Roughly 780 million years ago and a correspondingly distant 780 million light-years away, a strange stellar object was devoured by a black hole 23 times more massive than the sun. The strange object defies categorization, being more massive than any known collapsed star and less massive than any black hole ever detected, reports Dennis Overbye for the New York Times.
This places the misfit, still 2.6 times the mass of the sun, squarely in what’s called the “mass gap,” reports Rafi Letzter for Live Science. Collapsed stars, called neutron stars, have topped out at 2.14 times the mass of the sun and their generally accepted theoretical upper limit is 2.5 solar masses, according to the Times. Black holes on the other hand don’t seem to come smaller than five solar masses.
Part of the significance of this mass gap is that neutron stars and black holes each represent possible outcomes for dying high-mass stars. The deaths of such stars entail brilliant supernovae that are punctuated in a transformation of the star’s remaining hyper-dense core into either a neutron star or a black hole, wrote Jason Daley for Smithsonian in 2019. A more massive core turns the core into a light eating black hole and a less massive core will condense into a neutron star—meaning somewhere in the mass gap there may be a tipping point, a mass beyond which a black hole is preordained and below which a neutron star forms.
“We’ve been waiting decades to solve this mystery,” Vicky Kalogera, an astrophysicist at Northwestern University and one the authors of a new paper describing the discovery, tells the Times. “We don’t know if this object is the heaviest known neutron star or the lightest known black hole, but either way it breaks a record. If it’s a neutron star, it’s an exciting neutron star. If it’s a black hole, it’s an exciting black hole.”
Astronomers discovered the confounding object on August 14, 2019, using gravitational wave detectors in Italy and the United States called the International LIGO-Virgo Collaboration, reports Pallab Ghosh for BBC News. The detectors use lasers to measure the tiny ripples in the fabric of space-time created by the collision of massive objects elsewhere in the universe. The international team’s findings were published this week in the Astrophysical Journal Letters.
Charlie Hoy, an astronomer with Cardiff University who worked on the study, tells BBC News that the discovery may call for fundamental shifts in our understanding of these phenomena. “We can't rule out any possibilities. We don't know what it is and this is why it is so exciting because it really does change our field."
Christopher Berry, a gravitational wave astronomer at Northwestern University and the University of Glasgow and co-author of the new research, tells Megham Bartels of Space.com that figuring out what tips a dying star towards becoming a neutron star will help us understand how they work. "Neutron star matter is very difficult to model," he tells Space.com. "It's nothing we can simulate here on Earth, the conditions are too extreme."
And if the mass gap turns out to be smaller than previously thought, that will require tweaking the currently accepted astrophysical models, which could have broader ramifications for our understanding of the universe, Berry tells Space.com.
"This is testament to the fact that we are only just starting to explore the universe with gravitational waves," Berry tells Space.com. "We don't know what's out there. We've seen some of the more common sources now, we know what the typical type of gravitational waves are. But the full complexity, what the rare beasts in the jungle are, we're still trying to find out."