A neutron star weighs in at a billion tons per teaspoon. Typically, such a star is a 10-mile-wide ball made mostly of neutrons left over from a supernova explosion. Scientists had long thought matter could be squashed no more tightly, but now some suspect they may have discovered stars made of something denser: a hitherto theoretical stuff called "strange quark matter."
The evidence comes from two teams using NASA's orbiting Chandra X-ray Observatory. One measured radiation from 3C58, a neutron star 10,000 light-years from Earth, possibly created by a supernova noted in Asian records from 1181. The team expected 3C58 to be 1.5 million degrees Celsius, but found it to be less than half as hot. This could mean the star is made of denser material than they figured. Meanwhile, another team looked at RXJ1856, a star about 390 light-years from Earth, and found it substantially smaller than neutron stars are generally believed to be. Perhaps these are new kinds of stars: quark stars.
Quarks—the fundamental particles that make up protons and neutrons—come in six "flavors," grouped into whimsically named pairs: up/down, charm/strange, and top/bottom. They're bound together in larger particles called hadrons, which include protons and neutrons. Two up quarks and a down quark make a proton; one up and two downs make a neutron. Because other flavors are more massive, combinations involving them are unstable and break down into up and down quarks.
Except, perhaps, under "strange" circumstances. In 1984, physicist Edward Witten hypothesized that at extremely high densities, nuclear matter could melt into its component quarks, remaining stable as a mixture of up, down, and strange quarks. That proposal has led physicists to look for nuggets of strange quark matter in particle accelerators, while astronomers search for quark stars. Have the astronomers found them at last? They really aren't sure. They point out that there are other possible explanations for what they've seen. But they have more motivation than ever to keep looking.