Quark matters; birth of a strange dwarf.

A white dwarf is a dead, collapsed star. Nuclear fusion reactions no longer fuel its glow. Electrons and atomic nuclei in its crust and core lie so tightly packed that all atomic structure is lost.

Researchers now propose that a fraction of these diminutive stellar corpses may harbor cores that include strange quarks as one of their constituents. The presence of such quark cores would stabilize these stars, allowing them to exist over a wider range of masses and core densities than possible for white dwarfs composed of just ordinary matter.

Normally, a white dwarf packs the equivalent of the sun's mass into a ball roughly the size of a modest planet, while neutron stars -- the crushed relics of supernova explosions of giant stars -- can have similar masses but diameters of 40 kilometers or less. "The strange dwarf fills the gap between ordinary white dwarfs and neutron stars," says Norman K. Glendenning of the Lawrence Berkeley (Calif.) Laboratory.

Glendenning and his collaborators describe this "possible new class of very dense white dwarfs" in the May 1 Physical Review Letters.

The existence of compact stars containing strange matter hinges on the hypothesis that a quark nugget consisting of roughly equal proportions of up, down, and strange quarks may be more stable than an ordinary atomic nucleus, which contains only up and down quarks (SN: 3/4/89, p.138). Researchers have looked for strange matter in accelerator experiments and elsewhere but so far have come up with no unequivocal evidence either confirming or rejecting the hypothesis.

A strange dwarf consists of a tiny quark core surrounded by a hefty overlay of nuclear material, up to a few thousand kilometers thick. A thin layer of electrons surrounding the positively charged quark core keeps it out of direct contact with the crust of ordinary matter lying above it.

The nuclear material in the deep interiors of such stars can have a density up to 40,000 times higher than that found in typical white dwarfs, the researchers say.

Where could a strange dwarf's quark core come from? Glendenning and his colleagues speculate that if quark nuggets exist in the galaxy, they would readily contaminate any objects with which they come in contact. Over billions of years, enough of this material could accumulate in a main-sequence star to leave a core of strange matter when the star dies.