Messier 15
Ancient globular cluster in Pegasus with a collapsed core.
Messier 15 (also cataloged as NGC 7078 and nicknamed the Great Pegasus Cluster) is a globular cluster located in the constellation Pegasus. First spotted by Jean-Dominique Maraldi in 1746, it was later added to Charles Messier’s catalog of comet-like objects in 1764. With an age of roughly 12.5 billion years (give or take 1.3 billion), it ranks among the oldest globular clusters known.
The cluster lies about 35,700 light-years from Earth and spans roughly 175 light-years across. Its absolute magnitude is −9.2, making it about 360,000 times more luminous than the Sun. M15 is one of the densest globular clusters in the Milky Way. Its core has undergone a process called core collapse, resulting in a dense central cusp packed with stars, possibly surrounding a black hole. The cluster contains over 100,000 stars, including 112 variable stars and 8 pulsars. One of these pulsars, M15-C, is a double neutron star system. M15 also hosts Pease 1, the first planetary nebula ever found inside a globular cluster (discovered in 1928); only three others have been found in globular clusters since.
For amateur observers, M15 shines at magnitude 6.2, making it nearly visible to the naked eye under good conditions. Binoculars or a small telescope show it as a fuzzy star, while a telescope with at least a 6-inch (150 mm) aperture begins to resolve individual stars, the brightest of which are magnitude +12.6. The cluster appears about 18 arc minutes across (roughly three-tenths of a degree). It sits about 4° west-northwest of Epsilon Pegasi, the brightest star in Pegasus.
Two bright X-ray sources have been detected in M15 by the Uhuru and Chandra X-ray Observatory satellites: Messier 15 X-1 (also known as 4U 2129+12) and Messier 15 X-2. The former is thought to be the first X-ray source ever detected in the constellation Pegasus.
Quick Facts
- Epoch
- J2000
- Class
- IV
- Constellation
- Pegasus
- Ra
- 21 · 29 · 58.33
- Dec
- +12 · 10 · 01.2
- Dist Ly
- 10.944 ±
- Appmag V
- 6.2
- Radius Ly
- ~88 ly
- Metal Fe
- –2.37
- V Hb
- 15.83
- Age
- 12.0 Gyr
- Notes
- steep central cusp
Facts from the source article.
Lore & Background
Messier 15 is one of the most densely packed globular clusters 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. Home to over 100,000 stars, the cluster is notable for containing a large number of variable stars (112) and pulsars (8), including one double neutron star system, M15-C.
Reader's Guide
Messier 15 is significant as one of the oldest known globular clusters, offering insights into early stellar evolution and galactic history. Its core collapse and possible central black hole make it a key object for studying dense stellar dynamics. The presence of numerous variable stars, pulsars, and a double neutron star system provides valuable data for astrophysics. The discovery of Pease 1, the first planetary nebula in a globular cluster, highlights the cluster's unique role in stellar lifecycle studies. For amateur astronomers, 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. Larger telescopes reveal individual stars, the brightest of which are of magnitude +12.6. Its location about 4° WNW of Epsilon Pegasi makes it accessible to observers.
Did You Know?
- Messier 15 is one of the oldest known globular clusters, with an estimated age of 12.5±1.3 billion years.
- It contains Pease 1, the first planetary nebula discovered within a globular cluster, found in 1928.
- The cluster has a core that has undergone 'core collapse' and may contain a central black hole.
- Two bright X-ray sources, Messier 15 X-1 and X-2, have been detected in the cluster by satellites Uhuru and Chandra.
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