First Look at a Pulsar

In the 11 months since British astronomers announced the discovery of pulsars, scientists have done a brilliant detective job of piecing together the nature of the strange, regularly beeping radio sources. Their effort has been all the more remarkable because they have never actually seen a pulsar; all of their clues come from radio signals picked up by giant radio telescopes.

Now a new avenue of investigation has opened in the skies. Three University of Arizona astronomers have spotted a visible star that is located precisely where radio telescopes had detected a pulsar and is flashing at a rate identical to the pulsar's beeps. In all likelihood, they say, the flashing star is actually a pulsar.

Good Reasons. In setting out to make a visual sighting of a pulsar, Astronomers John Cocke, Michael Disney and Donald Taylor defied the beliefs of more experienced astronomers who were certain that the strange objects would be too small and distant to be seen through terrestrial telescopes. Undaunted, they pointed the 36-in. telescope at Arizona's Steward Observatory toward a small star in the Crab nebula, the glowing, cloudlike remnant of a supernova (stellar explosion) that was first witnessed from earth in 1054 A.D.

The Arizona astronomers had three good reasons for picking their target: 1) most scientists now believe that pulsars are neutron stars, small and incredibly dense spheres that are residues of exploded stars like the one that formed the Crab nebula; 2) a pulsar had recently been detected in the Crab by radio telescopes and 3) the Crab pulsar, or neutron star, beeps faster than any discovered to date. Thus it is presumably younger, hotter and brighter, and could be seen more easily than any other.

Corresponding Flashes. Although the target star seemed to be shining steadily, the astronomers fed its light into an electronic device that made 12,000 separate light-intensity measurements every second. They quickly discovered that the starlight increased to a peak about 30 times per second, a variation too rapid to be detected by the human eye. The flashes corresponded exactly to the radio pulses from the Crab pulsar, strongly suggesting that the target was indeed the pulsar. Unlike an earlier and apparently erroneous sighting of a flashing pulsar (TIME, May 31), this discovery was confirmed by the McDonald Observatory in Texas and Arizona's Kitt Peak National Observatory.

Astronomers do not yet know why the Crab pulsar flashes, but some suspect that its bursts of light are generated by charged particles in its intense magnetic field. Now that they have seen the light, scientists are attempting to take a clear spectrograph of the pulsar and to determine if the light itself is polarized. They are investigating a new report by astronomers at an observatory on Malta, who saw strange "ghost" flashes about 3DEG to either side of both the Crab pulsar and another nearby pulsar. With these new clues, scientists hope to be able to learn more about the physical characteristics of the neutron star and move closer to a complete solution of the great pulsar mystery.

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