M15, Globular Cluster in Perseus

Image by Danny Flippo.
Globular cluster M15 is among the more conspicuous of these great stellar swarms. At a distance of about 33,600 light years, its diameter of 18.0 arc min corresponds to a linear extension of 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. Its brightest stars are about of apparent magnitude 12.6 or absolute magnitude -2.8 or a luminosity of 1,000 times that of our Sun, and its horizontal branch giants are about of magnitude 15.6. Its overall spectral type has been determined as F3 or F4. The globular cluster is approaching us at 107 km/sec.

In amateur instruments, M30 appears somewhat smaller, perhaps about 7 arc minutes visually and 12.3 arc minutes photographically. On the other hand, the tidal radius of this globular cluster, beyond which member stars would escape because of the Milky Way galaxy's tidal forces is a bit larger: 21.5 arc minutes, corresponding to a distance of 210 light years from the cluster's center.

This globular cluster has the third rank in known variable star population, after M3 and Omega Centauri; a total of 112 variables have been identified. One of them is apparently a Cepheid of Type II (a W Virginis star).