Astronomers have discovered what they say might be a new form of star made of an exotic new type of matter predicted by theory but never seen. Their observations suggest that the matter in such a star is far denser than the already tightly-packed nucleus of atoms on Earth, causing its breakdown into nature's most fundamental particles, called quarks.

Two teams of astronomers peered through the orbiting U.S. Chandra X-Ray Observatory and saw what might at first glance be considered neutron stars shining in x-ray wavelengths.

These stars are between one-and-a-half and three times the mass of our sun and have exhausted their nuclear fuel and collapsed to merely 20 to 30 kilometers across. Composed only of the sub-atomic particles called neutrons, they are unbelievably dense, with a thimbleful of matter weighing a billion tons.

Until now, this has been the most extreme form of matter found.

But a team led by Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Massachusetts has discovered a star 400 light years away that is just 11 kilometers across, too small to be a neutron star. Mr. Drake says this means it could be made of smaller, denser exotic particles called quarks, the building blocks of neutrons and protons. "If you want to fit a bunch of neutrons into a smaller volume, you need to break those neutrons down into their constituent quarks," says Mr. Drake. "If you can undergo this phase transition, you create strange quark matter. Now, a star composed of strange quark matter would then be a smaller object."

Mr. Drake's finding is supported by the siting of a similar star 10,000 light years away by a team led by astronomer David Helfand of Columbia University in New York. Mr. Helfand's group concludes that their star's temperature is less than one-million degrees Celsius, far below the predicted value for a neutron star. "This requires a revision and the existence of new forms of matter in the cores of these stars."

Other astronomers say the discoveries change the view of the structure of matter and open a new window on nuclear physics. University of Chicago astrophysicist Michael Turner puts it this way. "They suggest the existence of a new state of matter that is made of undifferentiated quarks," he says. "If this is indeed the case, then astronomers have provided us with a stunning insight on quarks, the basic building blocks of matter."

Jeremy Drake warns, however, that he could be wrong about his star. What he interprets as an object too small for a neutron star may actually be an x-ray hot spot on a neutron star giving the appearance of something tinier. Normally, such a spot would rotate from view, giving the impression of x-ray pulses, like a lighthouse beacon. But this one does not, favoring the notion of the novel star.

Yet, Mr. Drake then adds a further note of caution. "There's a small chance that we could just miss this pulsation signature just by an unlucky orientation of the spin axis pointing straight down at us or by the unlucky positioning of that hotspot right on the pole of that star. But the chances of this happening are fairly small," he says.

Barring this possibility, Michael Turner of the University of Chicago says the two stars appear to be new members of the stellar family tree. And an expert on compact matter, Norman Glendenning of the Lawrence Berkeley National Laboratory in California, says they are apparently in a class by themselves. "This type of star is unlike another that has so far been observed," says Mr. Glendenning. "The present interpretation of the Chandra data therefore does indeed challenge our ideas about what we have been calling neutron stars."

The University of Chicago's Michael Turner suggests that all neutron stars might be partially or completely quark stars, but he points out that confirmation will require further observations.