Neutron star breaks mass record

New heavyweight champion overturns exotic theories

Astronomers have weighed a neutron star with nearly double the mass of the sun, the heaviest yet found. A mass that high rules out many theories that these ultradense remnants of supernova explosions contain anything other than ordinary matter, researchers report in the Oct. 28 Nature.

Some theorists have suggested that the high pressure in a neutron star’s core could break the matter there down into a soup of unbound quarks, the subatomic particles that combine to make protons and neutrons, or other exotic forms.

But that’s not a likely scenario if neutron stars as heavy as this one exist, says Coleman Miller, an astrophysicist at the University of Maryland in College Park who was not involved in the research. In most theoretical models, a quark star should collapse into a black hole before it ever reached two solar masses. Ergo, neutrons probably remain neutrons even under high pressure.

“This is a remarkable observation,” says Miller. “This is something that has been of burning interest to nuclear physicists because this is the only place in the universe to test how matter behaves at high density.This finding could inform models of other high-density systems, like the early universe, and refine the fundamental theories of how quarks interact, he says.

Normally, inferring a neutron star’s mass is difficult. But astronomers had luck on their side with J1614-2230, a neutron star roughly 3,000 light-years from Earth. J1614-2230 is a pulsar, so it emits a beam of radio waves as it spins. It is also one in a pair of companion stars that orbit each other.

Because the other star coincidentally passes close to the pulsar beam’s path, light from the pulsar is sidetracked by the companion’s gravity, as predicted in Einstein’s theory of general relativity. By measuring the slight delay of light, called the Shapiro delay, astronomers could determine the mass of the companion star.

Then, because the astronomers knew how fast the companion orbited the pulsar, they could calculate the pulsar’s mass to be 1.97 times the mass of the sun. Previously, the heaviest known neutron star weighed in at about 1.74 solar masses.

More heavy neutron stars are likely waiting to be discovered, says Paul Demorest, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Va., and coauthor of the Nature paper.

“It’s not that very massive ones were rare,” says Demorest. What is rare is finding a pulsar with a companion star that eclipses pulsing light.

Discovery of the high-mass neutron star doesn’t completely rule out an exotic interior — it’s possible that condensed quark matter could still survive deep inside, Demorest and collaborators write in the Nature paper. But the mass sets tight limits on the characteristics of matter within neutron stars. Theorists would have to tweak various quantities within narrow limits to permit some forms of exotic matter to exist.

“It could be allowed, but at some point Occam’s razor is going to come in,” says Miller, suggesting that a core of ordinary matter may turn out to be the simplest and most natural way to explain the star’s high mass.

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