A quick-spinning stellar corpse is the most massive of its kind ever seen. The dead star's extra bulk could rule out several theories about what these dense stellar objects are made of – and provide a celestial lab to explore exotic matter.

"For people who work in this field, it's huge," said neutron-star astronomer M. Coleman Miller of the University of Maryland, who was not involved in the new Green Bank Telescope study. "It's a big new addition to our information about a state of matter that we cannot explore in labs."

Weighing in at twice the mass of the sun, the new heavyweight champ – a pulsar dubbed J1614-2230 – is 20 percent more massive than any previously measured star of its class.

Pulsars are a special type of neutron star – the dense remains of ordinary stars that exploded as supernovas – that sweep the sky with a lighthouse-like beam of radio waves as they spin. As these radio beams swish past Earth, the stars appear to "pulse" at extremely regular intervals.

Neutron stars, true to their name, are formed almost entirely of neutrons, which can pack tightly into the densest form of matter known to exist without forcing the star to collapse into a black hole. But some theories suggest neutron stars could squish down even further by converting their neutrons to exotic types of matter. If neutron stars were packed with heavy, strange particles like hyperons or kaons, the stars would collapse under their own weight at much lower masses.

"If you're able to establish that there really is an object out there with high mass," Miller said, "it takes a lot of the predictions you would make with the exotic forms of matter and different particles, and says 'I'm sorry, you're wrong. Try again.'"

To take the extra-heavy pulsar's measurements, astronomers relied on a relativistic trick of the light.

Pulsars are usually among the most accurate clocks in the universe, blinking regularly tens to thousands of times per second. But J1614-2230 has a companion star, a white dwarf. When the radio pulses brush past the white dwarf, they slow down as if they were swimming through molasses, and take a longer time to get to Earth.

This effect, called the Shapiro delay, is due to Einstein's general-relativistic prediction that clocks run slower in a gravitational field, at least as seen from far away. The more massive the white dwarf is, the slower the pulses get.

Astronomer Paul Demorest of the National Radio Astronomy Observatory and colleagues used the Green Bank Telescope in West Virginia to watch how the times between pulses changed at different points in the pulsar's orbit around the white dwarf over the course of 8.7 days. A new instrument called GUPPI (Green Bank Ultimate Pulsar Processing Instrument) provided more precise measurements of the pulse delay than previous attempts could muster.

The astronomers used the mass of the white dwarf plus data on the pulsar's orbit to find the pulsar's mass: A whopping 1.97 times the mass of the sun. The next-most-massive neutron star was 1.67 times the mass of the sun, and most neutron stars cluster around 1.25 to 1.44 times the mass of the sun. The results are reported in the Oct. 28 Nature.

"The pulsar mass is quite a bit higher for this system than any that have been previously measured," Demorest said. "That changes our thinking about what is the maximum possible mass a neutron star can have."

Because the team used the Shapiro delay, the measurement is more reliable than previous attempts to measure neutron-star mass, Miller added.

"The Shapiro delay depends only on the mass, full stop, no other effects," he said. "It's much easier to interpret than others that have previously suggested higher masses."

The bulky star rules out all but a few models for the composition of neutron stars. Rather than containing exotic particles, the stellar corpses are probably made of plain neutrons and protons.

But that's hardly a disappointment to Miller. "It's cool," he said. "It represents a state of matter and a state of physics that we cannot reproduce on Earth. By these distant and safe observations, we're able to learn things about fundamental physical law that we could not learn otherwise."

One pressing theoretical question remains: How did the pulsar get so big? Is it slowly devouring its companion? Or was it just born this big?

"Either would be a valid explanation," Demorest said. "We just don't know which is correct yet."

Image: Bill Saxton/NRAO/AUI/NSF

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