In January 2020, those ancient waves reached the shores of our solar system, where they were picked up by ultrasensitive equipment at observatories in the United States and Italy. It marked the first time astronomers had ever detected a black hole swallowing a neutron star.
A mere 10 days later, astronomers detected the same phenomenon in another part of the universe, a black hole devouring its companion neutron star.
The discoveries, published Tuesday in the Astrophysical Journal Letters, represent a key breakthrough in gravitational astronomy, a budding field of research in which scientists examine tiny distortions of space-time for clues about how the universe works.
Researchers have previously documented two black holes colliding and two neutron stars crashing into each other. But while theoretical models had long predicted that black-hole-neutron-star mergers were possible, decades passed without any being observed.
“These are remarkable events, and we have waited a very long time to witness them. So it’s incredible to finally capture them,” Susan Scott, a physicist at Australian National University and co-author of the study, said in a statement. “Now, we’ve completed the last piece of the puzzle with the first confirmed observations of gravitational waves from a black hole and a neutron star colliding."
The study, which involved more than 1,000 scientists, will help researchers learn more about how these objects form and how common they are. It could even help unlock some mysteries of galaxy formation.
“There’s still so much we don’t know about neutron stars and black holes — how small or big they can get, how fast they can spin, how they pair off into merger partners,” said study co-author Maya Fishbach, a NASA Einstein Postdoctoral Fellow at Northwestern. "With future gravitational wave data, we will have the statistics to answer these questions, and ultimately learn how the most extreme objects in our universe are made.”
The stellar death spiral researchers observed is exactly the type of violent, high-energy event that generates gravitational waves, which cause the fabric of the universe to warp and undulate as they propagate through space-time.
First theorized by Albert Einstein more than a century ago, gravitational waves weren’t physically detected until 2015, when ripples generated by two colliding black holes were sensed by Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Louisiana and Washington state. The landmark discovery ushered in a new era of astronomy, allowing scientists to harness gravitational waves as observational tools.
Black holes and neutron stars have been studied for decades, but many of their properties remain a mystery. Both form after massive stars exhaust their fuel. Some collapse into black holes where gravity is so intense that not even light can escape. Others leave behind corpses just a few miles in diameter and made almost entirely of neutrons. After black holes, neutron stars are the densest known objects in the universe.
Scientists at LIGO and the Virgo gravitational-wave observatory in northern Italy — home to the most sensitive observational instruments ever constructed — detected the first collision of a black hole and neutron star on Jan. 5, 2020. The nature of the waves allowed them to infer the mass and ballpark the location. They determined the black hole was about nine times heavier than the sun, while the neutron star was about twice as heavy as the sun, according to the study.
The second merger was detected on Jan. 15, 2020. It involved a black hole six times the mass of the sun and a neutron star about 1½ times the sun’s mass.
In both cases, the neutron stars were consumed by their companions, according to the study. There were no associated flashes of light — and that was no surprise. There probably wasn’t any light show to see, researchers said, because the black holes were big enough to completely swallow the neutron stars.
“These were not events where the black holes munched on the neutron stars like the cookie monster and flung bits and pieces about,” Patrick Brady, a professor at the University of Wisconsin at Milwaukee and a LIGO spokesman, said in a statement. “That ‘flinging about’ is what would produce light, and we don’t think that happened in these cases.”
The first merger occurred about 900 million light-years from Earth, the second about a billion, according to the researchers. With these two observations documented, astronomers now estimate that about one such merger per month takes place within 1 billion light-years of Earth.
The study highlighted another intriguing point: Neither merger took place in the Milky Way galaxy, posing a host of questions for scientists going forward.
“With this new discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary,” Astrid Lamberts, a researcher at Observatoire de la Côte d’Azur, in Nice, France, said in a statement. “We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way.”