Messier 15 or M15 (also designated NGC 7078 and sometimes known as the Great Pegasus Cluster) 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 | = –2.37[7] dex |
| Estimated age | 12.0 Gyr[8] |
| Notable features | steep central cusp |
| Other designations | NGC 7078, GCl 120[9] |
![{\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}](http://wikimedia.org/api/rest_v1/media/math/render/svg/4c0821bd80891e071c08e7c7ee8e022baedf522c)
Characteristics
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][14] Just three others have been found in globular clusters since then.[15]
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.[11][14] 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 around 4° WNW of the brightest star of Pegasus, Epsilon Pegasi.[14]
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.[16][17] The former appears to be the first astronomical X-ray source detected in Pegasus.
Gallery
The central square arcminute of M15 imaged using the lucky imaging technique
Map showing the location of M15
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.
See also
References
External links
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