365-2966.2010.16813.x. Mass is from M_PD 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. arXiv:1108.4402. Bibcode:2011ApJ...742...51B. 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. arXiv:0801.0871. Bibcode:2008MNRAS.385.1998K. doi:10.1111/j.1365-2966.2008.12908.x
8. "SIMBAD Astronomical Database". Results for NGC 7078. Retrieved 2006-11-16.
9. 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. arXiv:astro-ph/0210158. Bibcode:2003AJ....125..376G. 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. arXiv:astro-ph/0109359. Bibcode:2001ApJ...561L.101W. doi:10.1086/324561.

External links

• Messier 15, SEDS Messier pages
• Messier 15, Galactic Globular Clusters Database page
• Globular Cluster Photometry With the Hubble Space Telescope. V. WFPC Study of M15's Central density Cusp
• Wikisky.org SDSS image of M15
• Gretton, Roy; Gray, Meghan. "M15 – Globular Cluster". Deep Sky Videos. Brady Haran.
• Messier 15 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Coordinates: 21^h 29^m 58.38^s, 12° 10′ 00.6″