Messier 15

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.Template:Cn At an estimated Template:Val billion years old, it is one of the oldest known globular clusters.<ref name="Malley"/>

Characteristics

M 15 is about 35,700 light-years from Earth,<ref name=Bhardwaj/> and 175 light-years in diameter. 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.<ref name="Gerssen2002"> Template:Cite journal</ref>

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. It also contains Pease 1, the first planetary nebula discovered within a globular cluster in 1928.<ref name="Gillettetal1989">Template:Cite journal</ref><ref name=OMeara1998 /> Just three others have been found in globular clusters since then.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref name=OMeara1998 /> 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). M15 is around 4° WNW of the brightest star of Pegasus, Epsilon Pegasi.<ref name=OMeara1998 />

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.<ref name=Forman> Template:Cite journal</ref><ref name=White> Template:Cite journal</ref> The former appears to be the first astronomical X-ray source detected in Pegasus.

Gallery

• M15 core lucky 10pc.gif

  The central square arcminute of M15 imaged using the lucky imaging technique

• M15map.png

  Map showing the location of M15

• New Hubble image of star cluster Messier 15.jpg

  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

• List of Messier objects
• X-ray astronomy

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
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