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
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
The findings also seemed to challenge the standard model of
neutron stars, until now the most extreme form of matter known to
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 ó up and
down ó 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
"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
"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
"If that is so," Dr. Glendenning said, "this star is in a class
quite by itself and will be an astonishing discovery of fundamental
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
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
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
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
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."