Last April NASA organized a press conference to herald some remarkable news: the possible discovery of two quark stars, objects even smaller than neutron stars. These long-theorized objects would be so dense that their neutrons would be squashed apart to form a sea of free quarks — a totally new state of matter. Now two Stanford University astronomers have bolstered critics who said the announcement was premature.
Timothy M. Braje and Roger W. Romani examined the spectrum of
one of the stars, RX J1856–3754, in X-rays, ultraviolet, and visible light. They find evidence that the spectrum comes from an ordinary, normal-sized neutron star about 27 kilometers in diameter with a smaller, hotter, brighter polar cap that creates the illusion of a smaller body. The quark-star interpretation is "strongly excluded" by their work, Braje and Romani write in a paper submitted to the Astrophysical Journal. They conclude that the star is probably a normal young pulsar whose radio beams are directed away from our line of sight.
But Jeremy Drake (Harvard-Smithsonian Center for Astrophysics), who proposed the quark-star model, isn't convinced. A small quark star might yet produce the observed spectrum, he says. "Model atmospheres [of such dense stars], with very strong magnetic fields for example, haven't been investigated in detail. There may be model atmospheres that fit the spectrum with only one temperature."