The larger picture shows the central portion of M15, photographed with Hubble's Wide Field Planetary Camera 2. The image is about 28 light- years across. The cluster center is in the upper right, on the highest- resolution part of the image. The inset is an enlargement of the innermost 1.6 light-years of the cluster. Images in ultraviolet, blue, and visual light were combined for this picture, so that the colors roughly correspond to the surface temperatures of stars in M15.
Hot stars appear blue, while cooler stars appear reddish-orange.
The density of stars rises all the way into the cluster center, marked by a green cross. Careful analysis of the distribution of these stars suggest that at some point in the distant past, the stars converged on M15's core, like bees swarming to their hive. A precise reading of the speeds at which stars move near M15's core would reveal whether the stars are packed so tightly because of the influence of a single massive object, or simply by their own mutual attraction. Stars would orbit more quickly in the gravitational grip of a black hole, which would be several thousand times more massive than our sun. Such measurements are time consuming but possible with Hubble.