bservations of two stars, one unusually small and the other unusually cold, have led astronomers to think they are seeing evidence of a new form of matter and a new kind of star, one possibly made of elementary particles known as quarks and denser than any cosmic object other than a black hole.

As a group, quarks are building blocks of larger particles and are normally bound together as protons and neutrons. Now it appears that a certain combination of these quarks are capable of a more independent existence as "strange quark matter" inside quark stars.

Astronomers said yesterday that if their interpretation was correct, the new findings were startling and exciting because they promised to open a new window on the nature of matter on the tiniest scales.

The findings also seemed to challenge the standard model of neutron stars, until now the most extreme form of matter known to scientists.

Quarks have never been unambiguously observed outside a nucleus in Earth-bound laboratories, although theories of free quarks have been discussed for 20 years. Neutron stars are the compact remnants of dying massive stars that explode and collapse, and nothing denser had been observed.

A teaspoon of neutron star material weighs a billion tons, or as much as all the cars, trucks and buses on Earth. Matter in the suspected quark stars would be far denser.

Physicists know of six kinds of quarks, two of which — up and down — make up ordinary matter like protons and neutrons. Theorists have long speculated that up and down quarks can be melded with a heavier kind known as the strange quark to form something called strange matter.

"The combined observational evidence points to a star composed not of neutrons, but of quarks in a form known as strange quark matter," said Dr. Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Dr. Sam Aronson, chairman of physics at Brookhaven National Laboratory on Long Island, where a high-energy particle collider is being used to study quarks, said the discovery, if confirmed, could lead to an understanding of the behavior of freely interacting quarks.

"No one has seen really free quarks," Dr. Aronson said.

Dr. Norman Glendenning, a senior scientist emeritus and quark specialist at Lawrence Berkeley National Laboratory in California, said at least one of the two stars could well be made only of quarks.

"If that is so," Dr. Glendenning said, "this star is in a class quite by itself and will be an astonishing discovery of fundamental significance."

But that does not mean the star is anomalous, other scientists said. If there are two quark stars, they said, such objects may turn out to be fairly common end-products of many exploded stars.

The research, described at a news conference at the National Aeronautics and Space Administration in Washington, was based primarily on data gathered by the Earth-orbiting Chandra X-ray Observatory. Dr. Drake is the lead author of a report to be published in June in The Astrophysical Journal.

Dr. Anne Kinney, director of astronomy and physics at NASA, cautioned, "I'd like to emphasize that this is evidence for, not proof of, a new form of matter."

The Chandra X-ray telescope, with help from the Hubble Space Telescope, studied two objects that were assumed to be neutron stars.

One is designated RXJ 1856, in the constellation Corona Australis, about 400 light-years from Earth. The other object is 3C58, in the constellation Cassiopeia and about 10,000 light-years away.

Analyzing the X-ray data, scientists found RXJ 1856 to be too small to be a neutron star. It is only seven miles in diameter, about half the diameter of a neutron star.

"We need a new theory to give us such a small star," Dr. Drake said.

Scientists also failed to detect the expected X-radiation from the hot surface of 3C58, indicating that its temperature must be far below that predicted for a neutron star.

The research on 3C58 was conducted by Dr. Patrick Slane and Dr. Steven Murray, both of the Center for Astrophysics, and Dr. David Helfand of Columbia University.

"Our observations offer the first compelling test of models for how neutron stars cool, and the standard theory fails," Dr. Helfand said. "It appears that neutron stars aren't pure neutrons after all. New forms of matter are required."

Each team qualified its conclusions.

Dr. Drake said his team's X-ray observations could be misleading if they were actually seeing a "hot spot" on a neutron star, making it only appear to be a small surface. The probability of this was considered small.

A stellar explosion observed by the Chinese and Japanese in 1181 presumably created 3C58, and this is the basis for calculating the object's predicted cooling rate.

The coordinates for the remnant from that explosion are not known precisely. If the temperature estimates actually apply to another object, this part of the research would be undermined.

But if it is the right object, scientists said, even a neutron star's density would not be enough to produce cooling particles fast enough to reduce its temperature to such a low level in the time since 1181. The object 3C58, Dr. Helfand said, would have to be as much as five times as dense for this to occur, indicating that at least the core of the object is made of something other than neutrons, perhaps "a new kind of exotic material."

Neutron stars, which have been known and observed for more than three decades, are formed out of the deaths of stars several times more massive than the Sun. After the star explodes, the core collapses to extremely high densities, which could be the force liberating the quarks.

"A neutron star, because it is so dense, may be the only natural place in the universe where quark matter exists," Dr. Glendenning said. "We may have discovered a way of learning if the existence of free quarks is true."

Dr. Michael Turner, a theoretical astrophysicist at the University of Chicago, suggested that the core collapse of an exploded star could at first produce a hot neutron star. Then, in a few seconds to a few days, the collapse could continue, producing even greater densities and thus freeing quark matter on its own.

If indeed there are quark stars, Dr. Turner said, they would probably be derived from collapsed stars that were 5 to 10 times the mass of the Sun.

The signatures of a quark star, Dr. Turner said, would seem to be its small size and rapid cooling.

"Regardless of how these mysteries are resolved, these precise observations are highly significant," Dr. Turner said. "They demonstrate our ability to use the universe as a laboratory where we can study some of the most fundamental questions of physics."