M15 - Globular Cluster, Pegasus
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Globular cluster M15 was discovered by Jean-Dominique Maraldi on September 7, 1746 while he was looking for De Chéseaux' comet; he described it as 'A nebulous star, fairly bright and composed of many stars'. Charles Messier, who cataloged it on June 3, 1764, and Johann Elert Bode couldn't make this out and described it as 'nebula without stars' so that it remained to William Herschel in 1783 to resolve this fine star cluster.

Globular cluster M15 is among the more conspicuous of these great stellar swarms. At a distance of about 33,600 light years, its diameter is about 175 light years, and its total visual brightness of 6.2 magnitudes corresponds to an absolute magnitude of -9.17, or roughly 360,000 times that of our sun.

M15 was the first globular cluster in which a planetary nebula, Pease 1 was discovered. It is one of only four known planetary nebulae in Milky Way globular clusters. M15 also contains the considerable number of 9 known pulsars

M15 can be found extremely easily: Find the 2nd mag star Epsilon Pegasi, and Theta Pegasi SE of it. Follow the line from Theta over Epsilon and find M15 3 1/2 deg W and 2 1/4 deg N of Epsilon. A 6th mag star is about 20' away to the East, another one of mag 7.5 about 5' to the NNE.

With its apparent visual brightness of magnitude 6.2, M15 is about at the limit of visibility for the naked eye under very good conditions. The slightest optical aid, small binoculars, reveals it as a round nebulous object. It appears as a round mottled nebula in 4-inch telescopes, with at best the very brightest stars visible, but otherwise unresolved in a fine star field. In larger telescopes more and more stars become visible the outer parts are resolved, with a more irregular, non-circular outline. The compact core, however, stays unresolved even in large amateur telescopes, but the brightest stars can be glimpsed even there. Chains and streams of stars seem to radiate out of this core in all directions, but less concentrated toward the West.

  Location J65 Celbridge Observatory
  Date/Time (UTC) 29 August 2006, 22:00
  Optics 200mm SCT, F6.3 reducer
  Filters None
  Imager Meade DSI-C
  Mount Astro-physics 1200GTO
  Exposure details 10 images * 30 seconds
  Software MaximDL, Photoshop 7
© David McDonald 2012 Updated 10 June 2012