Study reveals 14% of all massive stars are on collision course — and the results are weird

The origin story of one of the most bizarre phenomena in the universe is becoming increasingly clear, thanks to a new study published Monday in The Astrophysical Journal Letters.

The research sheds light on binary black holes — systems in which two black holes are interlocked in a close orbital dance around one another. Considering that they have the collective power of two of the most massive objects in the universe, binary black holes can produce some of the strongest gravitational waves ever detected, rippling space-time like nothing else we yet know of.

In the new study, scientists describe a new method to estimate how many massive stars will eventually collide and form binary black holes. And according to their calculations, 14 percent of all massive stars in the universe will eventually give birth to these strange, strong systems.

Massive stars are rarities in themselves — of the ones we know about, they tend to clock in at least 25 times the mass of our Sun and are located very far from Earth. To estimate how many result in binary black holes, the researchers used forensic data from past massive-star collisions to make their projections.

Binary black holes have been theorized for decades, but the first of these bizarre objects wasn't discovered until September, 2015, when the Laser Interferometer Gravitational-Wave Observatory (LIGO), detected gravitational waves from a binary black hole collision. That marked the first real-time observation of a black-hole merger.

The gravitational-wave signal event, dubbed GW150914, was the result of two black holes colliding in a galaxy more than one million light years from Earth. At the time, LIGO researchers estimated that, at its height, the power radiating from the black hole merger would have been more than 10 times that of the combined light power from every star and galaxy in the observable universe.

Since this breakthrough, scientists have discovered more than a dozen binary black holes.

Tracing black hole binaries' birth

"Researchers up until now have theorized the formation and existence for pairs of black holes in the universe, but the origins of their predecessors, stars, still remains a mystery," Karan Jani, astrophysicist at Vanderbilt University, and lead author of the study, said in a statement.

Binary black holes are believed to form when two massive stars collide, but scientists wondered how often this could take place in the lifetime of the universe.

In order to resolve this question, the team of researchers behind the new study accounted for the number of available stars in the universe, the process it takes for each of these stars to transition to an individual black hole, and the detection of the collision of those black holes as made by LIGO.

“We did a forensic study of colliding black holes using the astrophysical observations that are currently available,” Jani said. “In the process, we developed a fundamental constraint, or budget, which tells us about the fraction of stars since the beginning of the universe that are destined to collide as black holes."

The results of the study revealed that 14 percent of massive stars will eventually collide with one another and form binary black holes.

“That's remarkable efficiency on nature's part,” Jani said.

The researchers are hoping that their calculations will provide a framework that scientists can use to test the formation theories of stellar binary black holes, and trace the history of black holes throughout the entirety of the universe’s timeline.

Abstract: The binary black hole mergers observed by Laser Interferometer Gravitational-Wave Observatory (LIGO)–Virgo gravitational-wave detectors pose two major challenges: (i) how to produce these massive black holes from stellar processes; and (ii) how to bring them close enough to merge within the age of the universe? We derive a fundamental constraint relating the binary separation and the available stellar budget in the universe to produce the observed black hole mergers. We find that 14% of the entire budget contributes to the observed merger rate of (30+30) M_⊙ black holes, if the separation is around the diameter of their progenitor stars. Furthermore, the upgraded LIGO detector and third-generation gravitational-wave detectors are not expected to find stellar-mass black hole mergers at high redshifts. From LIGO's strong constraints on the mergers of black holes in the pair-instability mass gap (60–120 M_⊙), we find that 0.8% of all massive stars contribute to a remnant black hole population in this gap. Our derived separation–budget constraint provides a robust framework for testing the formation scenarios of stellar binary black holes.