A Mystery Ticking Slower

After Cambridge University astronomers announced the discovery of pulsars last February, scientists conjured up widely differing theories about the nature of the mysterious radio sources. There was unanimity about only one point: pulsars beeped with clocklike regularity. The pulses from space seemed so precisely timed that some scientists advocated their use as a universal time standard more accurate than even an atomic clock. Others suggested that the signals could provide a reliable timing device for astronauts on distant missions or for experimental checks on the theory of relativity. Now new discoveries have undermined these imaginative plans: pulsars, like clocks that need winding, are gradually running down.

The first inkling that pulsars might not be reliable timepieces came after Cornell University astronomers at Arecibo, Puerto Rico, trained their 1,000-ft. radio telescope on a newly discovered pulsar in the Crab Nebula, the glowing remnant of a supernova--or stellar explosion--that was seen from earth in A.D. 1054. Unlike most other pulsars, which have relatively low repetition rates (between one and four per second), the new find was ticking about 30 times per second. Carefully measuring the pulse rate in October and then again in November, the astronomers found that it was slowing down by about one beat in 2,000 every year.

Strange Bodies. Last week it became evident that the slowdown observed in the Crab may well be the rule rather than the exception among pulsars. Completing highly precise measurements on the first four original pulsars that they had discovered, British astronomers found that these, too, were running down --at the barely perceptible rate of about one beat in 10 million every year.

The discoveries made a prophet in his own time of Cornell Astrophysicist Thomas Gold, who last spring predicted that pulsars with faster rates would soon be discovered and that some might well be detected in the process of slowing down. The findings also supported the contention that pulsars are actually neutron stars, strange celestial bodies that were mathematically postulated by scientists in the 1930s but have not yet been proved to exist.

According to theory, neutron stars are formed during the cataclysmic processes that occur in a supernova. They consist entirely of neutrons densely packed into dim spheres that are about ten miles in diameter and weigh more than 10 billion lbs. per cubic inch. Astrophysicist Gold believes that a neutron star has an in credibly intense magnetic field that traps ionized gases expelled from the supernova. As the star and its magnetic field spin, the outmost of the trapped gases are whirled at almost the speed of light until they break away, producing an intense beam of radio waves--the regularly spaced pulses. At the same time, Gold theorizes, the ionized gases exert a drag on the magnetic field, and thus on the star itself, gradually slowing its rate of spin.

Ultimate Proof. Gold's theory neatly accounts for the pulsar in the Crab Nebula and one that pulses at eleven beats per second in another supernova remnant called Vela X. The theory suggests that these pulsars are relatively young and still spinning rapidly. Thus the remaining 24 pulsars discovered to date may well be neutron stars from older supernovas that are spinning--and thus pulsing--more slowly.

The ultimate proof of Gold's theory will come only when a neutron star is actually seen at a point where a pulsar has been located by radio telescopes. But neutron stars are small, emit very little light and are many light-years away. All of that makes optical sighting of any neutron star impossible within today's telescopic technology.

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