The first visible light image of a lone neutron star was captured this past September by NASA's Hubble Space Telescope. The image showed the star to be no larger than 16.8 miles across.

eutron stars are formed out of the deaths of massive stars (several times larger than the sun) so old they burned up all their nuclear fuel. Such stars throw off their outer layers in the mighty explosions known as supernovas, leaving only a core that is generally less than 10 miles in diameter but with a density of hundreds of millions of tons per cubic inch. How dense is that? A piece of neutron star the size of the period at the end of this sentence would weigh about as much as the ill-fated ship, Titanic.

According to Berkeley Lab's Norman Glendenning, an internationally recognized expert on compact stars, it is generally believed that quarks-the elementary particles that combine to form hadrons such as protons and neutrons-are liberated when matter is compressed to very high densities.

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Pressure within the core of a neutron star is thought to be so tremendous that nucleons (protons and neutrons) burst apart like the popping of balloons. This sets free the three quarks that combine to form each nucleon.

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

Some neutron stars spin rapidly-several hundred rotations per second-causing them to send out regular pulses of radio signals that earned them the name "pulsars." Radio signals from pulsars can be heard on Earth and could be used to confirm some of the theories about the universe in its earliest stages. Glendenning and his colleagues have postulated that the presence of quark matter in pulsars should be detectable by measuring their rates of spin. As pulsars age, their rotation slows. This "spin-down" means a loss of outward-pushing centrifugal force, which in turn means further compression of the pulsar's interior until nuclear matter is crushed into quark matter.

"First at the center and then in an expanding region, the relatively incompressible nuclear matter will be converted to the highly compressible quark matter phase," says Glendenning. "This conversion to quark matter (which has been likened to the consistency of soup) allows the pulsar to rapidly shrink."

The pulsar's sudden reduction in size results in a "spin-up," much like rotating ice skaters spin faster when they tuck their arms in close to their bodies. For example, a pulsar spinning at 200 rotations per second might, for a time, spin at 202 rotations per second.

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Glendenning and his colleagues estimate that converting the entire core of a pulsar from nuclear to quark matter should take about 100,000 years. Since there are currently 700 known pulsars, this means that about seven of them could be undergoing this transition now. Pulsar observations are still in their infancy, and many of the known pulsars were only recently discovered. Still, Glendenning and his colleagues believe that spin-ups as a result of nuclear-quark phase matter transitions are a very easy signal to detect and should be observable. The discovery of such spin-ups would be momentous, they say.

"It would prove that the essentially free quark state predicted for matter at very high energy densities actually exists," says Glendenning. "The detection of this state would give us a picture of an early phase of the Universe that is based on observation."

- Lynn Yarris