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Wednesday, September 22, 1999 Published at 11:35 GMT 12:35 UK


Chandra views stellar wreckage

The wreckage of a star that exploded thousands of years ago

By BBC News Online Science Editor Dr David Whitehouse

Nasa animation of supernova explosion (Nasa)
Images from the recently launched Chandra X-ray Observatory show previously unseen features in the wreckage of three exploded stars.

When a star explodes in a so-called supernova explosion, it can shine as bright as a galaxy of more well-behaved stars. Superhot gas at temperatures of millions of degrees is scattered throughout space.

Sometimes there can be left behind a superdense, small, rapidly-spinning object called a neutron star, which can continue to pump energy into the stellar wreckage.

[ image: The pulsar at the heart of PSR0540-69]
The pulsar at the heart of PSR0540-69
Two of the remnants Chandra has observed, designated G21.5-0.9 and PSR 0540-69, show the dramatic effects of this input of energy by the rapidly rotating, highly-magnetised neutron star, as well as the enormous shell structures produced by the explosions.

G21.5-0.9 is about 16,000 light years (1 light year = 10 trillion km/six trillion miles) distant. Chandra's image shows a bright central gas cloud surrounded by a much larger diffuse cloud.

Inside the inner nebula is a bright central source thought to be the rapidly rotating neutron star.

It acts like a powerful generator, creating intense electric voltages that accelerate electrons to speeds close to that of light. Its total output of energy is greater than a thousand Suns.

The appearance of the central nebula is thought to be due to the magnetic field which constrains the motions of the high energy electrons.

New information

"It is a remarkable image," said Dr. Patrick Slane of the Harvard-Smithsonian Center for Astrophysics. "Neither the inner core nor the outer shell has ever been seen before."

Sometimes, spinning neutron stars can give off bursts of radiation like a lighthouse in space. Astronomers call these objects pulsars.

[ image: E0102-72: The result of an explosion thousands of years ago]
E0102-72: The result of an explosion thousands of years ago
A previously known pulsar has been seen in the Chandra image of stellar remnant PSR 0540-69. This pulsar, located in a satellite galaxy to the Milky Way that is 180,000 light years distant, emits pulses of radio, optical, and X-radiation at a rate of 50 every second.

"The Chandra image gives us a much better idea of how this energy source works," said Dr. Stephen Murray, principal investigator for the High Resolution Camera, the X-ray camera used to make PSR 0540-69 image. "You can see X-ray jets blasting out from the pulsar in both directions."

The third new Chandra supernova image is E0102-72. Located in the Small Magellanic Cloud, another satellite galaxy of the Milky Way, E0102-72 is 190,000 light years from Earth.

This object, like G21.5-0.9 and PSR 0540-69, is believed to be the result of a supernova explosion of a massive star several thousand years ago.

"Chandra's gallery of supernova remnants is giving us a lot to think about," said Dr. Fred Seward, of the Harvard-Smithsonian Center for Astrophysics. "We are seeing many things we thought should be there, and many others that we never expected. It is great!"

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