NASA’s Chandra X-ray Observatory has found two stars – one too
small, one too cold – that reveal cracks in our understanding of the
structure of matter. These discoveries open a new window on nuclear
physics, offering a link between the vast cosmos and its tiniest
constituents.
Chandra’s observations of RXJ1856.5-3754 and 3C58 suggest that the
matter in these stars is even denser than nuclear matter found on Earth.
This raises the possibility these stars are composed of pure quarks or
contain crystals of sub-nuclear particles that normally have only a fleeting
existence following high-energy collisions.
By combining Chandra and Hubble Space Telescope data, astronomers
found that RXJ 1856 radiates like a solid body with a temperature of 1.2
million degrees Fahrenheit (700,000 degrees Celsius) and has a diameter of
about 7 miles (11.3 kilometers). This size is too small to reconcile with
standard models for neutron stars — until now the most extreme form of
matter known.
“Taken at face value, the combined observational evidence points to
a star composed not of neutrons, but of quarks in a form know as strange
quark matter,” said Jeremy Drake of the Harvard-Smithsonian Center for
Astrophysics (CfA) in Cambridge, Mass., and lead author of a paper on
RXJ1856 to appear in June 20, 2002 issue of The Astrophysical Journal.
“Quarks, thought to be the fundamental constituents of nuclear particles,
have never been seen outside a nucleus in Earth-bound laboratories.”
Observations by Chandra of 3C58 also yielded startling results. A
team composed of Patrick Slane and Steven Murray, also of CfA, and David
Helfand of Columbia University, New York, failed to detect the expected
X-radiation from the hot surface of 3C58, a neutron star believed to have
been created in an explosion witnessed by Chinese and Japanese astronomers
in 1181 AD. The team concluded that the star has a temperature of less than
one million degrees Celsius, which is far below the predicted value.
“Our observations of 3C58 offer the first compelling test of models
for how neutron stars cool and, the standard theory fails,” said Helfand.
“It appears that neutron stars aren’t pure neutrons after all — new forms
of matter are required.”
A teaspoonful of neutron star material weighs a billion tons, as
much as all the cars, trucks and buses on Earth. Its extraordinary density
is equivalent to that of the nucleus of an atom with all of the typical
space between the atoms and their nuclei removed. An atom’s nucleus is
composed of positively charged protons and neutral neutrons, particles so
small that 100 billion trillion of them would fit on the head of a pin.
Protons and neutrons are composed of even smaller particles called
quarks, the basic building blocks of matter. Enormous atom smashers are
designed to probe the forces between quarks and the structure of the nucleus
by smashing high-energy beams of nuclei into each other and observing the
violent aftermath for a fraction of a second.
Drake cautioned that the observations of RXJ1856 could be
interpreted as a more normal neutron star with a hot spot. Such a model is
under consideration by Fred Walter of the State University of New York,
Stony Brook, one of the discoverers of RXJ1856, which was originally found
in 1996 by the German Roetgen satellite. However, the hot spot model
requires a very special orientation of the star with respect to the Earth to
explain the absence of pulsations, which would be expected from the hot
spot. The probability of such an orientation is quite small.
“Regardless of how these mysteries are resolved, these precise
observations are highly significant,” said Michael Turner of the University
of Chicago. “They demonstrate our ability to use the universe as a
laboratory where we can study some of the most fundamental questions in
physics.”
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program, and TRW, Inc., Redondo Beach, Calif., is the prime
contractor. The Smithsonian’s Chandra X-ray Center controls science and
flight operations from Cambridge, Mass.
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