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Archive for Thursday, April 11, 2002

New stars defy theories on matter

April 11, 2002

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— An orbiting X-ray telescope has found two bizarre objects that may form a new star class and are perhaps filled with a new form of matter  findings that may challenge fundamental theories of particle physics and astronomy.

The objects, called RXJ1856 and 3C58, were thought to be neutron stars, but observations by NASA's Chandra X-ray telescope show that they are too small and too cold to be standard neutron stars.

"In both cases, we are seeing contradictions in these observations relative to standard theory," said Anne Kinney, director of astronomy at the National Aeronautics and Space Administration.

Neutron stars are the remnants of a supernova, a giant star that has collapsed and exploded after burning all of its nuclear fuel. Current theory is that a neutron star is composed of neutrons, the subatomic particles that make up most matter. Neutrons are made up of quarks, one of the most basic of particles.

The observations of RXJ1856 and 3C58 suggest that these stellar objects are smaller and cooler than neutron stars usually are. They may, in fact, be composed entirely of quarks or some other unknown, exotic matter.

At a news conference Wednesday, astronomers announced that RXJ1856 has a temperature of about 1.2 million degrees, too cool for a neutron star, and a diameter of about seven miles, much smaller than the standard. Yet the measurements show that the object has the mass and X-ray emissions of a larger neutron star.

"Conventional theory tells us this object should be a neutron star, comprised entirely of neutrons, with a diameter of about 12 to 20 miles in size," said Jeremy Drake, the lead observer for RXJ1856 and an astrophysicist at Harvard-Smithsonian Center of Astrophysics at Cambridge, Mass.

The smaller size suggests that the object is much more dense, perhaps dense enough to shatter its neutrons and set free the quarks they contain.

Drake said it is possible that the object is composed equally of up, down and strange quarks, all types of the fundamental particle.

Quarks are elemental particles that make up a neutron. The quarks in an ordinary neutron are of two types ÂÂ up and down. Drake said that data from the X-ray telescope suggest that RXJ1856 is composed in equal measure of up and down quarks, plus a particle called a strange quark, which is much denser.

A team led by David Helfand of Columbia University observed 3C58, the neutron star remnant of a supernova observed in the year 1181 by Chinese and Japanese astronomers, and found it to be much cooler than expected.

Helfand said that after a supernova explodes to form a neutron star, it rapidly gives off heat.

"The calculations predict that for a 820-year-old star, the temperature should be a little under 2 million degrees (3.6 million degrees Fahrenheit) and the surface should be glowing in X-rays and emitting about four times as much energy per second as the sun," said Helfand. "Our observations show that the temperature is far lower than that and the radiation is down by at least a factor of 10."

This low temperature "requires a fundamental revision" in what is understood about neutron stars, considered the most extreme form of matter observable in nature, said Helfand, and suggests that a new form of matter may be involved.

"It appears that neutron stars aren't pure neutrons after all  new forms of matter are required," he said. "Our observation suggests that the core of this star is made up of a new kind of matter."

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