The sun at the center of our solar system is a big-bodied behemoth, clocking in at more than 4 nonillion pounds (in the U.S., that’s 4 followed by 30 zeros). Now, multiply that mass by 2.14, and cram it down into a ball just 15 miles across. That’s an absurdly dense object, one almost too dense to exist. But the key word here is “almost”—because a team of astronomers has just found one such star.

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The newly discovered cosmic improbability, reported today in the journal Nature Astronomy, is a neutron star called J0740+6620 that lurks 4,600 light-years from Earth. It’s the most massive neutron star ever detected, and is likely to remain a top contender for that title for some time: Much denser, researchers theorize, and it would collapse into a black hole. Both neutron stars and black holes are stellar corpses—the leftover cores of stars that die in cataclysmic explosions called supernovae. The density of these remnants dictates their fate: The more mass that’s stuffed into a small space, the more likely a black hole will form. Neutron stars are still ultra-dense, though, and astronomers don’t have a clear-cut understanding of how matter behaves within them. Extremely massive neutron stars like this one, which exist tantalizingly close to the black hole tipping point, could yield some answers, study author Thankful Cromartie, an astronomer at the University of Virginia, told Ryan F. Mandelbaum at Gizmodo.

Cromartie and her colleagues first detected J0740+6620, which is a type of rapidly rotating neutron star called a millisecond pulsar, with the Green Bank telescope in West Virginia. The name arises from the way the spinning star’s poles emit radio waves, generating a pulsing pattern that mimics the sweeping motion of a lighthouse beam. During their observations, the researchers noted that J0740+6620 is locked into a tight dance with a white dwarf—another kind of dense stellar remnant. The two bodies orbit each other, forming what’s called a binary. When the white dwarf passes in front of the pulsar from our point of view, it forces light from J0740+6620 to take a slightly longer path to Earth, because the white dwarf’s gravity slightly warps the space around it. The team used the delay in J0740+6620’s pulses to calculate the mass of both objects. Previous measurements from the Laser Interferometer Gravitational-Wave Observatory (LIGO) suggest that the upper limit for a neutron star’s mass is about 2.17 times that of the sun—a figure that’s just a smidge above J0740+6620’s estimated heft. But with future observations, that number could still change. Harshal Gupta, NSF program director for the Green Bank Observatory, called the new paper “a very solid effort in terms of astronomy and the physics of compact objects,” Mandelbaum reports. “Each ‘most massive’ neutron star we find brings us closer to identifying that tipping point [when they must collapse],” study author Scott Ransom, an astronomer at the National Radio Astronomy Observatory, said in a statement. “The orientation of this binary star system created a fantastic cosmic laboratory.”

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