At the centers of galaxies one often finds a great increase in the density of stars, which suggests that a supermassive object such as a large black hole is present. Examples of these central peaks are found in the giant elliptical galaxy M87 and, at a much smaller scale, in our own galaxy (see the discussion of supermassive black holes in Chapter 25). Some 20-30 globular clusters also show strong central peaks of brightness. The most strongly peaked known at present is the bright globular M15, in the constellation Pegasus. It has been suggested that M15 may harbor a central black hole. This idea can be tested by measuring the density of stars near the center of the cluster, which can be compared with calculations of what the density should look like for objects of various masses at the center.
The problem with this simple idea is that the central regions of globular clusters are so crowded with stars that it has not been possible to resolve the individual star images. The high resolution provided by the Hubble Space Telescope has made better globular measurements possible, and the core of M15 has now been resolved into individual star images to within about 0.06 light years of the center. The following image shows the center of M15.
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