QUARK STARS represent one
segment on the sliding scale of collapsed stars stretching from white dwarfs
to black holes. In between lie neutron stars, in which self-gravitation
has forced electrons to merge with protons to form neutrons. At higher
density, some of the nuclear matter may exist in the form of hyperons,
heavy versions of neutrons which can be made artificially at accelerators
on Earth. Hyperons are normally unstable and quickly decay, but would survive
indefinitely inside neutron stars. Up to this point, the nucleons inside
a neutron star are still baryons; that is, they each consist primarily
of three quarks. But at higher density still, the baryons can melt, creating
the quark-gluon plasma state being sought at the CERN collider in Geneva
and (in the next few years) at the RHIC collider on Long Island. However,
if physicists don't hurry, astrophysicists might spot evidence for the
quark matter first. Rapidly spinning neutron stars (pulsars) gradually
shed energy and angular momentum in the form of radio emissions and an
electron- positron stellar wind. This causes the star to contract, jacking
up the pressure a bit, making conditions more favorable for the creation
of hyperons and quark matter. According to Norman Glendenning of LBL (nkg@csa.lbl.gov)
and his colleagues S. Pei (Beijing Normal University) and F. Weber (Ludwig-Maximilians
University of Munich) one in a hundred pulsars is undergoing the baryon-melting
phase transition. They suggest ways in which this transition could be detected,
and they look forward to the advent of a new "quark astronomy."(Norman
Glendenning et al., 1 September Physical
Review Letters; image available at Physics
News Graphics.)
