Cool star chills stellar theories

A Chandra image of neutron star KS 1731-260, remarkably cool after 12 years of being bombarded with hot gas from a companion star, which is not visible here. KS 1731-260 is the light blue object near the top of this image.

By Richard Stenger
CNN

(CNN) -- A star besieged by unbelievably hot waves of nuclear explosions is ten times cooler than expected, forcing astronomers to rethink their ideas about stellar physics.

For more than a decade, the energy of billions of hydrogen bombs per second battered a neutron star near the center of our galaxy.

Months after the assault ended, an astrophysicist used the Chandra X-ray Observatory to take the temperature of the beleaguered star, only to find that it burned no hotter than nearby stars that escaped the energy onslaught.

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"Twelve years of constant thermonuclear explosions. One would think that would heat things up," said Rudi Wijnands of the Massachusetts Institute of Technology.

"This leaves us wondering whether some neutron stars are in the freezer for a much longer time than previously thought ... or whether they cool down incredibly fast."

Neutron stars are the remnants of massive stars that explode in supernova. The dying stars shed their outer layers while their interiors implode and collapse into dense cores, which exert intense gravitational force.

Like other neutron stars, this particular one, known as KS 1731-260, draws gaseous material or fuel from a healthy companion star.

The swirling material from the donor star collides into the neutron star, setting off glowing explosions of intense X-ray energy.

Such gas siphoning periods can last for weeks or years. Astronomers expected KS 1731-260 to be particularly hot at the end of a 12-year episode, but it turned out to be only about the same temperature as neutron stars that consumed fuel from a neighbor for a week or month.

Astronomers have few clues as to why. Perhaps the neutron star was in a deep freeze for much longer than astronomers imagined.

"We may have identified a new type of neutron star system that can lie dormant for thousands of years," said fellow MIT astronomer Walter Lewin.

Wijnands announced his findings last week at a conference marking the second anniversary of Chandra, the most powerful X-ray telescope in orbit.