The subtle difference between when a massive dying star compresses into a core and when it collapses entirely may have been found. In a study published in Astrophysical Journal Letters, researchers at the Goethe University in Frankfurt say they’ve found the dividing line between compact objects called neutron stars and black holes.

When a massive star reaches the end of its life, it goes out with an immense bang called a supernova. From there, one of two known things Can happen: it either becomes a black hole, which has so much gravity not even light can escape, or a neutron star, which is a city-sized corpse of a formerly large star that’s made out of incredibly dense neutron matter.

But astrophysicists have struggled to find out exactly what variations cause a large star to compress into a dense stellar remnant, a neutron star, rather than the inescapable void of matter-eating fury that is a black hole. According to the Goethe researchers, the difference is simple: 2.16 solar masses. Any leftover object after a supernova that is less than 2.16 times the mass of the sun will star a neutron star, while anything more than 2.16 solar masses will become a black hole.

Most neutron stars are between one and two solar masses, and most black holes discovered so far (or at least suspected so far, since we can’t directly see something that gives off no light) are four solar masses or above.

So why is this important? Researchers are still studying the results of a new phenomena witnessed last year called a kilonova. It created ripples in the fabric of space-time that were detected from Earth. While it was widely reported to be a merger of two neutron stars, some researchers aren't sure if the larger object was in fact one of these dense stellar cores. The larger object was estimated to be between 1.36 and 2.26 solar masses, while the smaller was well within the mass range of an average neutron star.

If it is toward the upper end of that mass estimate, then we may have witnessed the merger of a black hole and a neutron star—which could be essential to the research taking place in the wake of the explosion, as astronomers delve deeper into what kind of object is left behind at the center of such an event. A large, unstable neutron star that swiftly became a black hole could have been left behind after the kilonova, if it was a merger of two neutron stars, or an entirely different event could have taken place where a black hole ingested the smaller neutron star. The latter type of event has been identified before, at least tentatively, in 2005.

The Goethe researchers suggest that adding even a little more mass to the object could cause it to collapse into a black hole—however, some researchers have theorized, but never proven, that another type of object exists between the mass of a neutron star and that of a black hole. But with an upper ceiling to neutron star mass determined, we can begin to figure out how a large star truly dies—and what it takes to make a star fully collapse into a light-eating inferno.

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