Evrim Yazgin
Cosmos science journalist
A new study has determined that the death of massive, spinning stars could send gravitational waves through the universe which can be detected on Earth.
The violent deaths of rapidly rotating stars 15–20 times the mass of the Sun is an event called a collapsar. When the star has used up all the fuel in its core through nuclear fusion, it implodes the explodes, leaving behind a black hole surrounded by a disc of leftover matter.
This material spirals into the black hole within minutes – so violently that it distorts the space around it and sends gravitational waves across the universe.
Simulations detailing this process are described in a paper published in the Astrophysical Journal Letters.
After the death of a massive, spinning star, a disk of material forms around the central black hole. As the material cools and falls into the black hole, new research suggests that detectable gravitational waves are created. Credit: Ore Gottlieb.
The researchers found that these ripples should be detectable using current instruments like the Laser Interferometer Gravitational-Wave Observatory which made the first observation of gravitational waves in 2015.
“Currently, the only gravitational wave sources that we have detected come from a merger of two compact objects – neutron stars or black holes,” says study lead Ore Gottlieb, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics in New York City, USA.
“One of the most interesting questions in the field is: What are the potential non-merger sources that could produce gravitational waves that we can detect with current facilities? One promising answer is now collapsars.”
The researchers found that collapsars can produce gravitational waves powerful enough to be spotted 50 million light-years away. This is less than a tenth the distance at which gravitational waves from merging neutron stars or black holes can be detected.
But the result does raise the possibility of detecting collapsars using gravitational waves, considering the nearest galaxy to us is the Andromeda galaxy, about 2.5 million light-years away.
Gottlieb says the result is a surprise because scientists thought the violent collapse of a dying massive, spinning star would be too chaotic to produce waves that could be picked out amid the background noise of the universe.
Gravitational waves produced by mergers are amplified by the fact that objects about to merge will orbit around each other tightly.
The new simulations show that collapsars can produce a similar effect due to the spiralling disc of matter around them.
“I thought that the signal would be much messier because the disk is a continuous distribution of gas with material spinning in different orbits,” Gottlieb says. “We found that the gravitational waves from these disks are emitted coherently, and they’re also rather strong.”
New gravitational wave detectors could find dozens of collapsars a year. But finding them depends on simulating more star mass and rotation profiles.
“In principle, we would ideally simulate 1 million collapsars to be able to create a generic template, but unfortunately, these are very expensive simulations,” Gottlieb says. “So, for now, we have to pick other strategies.”
Gottlieb’s team performed the calculations on a small subset of star types. Scientists can begin sifting through historical data to see if there are any matches in the archives.
Finding such events would help astrophysicists better understand the extreme physics of how stars collapse and black holes.
“These are things that we can otherwise not detect,” says Gottlieb. “The only way for us to study these inner stellar regions around the black hole is through gravitational waves.”
