Researchers may finally have hit on why pulsars, the rotating balls of neutrons that pepper the universe, spin the way they do. Simulations indicate that the key may be a wobbling shock wave that accompanies the explosion of a dying star.

When stars a few times heavier than our own sun run out of fuel, they collapse into ultradense pulsars. The hallmark of that collapse is a supernova explosion, which scours away much of the star's prior mass. In principle, the resulting pulsar should spin much more rapidly than its larger parent star, like a figure skater drawing her arms toward her to twirl faster.

"If you apply that [idea], the neutron stars are spinning so fast they would just break themselves apart," says astrophysicist John Blondin of North Carolina State University. "Something must slow them down." Researchers speculate that magnetic fields piercing the stars act as a brake, but they do not know if those fields can slow pulsars down to their observed range of speeds.

Blondin and his colleague Anthony Mezzacappa of Oak Ridge National Laboratory propose that a shock wave rumbling through the star during its collapse is the real culprit. Prior simulations of the process in 2-D had found that as the star's material condenses into a hard nugget, a powerful shock wave reverberates through its core and spreads outward.

The researchers' 3-D simulation picked up something the past models had missed: The shock wave rotates. Reporting in this week's Nature, they propose that this rotation causes the collapsing star to turn in the opposite direction that the shock wave moves.

After about a second, the rotation would be fast enough to account for typical pulsar speeds of one rotation every 300 milliseconds, they find. "The thing that got us excited," Blondin says, "is we naturally explain the observed pulsar periods."

But there is a hitch: Magnetic fields or some other effect would have to drastically slow down the core first, so the shock wave could dial it up to the right speed. That would require magnetic fields to be more effective brakes than current crude estimates suggest, says astrophysicist Roger Chevalier of the University of Virginia. He notes that those estimates already put pulsars in the right ballpark.

Blondin is optimistic, though. Without the shock wave effects, he says, "we have no theory that explains why they slow down to 300 milliseconds."

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