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'Star blast' rocks Milky Way

Washington, Feb. 19 (Reuters): A cosmic explosion just across the Milky Way from Earth gave off as much energy in one-tenth of a second as the sun does in 100,000 years, astronomers reported yesterday.

The blast observed on December 27 came from a neutron star ' a collapsed dead star with a sun-like mass squeezed into a sphere just 24 km across ' in the constellation Sagittarius (the Archer).

Even at a distance of 50,000 light-years, the explosion was powerful enough to bounce off the moon and disturb Earth's upper atmosphere, researchers said at a briefing at Nasa headquarters.

A light-year is about 10 trillion km, the distance light travels in a year.

It interfered with many satellites and overloaded receptors on some spacecraft, but was blocked by the atmosphere and had little practical effect on Earth except to disrupt some very low-frequency radio transmissions, the scientists said.

'Astronomically speaking, this burst happened in our backyard. If it were in our living room, we'd be in big trouble,' said Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics.

Neutron stars are rare enough, but this one was an exotic type known as a magnetar, with an ultra-strong magnetic field capable of stripping information from a credit card at a distance halfway to the moon.

If such a blast occurred within 10 light-years of Earth, it could destroy much of the ozone layer and cause mass extinction. Fortunately, the nearest known magnetar is about 13,000 light-years away, astronomers said.

It would have been impossible to see the explosion with the naked eye. It also was impossible for optical telescopes to spot because the blast was brightest in the gamma-ray range, not in the optical light range.

Instruments that measure gamma rays, X-rays and radio waves detected it as a huge spike in energy.

It is only the third time astronomers have detected this kind of blast in our galaxy, and this one is 100 times stronger than the previous two.

More information and images are available online at http://www.nasa.gov/vision/universe/watchtheskies/swift_nsu_0205.html.

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