Astronomers may have discovered two of the strangest objects in the universe--two stars that appear to be composed of a dense soup of subatomic particles called quarks. If the observations, announced today by NASA, hold up, the two would be the first credible examples of so-called strange stars, also known as quark stars. Such stars would present theorists with a chance to pin down some of the properties of exotic matter.
A strange star is the last incarnation of a medium-mass sun. (Only the heaviest stars become black holes.) When a star dies, it collapses under the influence of its own gravity. If the corpse is medium mass--more than about 1.44 times the mass of the sun--its gravity squeezes together electrons and protons in the stellar material, forming neutrons. At still greater masses, in theory, neutrons might break down into their component quarks. Under enough pressure, half of the neutrons' "down" quarks might turn into strange quarks, creating an even more compact type of matter.
The first star, RXJ1856, is a neutron star about 400 light-years away in the constellation Corona Australis. What's weird is it's small size. Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues took its measure by analyzing the light coming from the star. This enabled them to determine its temperature, which in turn predicts how many x-ray photons it emits. Since x-ray emission is proportional to a star's size, the team was able to estimate the star's size by using measurements of x-ray brightness collected by the orbiting Chandra X-ray Observatory. And that proved to be the rub. "It's about 50% smaller than the range of sizes neutron stars can be," says Drake. Such dense matter, theorists believe, could exist within a strange star--and nowhere else that they can easily imagine.
The second star, 3C58, is about 10,000 light-years away in the constellation Cassiopeia. Born in a supernova explosion that Chinese and Japanese sky-watchers noted in August 1181, the star had cooled down faster than a neutron star is expected to. It's twice as cold, and 16 times as dim, as it ought to be, says David Helfand, an astronomer at Columbia University and a member of the Chandra observation team, but strange stars would probably cool off faster than neutron stars do.
If these two candidates are indeed strange stars, they should help astronomers better understand the nature of subatomic particles. Stars, after all, can squeeze an atomic nucleus to densities unachievable in the lab. But that's only if they truly are strange stars. "It's a very big 'if' right now," says Michael Turner, a cosmologist at the University of Chicago.