Just neutrons, no quarks. (Neutron Star Stuff).

Neutron stars cram more mass than that of the sun into a sphere as wide as a city. A teaspoon's worth of a neutron star weighs in at a billion tons. Exotic though they may be, neutron stars are not what physicists would call strange, according to a study reported this week.

To find out what these ultradense stars are made of, Jean Cottam of NASA's Goddard Space Flight Center in Greenbelt, Md., and her colleagues used an X-ray satellite to determine how light is warped by the extreme gravity of a neutron star partnered with an ordinary star some 30,000 light-years from Earth. This pairing is known as EXO0748-676.

According to the general theory of relativity, light escaping from any strong region of gravity loses energy. The energy loss shifts the light to longer, or redder, wavelengths. Cottam and her colleagues have for the first time measured the gravitational redshiff of light passing through the centimeter-high atmosphere of a neutron star.

The redshift induced by the star's extreme gravity depends on the ratio of its mass to its radius. This ratio provides a measure of the star's internal pressure relative to its density. From that number, astronomers can investigate whether the interior of a neutron star is made of just neutrons or includes exotic particles.

According to a widely accepted model for the structure of a neutron star, its gravity squeezes protons and electrons together to make a compact ball of neutrons. But some scientists have speculated that the neutrons are squeezed further, dissolving into quarks, which are the building blocks of elementary particles. A resulting quark star, for example, would consist of up and down quarks, which make up protons and neutrons, and also strange quarks, which are heavier and not found in ordinary matter.

From their redshift measurements, Cottams team calculates a mass-to-radius ratio of 0.152 solar masses per kilometer. That ratio is just right if the star is composed of neutrons, but it's inconsistent with the most plausible quark models, say Cottam and her collaborators, Frits Paerels of Columbia University and Mariano Mendez of the SRON National Institute for Space Research in Utrecht, the Netherlands. Their report appears in the Nov. 7 Nature.

The findings "look very solid both in terms of the data and their interpretation," says Lars Bildsten of the University of California, Santa Barbara. He adds that the new argument is far more convincing than previous claims that such stars might be composed of quarks. Those conclusions were based on the estimated temperatures of two neutron stars (SN: 4/20/02, p. 246).

Even with the high resolution of the European Space Agency's X-ray Multi Mirror (XMM) Newton satellite, Cottam's measurements required the brilliance provided by 28 thermonuclear blasts. These blasts were generated when a critical mass of material from the ordinary companion star piled up on the neutron star's surface. During the blasts, XMM-Newton measured the spectra of X rays passing through highly ionized iron atoms in the neutron star's atmosphere.

Previous attempts to measure a neutron star's redshift focused on a star with an enormous magnetic field. Strong fields, however, induce their own redshift. Since the fields from neutron stars aren't precisely known, the magnetic component of the stars' redshift can't be clearly separated from the gravitational component, notes Cole Miller of the University of Maryland in College Park. In contrast, the object studied by Cottam's team has such a weak magnetic field that its redshiff results entirely from gravitational effects.