Quark Star Search

By monitoring the radio pulses that emanate from spinning neutron stars, astronomers might be able to watch them turning into something even more exotic--quark stars. So argues a team of physicists in the 1 September issue of Physical Review Letters, in which they present calculations of how a neutron star's spin would increase as its core changed into a soup of free quarks, the most basic constituent of most matter. If the telltale signal is detected, physicists would have their first glimpse of the kind of matter that filled the universe milliseconds after the big bang.

As the collapsed remnants of large stars, neutron stars are incredibly dense. For more than 20 years, physicists have thought that crushing pressures in their cores might be able to break neutrons into their constituent quarks. Unfortunately, quark stars would look the same as neutron stars. However, Norman Glendenning of the Institute for Nuclear and Particle Astrophysics at the Lawrence Berkeley National Laboratory (LBNL) in California and his collaborators Shouyong Pei and Fridolin Weber think it might be possible to catch a neutron star in the act of becoming a quark star.

The diagnosis hinges on changes in density. As a pulsar--a rapidly spinning neutron star--gradually slows down, the centrifugal force trying to throw material outward diminishes, and the star becomes denser. If this density crosses a critical threshold and the neutrons break up into quarks--which are much more compressible than ordinary matter--the star will compress under its own gravity. Like an ice skater pulling in her arms, the star will speed up before slowing again.

Pulsars can pick up their pace for other reasons, but Glendenning says that quark stars would accelerate much more dramatically. He's also optimistic that an accelerating pulsar could be spotted, as his calculations suggest the speedup should last for 100,000 years or so.

The discovery of quark matter in neutron stars would encourage experimental physicists that the stuff can also be created or observed in the laboratory. "It will give a big spur to accelerator-based work on Earth," says Arthur Poskanzer, a nuclear physicist at LBNL, who searches for quark matter at CERN.