Messier 15

Messier 15
M15 photographed by HST. The planetary nebula Pease 1 can be seen as a small blue object to the lower left of the core of the cluster.
Observation data (J2000 epoch)
Class	IV
Constellation	Pegasus
Right ascension	21h 29m 58.38s[1]
Declination	+12° 10′ 00.6″[1]
Distance	33.6 kly (10.3 kpc)
Apparent magnitude (V)	+6.2
Apparent dimensions (V)	18′.0
Physical characteristics
Radius	~88 ly[2]
VHB	15.83
Estimated age	13.2 Gyr
Notable features	steep central cusp
Other designations	NGC 7078, GCl 120[1]
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 13.2 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.^[3] 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.^[4]

Home to over 100,000 stars,^[3] 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^[5] in 1928. Just three others have been found in globular clusters since then. [1]

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.^[3] 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.^[3]

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.^[6][7] 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

See also

• X-1 X-ray source
• X-ray astronomy

References

1. ^ ^{a b c} "SIMBAD Astronomical Database". Results for NGC 7078. http://simbad.u-strasbg.fr/Simbad. Retrieved 2006-11-16. 
2. ^ distance × sin( diameter_angle / 2 ) = 88 ly radius
3. ^ ^{a b c d} http://www.astropix.com/HTML/SHOW_DIG/M15_Pease1.HTM
4. ^ 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". Astronomical Journal 125 (1): 376–377. arXiv:astro-ph/0210158. Bibcode 2003AJ....125..376G. doi:10.1086/345574. http://iopscience.iop.org/1538-3881/125/1/376/pdf/1538-3881_125_1_376.pdf. 
5. ^ 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. 
6. ^ 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. 
7. ^ 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
• 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″