An extremely dense celestial object thousands of light-years away is serving as a natural nuclear physics experiment, providing clues to processes that cannot be reproduced in the lab.

Astrophysicists look to neutron stars, extraordinarily compact remnants of massive stars, to find out how matter behaves at the highest densities that it can withstand. When a star collapses to a neutron star, what remains is an object that squeezes more mass than is contained in the entire solar system into a sphere the diameter of a midsize city. A number of hypotheses have been formulated to explain what might be at the core of these ultradense objects, from simple neutrons and protons to more exotic admixtures containing quantum particles that are incredibly short-lived on Earth.

"It's hard to predict what matter might do at these densities," says Paul Demorest, a radio astronomer at the National Radio Astronomy Observatory in Charlottesville, Va. "It's basically the most dense form of matter that we know of that is not a black hole."

Now a neutron star known as J1614-2230 is providing some answers to what the object and its ilk are made of. A new, precision mass estimate for J1614-2230, which pegs the stellar remnant as having nearly twice as much mass as the sun, rules out several of the proposed models for a neutron star's interior, according to a study in the October 28 issue of Nature. (Scientific American is part of Nature Publishing Group.)

With the benefit of one of the world's largest radio telescopes and a bit of good luck, Demorest and his colleagues derived the mass by timing the arrival of regular electromagnetic pulses coming from the neutron star. J1614-2230 is a pulsar, a neutron star emitting a beam of radiation that reaches Earth in bursts as the pulsar spins and its beam sweeps across astronomers' telescopes like a lighthouse casting its beam across nearby ships. The pulsar targeted by the researchers is a rather fast spinner, completing more than 300 revolutions per second. But each pulse does not arrive exactly on time; when the beam passes by a massive object on its roughly 3,000-light-year journey from J1614-2230 to Earth, its arrival is delayed due to an effect of general relativity known as gravitational time dilation.

At the 100-meter Green Bank Telescope in West Virginia, Demorest and his colleagues timed the arrival of J1614-2230's radio pulses as the pulsar cycled through its orbit with a binary companion, a less massive stellar remnant known as a white dwarf. The white dwarf, which is half as massive as the sun, has enough mass to delay the arrival of pulses by dozens of microseconds, thanks to the fortuitous alignment of the orbital plane of the pulsar–white dwarf binary with the line of sight from Earth. (When the relative positions of the white dwarf and pulsar bring them into line with Earth, the beam of radiation from J1614-2230 passes very close to the white dwarf on its way here, maximizing the delay.) "We got a bit lucky with this system in that it's oriented almost exactly edge-on as viewed from Earth," Demorest says.

By tracking the evolution of the gravitationally induced lag—a phenomenon known as the Shapiro delay—through a full 8.7-day orbit of the pulsar–white dwarf system, the researchers were able to produce precision estimates for the objects' masses. The estimate for the pulsar J1614-2230 (which the study's authors conclude has 1.97 times the sun's mass) is the largest precision estimate to date for a neutron star. That measurement provides some guidance as to which of the many proposed neutron star compositions could be valid, because some models do not allow for an object so hefty.

"Having one this massive rules out a whole bunch of fully legitimate, fairly mainstream predictions," says Cole Miller, an astrophysicist at the University of Maryland, College Park, who wrote a commentary in the same issue of Nature accompanying the research. Some of those included contributions from hyperons, particles just a bit heavier than neutrons that are exceedingly short-lived on Earth, or quarks de-confined from the protons and neutrons they usually conspire to form. "I would say that the relatively high mass of this object is a bit of a vote in favor of a core dominated by neutrons and protons, with some electrons," Miller says.

James Lattimer, an astronomy professor at Stony Brook University in Long Island, N.Y., says that the J1614-2230 measurement "sets a significantly new limit" for the mass of neutron stars and that its high signal-to-noise ratio sets it apart from similarly high masses derived in the past. He notes that although the research rules out most hyperon models proposed to date, alternate models might be devised that mesh with the new observations. Similarly, for quark matter the research makes it "more difficult to have de-confined quarks in a neutron star's interior, but that is certainly not ruled out," Lattimer says.

Just what makes up the inside of a neutron star has more import than simply proving one researcher's model correct and another flawed. "This is very fundamental nuclear physics," Miller says. "By looking at neutron stars and other astrophysical objects, we are able to probe regimes that are simply inaccessible here on Earth. This is historically where we've really learned the most, where we've gone beyond the bounds of lab experiments."