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By Deborah Zabarenko
Reuters

WASHINGTON (April 10) - Two weird stars -- one too cold, the other too small to fit known astronomical models -- show evidence for a completely new form of matter, astronomers said on Wednesday.

Scientists believe these stars could be made not of atoms, or even of the sub-atomic particles called neutrons, but of free-floating sub-sub-atomic particles called quarks, and strange quarks at that.

NASA's Chandra X-ray Observatory considered the oddball objects by looking at the high level of X-rays they emit. At first, astronomers thought these might be neutron stars, which before this discovery were the most extreme form of matter known.

Neutron stars are left after big stars explode in blasts called supernovae, and their cores collapse in on themselves. Neutron stars are almost unimaginably dense: a teaspoon of neutron star material weighs a billion tons, or as much as all cars, trucks and buses on Earth.

That is because they are composed only of neutrons crammed together, unlike every bit of earthly matter, which is made up of atoms containing neutrons, protons and electrons with lots of space in between.

Astronomers believe the two stars they studied could be even denser that that. Instead of being made of neutrons, they could be made of quarks. Neutrons in a neutron star are made of quarks, but bundled together in relatively roomy groups of so-called confined quarks.

The two stars under observation could be made up of free quarks huddled together, which take up even less space than confined quarks. If that proves true, they would be what astronomers call strange quark stars, objects which have existed so far only in theory.

SMALL, COLD AND EXOTIC

One piece of evidence for this is one of the stars' extremely small size, Jeremy Drake said at a National Aeronautics and Space Administration briefing.

"Until now we've sought to understand nature on the tiniest of scales, involving experiments to look at matter in finer and finer detail," said Drake, of the Harvard-Smithsonian Center for Astrophysics. He said these current observations "might provide a new window on the nature of matter on the tiniest of scales."

His team studied an object known as RXJ 1856, in the constellation Corona Australis, about 400 light-years from Earth. A light-year is the distance light travels in a year, about 6 trillion miles.

Astronomers figured this was a neutron star, but then used the Chandra observatory and the Hubble Space Telescope to determine its size, which was 10 miles or less in diameter -- below the lower size limit for neutron stars, which range from 12 miles to 20 miles across.

One way to produce such a tiny star, Drake said, would be to squeeze a neutron star down to its constituent quarks, creating a strange quark star.

In the case of the second odd star, astronomer David Helfand of Columbia University studied an object known as 3C58, which is located in the constellation Cassiopeia and is about 10,000 light-years from Earth.

Astronomers in Asia became aware of this object in 1181 when it flamed out as a supernova, Helfand said. Going on this historical record, present-day astronomers calculated that the remnant star should have cooled down to about 2 million degree Celsius (35.6 million F) by now. In fact, Helfand said, it is only about 1 million degrees C., making it too cool for a neutron star.

Even a neutron star's density would not be enough to squeeze particles out of this object fast enough to cool it down to this temperature, Helfand said. 3C58 would have to be as much as five times as dense for this to happen.

"Our observation suggests that the core of this object is made of a new kind of exotic material," Helfand said.

Reuters 04/10/02 13:26 ET

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New Form of Matter?