Bill Saxton; NRAO/AUI/NSF

ScienceShot: Braking a Dead Star's Wild Spin

Millisecond pulsars spin so fast that the rotation velocity of the speediest one could get you to the moon in 7 seconds. But a new study indicates that these dead stars, which measure just 25 kilometers across but harbor more mass than our sun, would spin even faster if it weren't for something slowing them down. The objects reach such high speeds—spinning up to 716 times a second—because gas falling onto them from an orbiting star spins them up the way falling water makes a water wheel turn. Now calculations presented online today in Science show that as this mass transfer ends, the pulsar's magnetosphere—the region of space where its magnetic field traps charged particles—expands and thus slows the spin in the same way spinning ice skaters do when they extend their arms. As a result, the pulsar can lose over 50% of its rotational energy. Without this process, millisecond pulsars would be even more extreme, possibly sporting periods under a millisecond. These exotic objects aren't just entries in the Guinness Book of Cosmic Records. Material from the companion star can encircle the pulsar in a disk that forms planets. In fact, the millisecond pulsar shown here, named PSR B1257+12, was the site of the first planets found beyond our solar system back in 1991.

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