A star that may tear itself apart

For the past month, the fastest pulsar in the sky has riveted astronomers' attention. It looks like a star about to fly apart.

A pulsar is an object that sends out pulses of radio waves and sometimes of light with clocklike precision. Many astronomers think the object is spinning, sending out beams that sweep past observers like those of a lighthouse.

The pulses are coming 642 times a second. That's roughly 20 times the rate of the fastest previously known pulsar - the 900-year-old remnant of a supernova star explosion that produced the Crab Nebula.

The new pulsar appears to spin so fast that there has been much speculation since its discovery that it may be close to flying apart. Furthermore, it seems scarcely to be slowing down at all. ''Normal'' pulsars, such as that in the Crab , start out with maximum spin and then slow down at a measurable rate.

Carl E. Heiles of the University of California at Berkeley, one of the pulsar's discoverers, recently has suggested that the new pulsar may work the other way around. It may have started out rotating slowly and have subsequently been speeded up by the influence of a companion star until it now is on the verge of rotational breakup.

Pulsars are thought to form in the supernova explosion of a star. This outburst throws off a shell of dust and gas, leaving a remnant so highly compressed that the electrons of its atoms are squeezed into the atomic nuclei to create a dense mass of neutrons. This is called a neutron star.

When newly formed, such a neutron star should not only be spinning at its fastest rate, it should also be hot enough to emit lots of X-rays. One of the puzzling features of the new pulsar is that no X-rays have yet been detected from it. This would indicate that it has been cooling down for millions of years. Yet its fast spin implies a very young object.

Heiles suggests that the neutron star is indeed old, formed 10 million years ago in the explosion of one member of a double star. Then, about a million years ago, it began to draw in gas from the other star. This inflowing gas has spun it up to its present high rotation.

The discovery of this unusual pulsar was the culmination of several years of growing interest in the radio source known as 4C21.53. Its identification as a pulsar came out of research by Donald C. Backer, Shrinivas Kulkarni, and Carl Heiles of Berkeley; Michael Davis of the Arecibo Ionospheric Observatory in Puerto Rico; and Miller Goss of the University of Groningen in the Netherlands. It now has the pulsar designation 1937+215.

Since announcement of the discovery Nov. 18, it has attracted worldwide scientific attention. The journal Nature calls it ''obviously a landmark in astrophysics.'' It will be the focus of intense research in 1983.

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