| 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. |
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| 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.
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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]
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]
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.
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