For more than a decade astrophysicists have wondered why nature appears to show an odd restraint in the way it slays stars. In life, they range from pip-squeaks to behemoths. Small ones simply burn out and fade away, but something more curious happens to the jumbo-size variety. When such a star dies, its great bulk causes its innards to implode as a core-collapse supernova. The process sparks a cataclysmic explosion and compresses some of the remains into astrophysical exotica—often a neutron star or, for the very heaviest suns, a black hole. Yet a pronounced rift appears to divide the weight classes of these two types of massive stellar corpses. Although astronomers have spotted neutron stars weighing up to around two solar masses and black holes as light as five, middleweight cadavers have gone entirely missing—until now.
In June 2020 the Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration announced the first conclusive detection of a stellar remnant falling into the so-called mass gap between neutron stars and black holes. After months of calculations, researchers at LIGO and the Virgo gravitational-wave detector in Italy concluded that such waves rippling through Earth in August 2019—an event dubbed GW190814 that was initially classified as a black hole consuming a neutron star—actually came from a 23-solar-mass black hole swallowing a mysterious 2.6-solar-mass object. Whether the smaller body is the heaviest known neutron star or the lightest known black hole—or a truly exotic beast, such as a star made of particles distinct from those of normal stars—its existence suggests that the theories describing the most extreme stellar fates need updating.
“I would rank this as definitely the most exciting announcement we’ve seen from LIGO since the original binary black hole discovery and then the first detection of a neutron star collision,” says Duncan Brown, a gravitational-wave astronomer at Syracuse University, who was not involved in the research. “We’re probing a new piece of astrophysical understanding of the universe.”
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The new finding hints that the cosmos may enjoy a wider freedom in how it disposes of stars than researchers had supposed. Whether that leeway means atomic building blocks have enough brawn to support more monstrous neutron stars or that supernovae can forge tinier black holes, LIGO’s detection shrinks the gulf between those two most plausible scenarios.
“The idea of a mass gap as a true gap with nothing in it, I think, is getting progressively destroyed,” says Philippe Landry, a LIGO member at California State University, Fullerton. “This is going to be one nail in the coffin.”
From a fundamental physics perspective, the line separating neutron stars from black holes is razor-thin. If you toss an apple onto a neutron star at the limit of what its constituent neutrons can bear, it will abruptly collapse into a black hole. The heftiest known neutron star weighs 2.14 times the mass of our sun. And nuclear theorists suspect the objects can grow somewhat heavier, with the most optimistic models putting the complete breakdown of matter at 2.5 solar masses. Based on such theories, the LIGO collaboration calculated that the chance of the lighter partner in GW190814 being a neutron star is less than 3 percent. A neutron star that heavy, Landry says, would be a “complete game changer.”
Although they would never be able to collect any winnings, most astrophysicists would bet that 2019’s merger involved a big black hole gobbling up an improbably tiny one. But while nuclear theory makes that scenario more plausible than one involving a single black hole and a neutron star, it still challenges the best theories of how such systems come to be. “Basically,” Landry says, “something’s got to give.”
As far as astrophysicists know, making a pint-size black hole—and then feeding it to a larger one—should pose a nearly insurmountable obstacle to the universe. A conceivable way of creating such a diminutive partner is to mash together two bulky neutron stars, an event LIGO witnessed in 2017. What are the chances, however, that a special matchmaking environment, such as a chaotic galactic center dense with stellar corpses, brought two neutron stars together—and also managed to set up the resulting mass-gap black hole with a much larger companion? “There’s nothing that forbids it,” says Feryal Özel, an astrophysicist at the University of Arizona, who is not involved with LIGO. “But it’s more of a soap opera.”
To many, the simplest option is that the mini black hole was born—as most black holes are—directly from the heart of a dying star. The basics of stellar death are simple: a star blows off a massive shell, leaving its core to crumple into a black hole or neutron star. But predicting the exact aftermath of a messy explosion involving gravity, thermodynamics and particle physics represents something of a cosmic final exam that astrophysicists are still working on. “Pick up the comprehensive encyclopedia of physics,” Brown says, “and you will probably need almost every piece of physics in there to model a supernova.”
So when a team led by Özel analyzed groups of known neutron stars and black holes in 2010 and concluded that none were likely to lie between two and five solar masses, supernova researchers jumped at the chance to place more meaningful limits on the inscrutable process. A rough picture emerged in which fast, violent explosions expelled most of a star’s material cleanly, leaving the naked core to contract into a classic neutron star of perhaps two solar masses. In gentler cataclysms, however, some debris failed to escape and crashed back onto the neutron star. This material might (conveniently) add at least three solar masses, producing a black hole weighing—at minimum—as much as five suns.
Thus, the vacant zone became a tool for theorists, says Vicky Kalogera, an astrophysicist at Northwestern University and a LIGO member. She and her colleagues asked themselves, “What do I need to do to the core-collapse mechanism to create the gap?” she says.
LIGO’s mass-gap discovery hints that perhaps they need not bother: a 2.6-solar-mass stellar black hole would suggest that no such rigid regulations apply. Supernovae may be free to search out the fundamental line between neutron stars and black holes after all, removing one onerous stipulation from the astrophysical final exam. “I think this thing pretty much says there isn’t a mass gap,” Brown says.
Nevertheless, the divide between neutron stars and black holes may well persist as a cosmic proclivity rather than a rule. If you looked naively at star sizes, Özel says, you would conclude that black holes should be everywhere. Their relative absence still suggests to her that supernovae likely conspire against them to some degree. “It could be that these objects are very difficult to make, but once in a blue moon, a supernova explosion lands you there,” she says. Plus, researchers will still have to figure out why a mass gap seems to separate black hole and neutron star duos seen in x-rays but not those detected by gravitational waves.
Hard answers will come only with more detections of seemingly impossible objects. LIGO can tell tiny black holes from big neutron stars if the partners have a similar mass or give off a visible flash when they merge (colliding black holes are not expected to detonate with a burst of light, as merging neutron stars do, at least in principle). Even if ambiguous events continue to pile up, however, just watching where the masses fall will reveal a lot about what nature does with its leftover stars.
“After one weird system is discovered, then we need more of them,” Kalogera says.