Cosmic Kickball

What makes a pulsar spin?

Saturday, August 01, 1998

Pulsars, the dense spinning remnants of exploded stars, contain about the same mass as the sun crushed into a wad of neutrons less than 10 miles wide. These stellar nuggets spin as much as several hundred times a second, a spin that astronomers have believed was inherited from the rotating core of the pulsar's exploded parent star. Recently, however, two astrophysicists proposed that the conventional explanation for what makes pulsars spin is probably wrong.

Henk Spruit, of the Max Planck Institute for Astrophysics in Garching, Germany, and Sterl Phinney at Caltech had been studying the rotation of giant stars. "During the evolution of a star about to go supernova, the outer part of the star expands out to about 100 times the radius of the sun, and the inner part that's going to make the pulsar shrinks to less than 1,000 miles across," Phinney says. And like a spinning skater moving her arms in or out, the outer parts slow down and the inner parts speed up enormously.

But Spruit and Phinney have made detailed calculations showing that fluid friction within the star eventually equalizes the spin so the outer layers speed up at the expense of the inner core. By the time the star explodes, the core is spinning too slowly to make a pulsar. So what makes pulsars spin? Spruit and Phinney studied the physics of the collapsing stellar core. In the conventional model, as the core is crushed and transformed into a block of neutrons, it releases a torrent of subatomic particles called neutrinos. This neutrino jet can kick pulsars across the galaxy at speeds of several hundred miles per second.

Spruit and Phinney discovered that this neutrino outburst, if not focused exactly on the pulsar's center, would spin the pulsar, just as a kicked soccer ball spins as it flies toward the goal. The spinning core of the old parent star, says Phinney, has nothing to do with the spin of its pulsar progeny.

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