Wednesday, September 12, 2007

Teardrop in the Sky

A spinning star found feeding on its stellar companion, whittling it down to an object smaller than some planets. Pulsars are the cores of burnt out "neutron" stars that spin hundreds of times per second.

The object’s minimum mass is only about 7 times the mass of Jupiter. But instead of orbiting a normal star, this low-mass body orbits a rapidly spinning pulsar every 54.7 minutes, at an average distance of only about 230,000 miles (slightly less than the Earth-Moon distance).

"This object is merely the skeleton of a star," says study team member Craig Markwardt of NASA's Goddard Space Flight Center in Maryland. "The pulsar has eaten away the star's outer envelope, and all that remains is its helium-rich core."

The system was discovered in early June when NASA's Swift and Rossi X-ray Timing Explorer (RXTE) satellites picked up an outburst of X-rays and gamma rays in the direction of the Milky Way galactic center in the constellation Sagittarius, and named SWIFT J1756.9-2508.
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Scientists think that several billion years ago, the system consisted of a very massive star and a smaller star about 1 to 3 times the mass of our sun. The bigger star evolved quickly and exploded as a supernova, leaving behind a spinning stellar corpse known as a neutron star. Meanwhile, the smaller star began to evolve as well, eventually puffing up into a red giant whose outer envelope encapsulated the neutron star.

This caused the two stars to draw closer together, while simultaneously ejecting the red giant's envelope into space.

After billions of years, little remains of the companion star, and it's uncertain whether it will survive. "It's been taking a beating, but that's part of nature," said study team member Hans Krimm, also of NASA Goddard.

Today, the two objects are so close to each other that the neutron star's powerful gravity siphons gas from its companion to form a spinning disk around itself. The disk occasionally dumps large quantities of gas onto the neutron star, creating an outburst like the one detected in June.

Image Credit: Aurore Simonnet/Sonoma State University

Hubble Captures Stars Going Out in Style
Planet Survives Star's Death Throes from LiveScience
The Universe through the looking glass from NASA Science

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