M15 is one of the brightest and finest globular clusters in the northern section of the sky and the best deep-sky object in the constellation of Pegasus. It is only marginally fainter and smaller than M13, the finest northern globular cluster. M15 is relatively easy to find, located 4 degrees to the northwest of magnitude +2.4 star Enif (Epsilon Pegasi - e Pegasi) and positioned on one edge of a right-angled triangle made up of stars of 6th, 7th and 8th magnitudes.
Approaching naked eye visibility under excellent conditions, this globular cluster is easily observed with binoculars or finder scopes, appearing as a magnitude +6.2 fuzzy star. A 100mm (4-inch) telescope at low power (40x) reveals a uniformly lit disk while high powers (>100x) hint at resolution of some of the outer stars. When viewed through a larger 200mm (8-inch) telescope, M15 appears as a large bright diffuse ball surrounding a dense compact centre region, with many individual stars resolved in the outer halo. The brightest of these stars are of magnitude +12.6. Larger telescopes do even better. A 300mm (12-inch) scope resolves many stars across the complete disk, creating a spectacular 3-dimensional effect. In total, the globular has an apparent diameter of 18 arc minutes, however in amateur telescopes it appears somewhat smaller, perhaps only 8 arc minutes visually.
M15 is one of the most densely packed globulars known in the Milky Way galaxy. Its core has undergone a contraction known as "core collapse" resulting in an enormous number of stars surrounding what may be a central black hole. It is also unusual in that it is one of only four known globulars (along with M22, NGC 6441 and Palomar 6) that contains a planetary nebula. The planetary nebula is named Pease 1.
The cluster was discovered by Italian born French astronomer Jean-Dominique Maraldi on September 7, 1746 and is located 33,600 light-years from Earth. With an age of at least 12.0 billion years, it's thought to be one of the oldest known Milky Way globular clusters.
M15 Data Table
|Object Type||Globular Cluster|
|RA (J2000)||21h 29m 58s|
|DEC (J2000)||12d 10m 00s|
|Apparent Size (arcmins)||18 x 18|
|Radius (light years)||88|
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