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Some Black Holes May Be ‘Quark Stars’

Think black holes are strange? Understandable, considering these powerhouses of the universe (many times heavier than our sun) are collapsed stars with gravity so strong that even light cannot escape their grasp.

But maybe they're not "strange" enough, some astrophysicists suggest. "Stellar" black holes, ones only a few times heavier than the sun, may actually be something even weirder called a quark star, or "strange" star.

A physics team led by Zoltan Kovacs of the University of Hong Kong sizes up the issue in the current Monthly Notices of the Royal Astronomical Society. Quark stars are only theoretical right now, but "the observational identification of quarks stars would represent a major scientific achievement," Kovacs says.

If quark stars exist, it could prove a theory that normal matter - the stuff of people, planets and stars - isn't stable and could help explain the existence of the "dark matter" that fills much of the universe.

First suggested in 1970, a strange star is a collapsed star that doesn't quite crumple enough to turn into a full-fledged black hole and yet is too heavy to become a so-called neutron star (at least 1.4 times heavier than the sun.) Neutron stars do exist, as astronomer Jocelyn Bell showed with the discovery of a pulsar, a spinning neutron star that streams particles from its poles.

In a quark star, gravity would be so strong that it squeezes the subatomic particles called quarks right out of the protons and neutron building blocks of the original star's atoms. That would leave behind a solid mass of quark stuff called strange matter, hence the name "strange star."

Earlier in the decade, astronomers suggested that a neutron star called RX J1856, about 400 light-years away (one light-year is about 5.9 trillion miles) was about one-third too small and might be a quark star. But a 2004 Nuclear Physics B journal report showed the star's intense magnetic field explained its size, so it really was a neutron star.

So, if size alone won't reveal a quark star, what will? In the new study, Kovacs and his colleagues, Cheng Kwong-sang and Tiberiu Harko, analyze the disks of dust and gas circling supposed black holes. Whipped to high speeds by the intense gravity of a black hole, these disks are thought to heat to high temperatures and emit powerful radiation. For a quark star, the radiation would be about 10 percent less than predicted around a black hole, they find. And a quark star would give off a dim light (called bremsstrahlung emission), unlike a black hole, emitted by a thin layer of electrons on its surface.

Don't expect to see bremsstrahlung emissions any time soon, Kovacs says; observations will take years to make, because the best targets are so distant.

"If we just found one object that was made of strange matter and was stable, that would be an amazing result," says astrophysicist Mark Alford of Washington University in St. Louis.

The find would confirm a "strange-matter hypothesis," suggesting that normal matter will decay into strange matter if it comes into contact with some of the stuff. "It would be a great surprise to most physicists, and most people I think, to discover that matter as we know it is not stable, and it all really 'wants' to turn in to strange matter," Alford adds. So, in theory, like "Ice-9" in Kurt Vonnegut's novel "Cat's Cradle," strange matter could eat up the universe.

But Manjari Bagchi of India's Inter University Centre for Astronomy and Astrophysics in Pune says studying pulsar pairs will reveal whether quark stars exist sooner, based purely on their orbits, not on the brightness or dimness of the stars. Or finding a neutron star that weighs less than the sun would mean that it has to be a quark star, he says, because neutron stars wouldn't have enough gravity to hold their neutrons together at that size.

If strange matter exists, though, Alford suggests it might be the culprit for the dark matter observed only by its gravitational effects. Although dark matter can't be seen (it's literally dark to telescopes), it outweighs normal matter by about six times, judged by its gravitational effects throughout the universe. Some dark matter might just be "strangelets" roaming the cosmos, blasted free from quark stars.

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