Published online 27 October 2010 | Nature | doi:10.1038/news.2010.565

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The largest pulsating star yet observed casts doubts on exotic-matter theories.

Radio pulses from a neutron star suggest exotic particles are asbent from its core. Bill Saxton, NRAO/AUI/NSF

Neutron stars are living up to their name. Measurements of radio waves emanating from the most massive pulsating star yet discovered suggest that it is, indeed, made up of neutrons, rather than exotic particles, as some theories propose.

Neutron stars are the corpses left behind after certain normal stars explode as supernovae. According to standard astronomical models, matter is squeezed down so tightly within their cores that nuclei break apart into their constituents, with protons and electrons crushed together into neutrons — hence the stars' name. But in reality, astronomers know little about what happens to matter at such high densities, says Paul Demorest, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Virginia, which has led some to suggest that 'neutron star' may be a misnomer. Rival models suggest that the objects might be made up of the constituents of neutrons — free quarks — or other types of exotic matter, such as 'hyperons'.

One way to test which theory is correct is to calculate the masses of neutron stars, because each model predicts a different upper limit to their mass. In Nature today1, Demorest and his colleagues report a neutron star with a mass that is almost twice that of the Sun — beating the previous record-holder by more than 13%2.

The researchers used the National Radio Astronomy Observatory Green Bank Telescope in West Virginia to examine a rotating neutron star, called J1614-2230, which orbits a companion 'white dwarf' star as part of a binary star system. The neutron star emits radio pulses at regular millisecond intervals, but as each pulse passes by the white dwarf on its way to Earth it is delayed in a way that depends on the mass of the dwarf and varies as the stars orbit each other.

This 'Shapiro delay' is an effect of general relativity, which states that the gravitational field of massive objects causes clocks to tick more slowly and radio pulses to be retarded. By combining measurements of the Shapiro delay, the period of the stars' orbit and the speed of the neutron star, the scientists calculated the mass of the neutron star.

Mass mystery

There have been tentative reports of other heavy neutron stars before, but follow-up measurements looking for the Shapiro delay have failed to confirm the high mass, says David Nice, an astronomer at Lafayette College in Easton, Pennsylvania. "This is a beautiful demonstration of the Shapiro effect that clears up whether heavier neutron stars exist," he says.

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The reported mass of the neutron star is above that predicted by almost all current exotic models3, weakening the possibility that neutron stars are made from anything other than neutrons, says Demorest.

However, James Lattimer, an astronomer at the State University of New York at Stony Brook, notes that it would be relatively easy to tweak the parameters that control the strength of interaction between the particles in exotic models to bring them back in line with the researchers' findings. "To rule out these exotic models fully you also need to know the radius of the star," he says.

Work to calculate the star's radius is underway. But, nonetheless, the neutron star's high mass could force astronomers to rethink their models of how binary systems evolve, says Lattimer. Computer simulations tend to predict that neutron stars in binaries will have masses between around 1.2 and 1.5 times that of the Sun. Models suggest that a small amount of matter can be transferred to the neutron star from its companion, but even that could not account for the extremely large mass of this neutron star, says Lattimer. For this reason, it will be important to corroborate this result by looking for other heavy neutron stars in binary systems. "It's a mystery that will have to be looked into."