WHEN STARS COLLIDE.

ASTROPHYSICAL collisions; GLOBULAR clusters; NEUTRON stars; STELLAR evolution; X-ray binaries; BLACK holes; FABIAN, Andrew C.; PRINGLE, James E.; REES, Martin J., 1942-

The article discusses stellar collisions. Collisions can occur in star clusters, especially globular clusters, where the density of stars is high and where gravitational interactions heighten the odds of impact. Globular clusters contain stars called blue stragglers that are best explained as the outcome of collisions. And globulars contain an anomalously high number of x-ray sources--again the likely product of collisions. Dense star clusters are a veritable demolition derby. Within these tight knots of stars, observers in recent years have discovered bodies that are forbidden by the principles of ordinary stellar evolution--but that are naturally explained as smashed-up stars. Collisions can modify the long-term evolution of entire clusters, and the most violent ones can be seen halfway across the universe. The 1963 discovery of quasars was what inspired skeptical astronomers to take stellar collisions seriously. Andrew C. Fabian, James E. Pringle and Martin J. Rees of the University of Cambridge suggested that a grazing collision or a very near miss could cause two isolated stars to pair up. Normally a close encounter of two celestial bodies is symmetrical: they approach, gather speed, swing past each other and, unless they make contact, fly apart. But if one is a neutron star or a black hole, its intense gravity can contort the other, sapping some of its kinetic energy and preventing it from escaping, a process known as tidal capture. The neutron star or black hole proceeds to feast on its ensnared prey, spewing x-rays. If the close encounter involves not two but three stars, it is even more likely to produce an x-ray binary. INSET: PROCESSES THAT MAKE COLLISIONS MORE LIKELY.