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Sizing Up a Neutron Star

9 September 2004 (All day)
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Flickering burst. Bright explosions from a neutron star orbiting a companion (inset, in artist's conception) helped astrophysicists deduce the size of a neutron star for the first time.

NEW ORLEANS, LOUISIANA--A neutron star seems stranger than fiction, with more than a sun's worth of mass crushed into a sphere weighing 3 billion tons per teaspoon. Now, the first solid measurement of a neutron star's size has made that tale more real. The diameter--about 23 kilometers--matches predictions for a star consisting of a dense soup of neutrons rather than more exotic particles.

Neutron stars are the corpses of huge stars that die in supernovas. Theorists think they contain the densest matter we can see, because anything more compact would collapse into a black hole. Popular models call for a superfluid inferno of nearly pure neutrons, about 20 kilometers wide and 1.5 times as massive as our sun. Some physicists have proposed that the objects could be smaller if their cores contain an even denser brew of quarks, Bose-Einstein condensates, or nuggets of "strange matter." But neutron stars are so far away that gauging their size has been impossible.

In the new study, graduate student Adam Villarreal of the University of Arizona, Tucson, and astrophysicist Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Maryland, examined light from a neutron star that pulls gas from a companion. The gas piles up on the spinning star and ignites in fierce bursts every few hours. Peering from halfway across the Milky Way, a NASA x-ray satellite detected 38 such bursts during occasional observations over 7 years. When the researchers combined all bursts into a single statistical analysis, they concluded that the explosions flicker 45 times every second. The neutron star must spin at that rate, Villarreal and Strohmayer reported here on 8 September at a meeting of the American Astronomical Society's High-Energy Astrophysics Division.

Then, the team relied on an estimate of the strength of the neutron star's gravitational field, published in 2002 by other scientists. That measurement yielded a wide range of plausible sizes and masses. The new spin rate limits the diameter to between 19 and 30 kilometers, with a most likely value of 23 kilometers, Strohmayer says.

The deductions are complex but reliable, says astrophysicist M. Coleman Miller of the University of Maryland, College Park. "This does constrain the radius in the most interesting range," Miller says. "A 'strange matter' star would have to be much smaller than this."

Related sites
More information about the study, including an animation
Miller's primer on neutron stars
Rossi X-ray Timing Explorer, used in the research

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