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 - ε 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 a finder scope, 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, M15 is 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|