The measurement may lead to a better understanding of nature's building blocks -- protons, neutrons and their constituent quarks -- as they are compressed inside the neutron star to a density trillions of times greater than on Earth.

Tod Strohmayer of NASA Goddard Space Flight Center in Greenbelt, Md., and Adam Villarreal, a physics graduate student at the University of Arizona, present their results today at the American Astronomical Society's High Energy Astrophysics Division meeting in New Orleans.

Strohmayer and Villarreal estimate that the neutron star is about 1.8 times as massive as the sun -- slightly more massive than expected -- with a radius of about 7 miles (11.5 kilometers). It is part of a binary star system named EXO 0748-676, located about 30,000 light-years away in the southern sky constellation Volans, or Flying Fish.

The scientists used NASA Rossi X-ray Timing Explorer data to measure how fast the neutron star spins. Spin-rate was the unknown factor needed to estimate the neutron star's size and total mass. Their results agree with a mass-to-radius ratio estimate made from European Space Agency (ESA) X-ray satellite observations in 2002.

The long-sought mass-radius ratio defines the neutron star's internal density and pressure relationship, the so-called equation of state.

"Astrophysicists have been trying for decades to constrain the equation of state of neutron star matter," Villarreal said. "Our results hold great promise for accomplishing this goal. It looks like equations of state which predict either very large or very small stars are nearly excluded."

Knowing a neutron star's equation of state allows physicists to determine what kind of matter can exist within that star. Scientists need to understand such exotic matter to test theories describing the fundamental nature of matter and energy, and the strength of nuclear interactions.

"We would really like to get our hands on the stuff at the center of a neutron star," said Strohmayer. "But since we can't do that, this is about the next best thing. A neutron star is a cosmic laboratory and provides the only opportunity to see the effects of matter compressed to such a degree."

A neutron star is the core remnant of a star once bigger than the sun. The interior contains matter under forces that perhaps existed at the moment of the Big Bang but which cannot be duplicated on Earth.

In this system, gas from a "normal" companion star, attracted by gravity, plunges onto the neutron star. This triggers thermonuclear explosions on the neutron star surface that illuminate the region. Such bursts often reveal the spin rate of the neutron star through a flickering in the X-ray emission, called a burst oscillation.

Strohmayer and Villarreal detected a 45-hertz burst oscillation frequency, which corresponds to a neutron star spin rate of 45 times per second. This is a leisurely pace for neutron stars, which are often seen spinning at more than 600 times per second.

They next capitalized on EXO 0748-676 observations with ESA's XMM-Newton satellite, led by Jean Cottam of NASA Goddard in 2002. Cottam's team detected spectral lines emitted by hot gas, lines resembling those of a cardiogram.

These lines had two features. First, they were Doppler shifted. This means the energy detected was an average of the light spinning around the neutron star, moving away from us and then towards us. Second, the lines were gravitationally redshifted. This means that gravity pulled on the light as it tried to escape the region, stealing a bit of its energy. The gravitational redshift measurement offered the first estimate of a mass-radius ratio, because the degree of redshifting depends on the mass and radius of the neutron star.

Strohmayer and Villarreal determined that the 45-hertz frequency and the observed line widths from Doppler shifting are consistent with a neutron star radius between 9.5 and 15 kilometers (between about 6 and 9 miles) with the best estimate at 11.5 kilometers (about 7 miles). They used the radius and the mass-radius ratio to calculate the neutron star's mass between 1.5 and 2.3 solar masses, with the best estimate at around 1.8 solar masses.

The result supports the theory that matter in the neutron star in EXO 0748-676 is packed so tightly that almost all protons and electrons are squeezed together to become neutrons, which swirl about as a superfluid, a liquid that flows without friction. Yet the matter isn't packed so tightly that quarks are liberated, a so-called quark star.

"Perhaps most exciting is that we now have an observational technique that should allow us to measure the mass-radius relations in other neutron stars," Villarrael said.

A proposed NASA mission called the Constellation X-ray Observatory would have the ability to make such measurements, but with much greater precision, for a number of neutron star systems.