Astronomers today presented pictures taken with the Hubble Space Telescope of the heart of M15, a dense cluster of stars within our own Galaxy. The pictures show for the first time that M15 is in the process of recovering from a deep implosion of its core regions, caused by a massive gravitational instability. Many other star clusters may have experienced a similar collapse, in which their central stars crowd into a compact aggregate, causing a sharp rise in central density. This process may also happen in the dense centers of galaxies, where it may lead to the formation of massive black holes. The analysis of the Hubble images was presented by Dr. Tod R. Lauer of the National Optical Astronomy Observatories, Tucson, Arizona, Dr. Jon A. Holtzman of Lowell Observatory, Flagstaff, Arizona, Dr. Sandra M. Faber of Lick Observatory, Santa Cruz, California, and fellow members of the Hubble Wide Field/Planetary Camera imaging team, at the American Astronomical Society meeting in Philadelphia, Pennsylvania.
M15 is a globular cluster, which is a dense spherical swarm of nearly a million stars bound by their mutual gravity. Globular clusters are very old objects, scattered about the galaxy in a loose halo that reflects the galaxy's shape at formation. M15 is roughly 200 light years in diameter and is located in the constellation Pegasus at a distance of 42 thousand light years. It is easily visible in a small telescope, but not even the most powerful telescopes on Earth have been able to resolve the mysterious, dense structure containing the central-most concentration of stars.
Images taken by the Hubble now show the center of M15 in clear detail, and have allowed Lauer, Holtzman, and Faber to describe the present physical conditions at the heart of the cluster. Stars in the center of M15 are found to be packed together with a density nearly a million times that of stars in our own region of the Galaxy. The Hubble pictures show that there are nearly 7,000 stars within the central core of the cluster. This core is tiny, only 0.8 light years across. For comparison, this is just one fifth the distance between the Earth and the nearest star outside the Solar System.
The mystery at the heart of M15 comes from the complex way gravity shapes systems comprising more than just a few objects. Examples of the unusual structures that result from gravitational interaction in complex systems include the fine banding and braiding of the particles that make up Saturn's rings and the spiral structure of many galaxies. In the case of a globular cluster, there is no stable configuration that its swarm of stars can assume, and the core of the cluster is unstable to central gravitational collapse. As stars orbit within the cluster, their orbital paths sometime bring them close to one another. As two stars interact gravitationally, one of the stars tends to lose energy and fall deeper into the cluster, while the other gains energy and moves outward; the more massive a star is, the more energy it tends to lose and the deeper it falls into the center. The cluster's life story is one of steady evolution as the heavier stars slowly fall inwards, causing the center of the cluster to become increasingly denser and more compact.
It was once believed that this process would run away, ending in the formation of a single, central, giant black hole; indeed that may happen in the centers of galaxies. Astrophysicists now believe that the collapse process in star clusters may be reversed when pairs of stars approach each other closely enough to form tightly bound binary systems. Other stars in the cluster center then interact with the binary pairs, drawing gravitational energy from them and reversing the implosion of the cluster. If the cluster could be viewed in fast motion, it would appear to collapse and then bounce back as the energy source of the binaries suddenly becomes important.
The Hubble pictures show clearly that the post-collapse expansion in M15 is well along. From ground-based observations, it was already known that M15 has an extremely dense and compact center; it was the best candidate for a cluster in which central collapse might be in progress. Ground-based imagery is not sufficient to set limits on the central stellar density, produce an accurate map of the central distribution of stars, or deduce the present evolutionary phase of the cluster. The best ground-based data show only a smooth rise in the concentration of stars towards the center, with no signs of the density leveling off at the center. In addition, it is difficult to interpret the ground-based pictures; although there are thousands of stars at the center of M15, its light is dominated by just a few red giant stars that greatly outshine everything else. With the Hubble, the resolution of the images is sharp enough (even as provided be the present optics) that Lauer, Holtzman, and Faber could use computer modeling techniques to remove these bright stars from the images. They were thus able to isolate the smooth background structure of the cluster comprising the remaining, less luminous stars. The surprising result is that M15 has evolved much further past the point of maximum central density than the ground-based data had suggested. Despite its high density, M15's center is less dense, by at least a factor of 100, than it would appear just at the moment of extreme core collapse. This suggests that M15 has already passed its point of maximum collapse and is expanding outward, powered by binary stars formed during the collapse.
The Hubble pictures may also rule out the possibility that there is a massive black hole at the cluster center. Some astronomers have suggested the presence of a black hole with a mass of up to 1,000 solar masses, based on the high velocities observed for stars at the center of the cluster. If a black hole were present, however, the Hubble images should show a strong concentration of stars closely bound to it. The smooth, diffuse nature of the center as seen by the Hubble appears to contradict this picture. While this finding might be viewed as disappointing to those who had hoped for proof of a massive black hole, it shows the true power of the Hubble in its ability to test conclusions drawn from a limited ground-based perspective. The Hubble has provided the first complete look at the central structure of an evolving cluster of stars, and in doing so has allowed astronomers to pinpoint the present evolutionary phase in the cluster's life.
The National Optical Astronomy Observatories comprise three major facilities: Kitt Peak National Observatory, Arizona; Cerro Tololo Inter-American Observatory in Chile; and the National Solar Observatory, with telescopes at Sacramento Peak, New Mexico, and Kitt Peak, Arizona. These facilities are operated for the U.S. national community of astronomers, to whom they are allocated on the basis of the relative merit of research proposals. NOAO is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.