A dizzying orbit for a binary star.

A dizzying orbit for a binary star

The closer two orbiting bodies are, the faster they go around. Johannes Kepler determined that back in the 17th century, and it's still true. Known periods of binary stars -- pairs of stars gravitationally bound to each other and orbiting their common center of mass -- range from years for fairly distant pairs to the recently discovered shortest astronomical orbital period on record -- 11 minutes.

The 11-minute orbit belongs to a binary system consisting presumably of a neutron star and a whte dwarf star orbiting each other at a distance of 80,000 miles, about a third of the distance between the earth and the moon. Catalogued as 4U 1820-30 and located about 20,000 lightyears from earth, the system is a strong emitter of X-rays. William Priedhorsky of Los Alamos (N.M.) National Laboratory and L. Stella and N. E. White of the European Space Agency (ESA) found the orbiting system in data taken by ESA's orbiting X-ray observatory, Exosat. "We know of no other double star system with stars this close," says Priedhorsky.

Neutron stars and white dwarfs are two possible end stages for the lives of ordinary stars. They result from a star's collapse, or collapse and explosion, and are, by stellar standards, tiny relics of former greatness. Either one is only about 10 miles across. However, the present orbit of 4U1820-30 would fit well inside any ordinary star, and that raises questions about the relation of its constituents to each other at earlier stages when they each would have been hundreds of thousands of miles across.

F. Verbunt of the Max Planck Institute at Garching, West Germany, proposes that the system formed by a kind of stellar cannibalism: A red giant -- a star in a late but not terminal stage of life -- collided with a neutron star and swallowed it. The neutron star descended into the atmosphere of the red giant, and as it orbited there, it triggered an energy release that eventually blew off the outer layers of the red giant, leaving behind the tiny white dwarf. In other words, the orbit of 4U1820-30 was once inside a red giant star.

Under ordinary circumstances the probability of such a collision would be virtually nil, but to support his supposition, Verbunt points out that 4U1820-30 is located in a globular cluster, one of the very dense clusters of stars that are scattered around the edges of our galaxy. In a globular cluster, where stars are a million times as densely packed as they are in the galaxy at large, such a collision is much more likely. This globular cluster is NGC 6624, located in the constellation Sagittarius. It can be seen with binoculars or a small telescope.