A Celestial Odd Couple

By MICHAEL D. LEMONICK.

Astronomers are used to dealing in extremes, but no one had ever discovered anything like this before: a pair of superdense stars a mere 80,000 miles apart, locked in a gravitational embrace, one orbiting the other every 11 1/2 minutes and spewing X rays at a temperature of 50 million degrees Fahrenheit. Says Nicholas White of the European Space Agency's EXOSAT observatory in Darmstadt, West Germany: "What we've got is a system you could fit between the earth and the moon that generates 100,000 times more luminosity than the sun."

That luminosity is largely manifested not in visible light but in invisible X rays that every 11 1/2 minutes, like clockwork, momentarily become more intense. It was the brilliant radiation that enabled EXOSAT, a European X ray- detecting satellite, to find the binary stars, which were hidden from ordinary telescopic view in a globular cluster of stars 20,000 light years away from the earth.

The newly discovered stars are among the more bizarre inhabitants of the celestial zoo. According to White and his colleagues, one member of the strange duo is a neutron star -- the burned-out remnant of a large star that has collapsed under its own gravity and then exploded, leaving behind a spinning, tightly packed ball of neutrons. Incredible as it seems, that ball, which is more massive than the sun, is only ten miles or so in diameter and is so dense that a cubic inch would weigh 100 billion tons on earth. Its partner in the celestial dance is a white dwarf, a dying star once comparable in size and mass to the sun that has burned up its fuel and shrunk to about three times the size of the earth. But the dwarf still glows like an ember as it slowly radiates away the last of its heat, and, while nowhere near as squeezed as a neutron star, it is still thousands of times as dense as the sun.

Put the two close together, and the fireworks begin. The monumental gravity of the neutron star raises such high tides on its companion that gases are torn wholesale from the white dwarf's surface and pulled into orbit around the neutron star, forming a so-called accretion disk. Some of that material continuously spirals down to smash into the surface of the neutron star -- at a rate of a trillion tons a second -- striking so violently that it literally explodes. Says Co-Discoverer William Priedhorsky of Los Alamos National Laboratory: "A neutron star can convert about 10% of the mass that falls on it into radiation. If you toss on a marshmallow, you get out the energy of a Hiroshima bomb." A trillion-ton marshmallow every second makes an even bigger splash; the stupendous energy from this perpetual explosion radiates outward | as a steady torrent of X rays.

Priedhorsky, White and Colleague Luigi Stella, also of the European Space Agency's observatory, were in Darmstadt analyzing data from the now defunct EXOSAT satellite when they made their discovery. They had been looking for variations in the luminosity of a familiar X-ray source known to astronomers as 4U1820-30. The very fact that the mysterious source emitted X rays implied that it was a binary system consisting of a star feeding matter and energy to a ravenous neutron-star companion -- and variations in X-ray intensity would provide clues to the nature of that system. In most of the known X-ray binary systems, the periodic irregularities caused by the orbital motions of the stars are several hours apart; thousandth-of-a-second variations, on the other hand, are generated by the rapid rotation of the neutron star.

Not this time. "Priedhorsky noticed an occasional ripple in the data," says White, "so we ran a quick analysis, and to our astonishment, there was a peak every 685 seconds." Reviewing data from earlier observations, the team found evidence for the same pattern going back at least ten years, and the fact that the period had remained virtually unchanged in that time confirmed that the variations were orbital in character. If they had been caused by the rotation of the neutron star, they would gradually have sped up. Reason: angular momentum of the material zooming in from the accretion disk is added to the momentum of the neutron star, increasing its spin rate. Given the 11 1/ 2-minute orbital period of the star system and the mass of known neutron stars (they are all approximately the same), the scientists were able to calculate that the stars were separated by only 80,000 miles.

The consensus among binary-star experts is that the system was created when two previously unrelated stars collided or had a near miss. Scientists believe such encounters are common in globular clusters, where stars are packed a million times as tightly as they are in the neighborhood of our solar system. And all seem to agree that the discovery will serve as a cosmic laboratory, yielding information about the dynamics and evolution of interacting stars. Says Princeton Astrophysicist Bohdan Paczynski: "It is a type of binary that has not been seen before -- a white dwarf close enough to lose mass to a neutron star. It is almost a dream system to study because it is so simple. Usually, nature is not so generous."

With reporting by James L. Graff/Munich and Edwin M. Reingold/Los Angeles