Recently Discovered Neutron Star Is Almost Too Massive to Exist

The star J0740+6620 is 2.14 times the mass of our sun but just 12 miles in diameter, approaching the density of a black hole

Massive Neutron Star

Astronomers at the Green Bank Observatory in West Virginia have located the most massive neutron star on record, so dense that it may be close to the cusp of collapsing into a black hole.

Neutron stars are one of the evolutionary end points for high-mass stars. After they’ve spent most of their nuclear fuel near the end of their lives, the stars explode in bright supernovas, leaving behind an ultra-dense core of material. If that core is of a certain mass, it becomes a neutron star under the pressure of gravity. If it’s beyond a certain mass, it will collapse into a black hole. But researchers aren't exactly sure of the dividing line between the two—yet.

Astronomers are interested in neutron stars for various reasons. Most of these dense stars are less than 15 miles in diameter, but a single sugar cube worth of the star would weigh 100 million tons here on Earth. Certain neutron stars called pulsars emit beams of radio waves from their magnetic poles and rotate at a steady rate, which is why astronomers call them “cosmic lighthouses.”

In fact, the regularity of pulsars makes them useful in the hunt for elusive low-frequency gravitational waves. Any slight disruption in a pulsar’s regular rotation could be evidence of a gravitational wave passing through.

Green Bank Observatory and the Arecibo Observatory in Puerto Rico have been cataloging pulsars for the last dozen years, reports Ryan F. Mandelbaum at Gizmodo. As part of their survey, they happened upon MSP J0740+6620, a pulsar about 4,600 light-years from Earth, according to a new study in the journal Nature Astronomy.

When they focused on the pulsar, they found that it had a companion white dwarf, or the cooling core of a smaller dead star. The two objects orbit each other, which helps scientists calculate the objects’ masses. When the white dwarf passes in front of the pulsar, it changes how the pulsar’s light moves through space, creating a time delay in its regular pulsing light.

By measuring that delay, researchers can determine the pulsar’s mass using a method called a Shapiro time delay. The team found that MSP J0740+6620 is about 2.14 times more massive than our own sun. That makes it the largest neutron star ever recorded—and close to the theoretical limit for the objects. But the mega-sized neutron star isn’t interesting just because it’s big.

“The orientation of this binary star system created a fantastic cosmic laboratory,” co-author Scott Ransom of the National Radio Astronomy Observatory said in a press release. “Neutron stars have a tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse. Each 'most massive' neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mindboggling densities.”

The finding could also help astrophysicists answer some big questions about neutron stars. For instance, researchers aren’t sure what’s going on inside the superdense cosmic objects, whether the neutrons inside them flow freely like a fluid or if they’re stuck in place. It’s also possible that the intense gravity crushes the neutrons into a stew of quarks and other exotic particles. And the more researchers learn about massive neutron stars, the closer they are to discovering the “tipping point” at which gravity runs wild, creating a black hole.

“These stars are very exotic,” co-author Maura McLaughlin of West Virginia University says in another press release. “We don’t know what they’re made of and one really important question is, ‘How massive can you make one of these stars?’ It has implications for very exotic material that we simply can’t create in a laboratory on Earth.”

The study is also a step forward in overturning previous thinking about neutron stars.

“For a long time we thought that neutron stars could only be around 1.4 times the mass of the sun,” Thankful Cromartie, the study’s lead author and West Virginia University graduate student, tells Catherine Thorbecke at ABC News. “[This study] is a pretty big leap forward in terms of discovering more and more massive neutron stars. I think the discovery is very compelling because it shows that we can use astrophysical observations as kind of a laboratory in space to do physics that we can’t do on Earth. We can’t exactly make neutron stars here on earth so the only way we have access to this astrophysics is by observing these neutron stars. I think it's a pretty darn cool tool to have access to.”

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