On April 25, 2019, scientists at Kenyon working globally with the Laser-Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration (LSC) detected gravitational waves at an observatory in Livingston, Louisiana, signalling a ripple in spacetime caused by the collision of two neutron stars in another galaxy.
This isn’t the only time a neutron star collision has been detected from Earth. The first observed collision took place in 2017.
“It was a very loud signal,” commented Les Wade, a physics professor who co-runs the LIGO Lab at Kenyon with his wife Madeline Wade. “What was really special about [the 2017 collision] was that we saw it with the gravitational waves and then we were able to find the collision with telescopes from all other frequencies of light.”
The collision in 2019, which researchers refer to as GW190425, was not quite as theatrical—it lacked the flash across the electromagnetic spectrum that made the 2017 collision so exciting. GW190425 objects were also notably different because of their mass.
“Neutron stars can only be supported up to a maximum mass,” Wade said. “One of the neutron stars was pushing that maximum mass boundary, which is pretty interesting.” According to a paper published by the LSC in January 2020, neutron stars are normally about 1.5 times the mass of the sun. In GW190425, however, the heavier of the two stars may have been up to 2.52 solar masses, making this potentially the heaviest binary neutron star collision ever observed.
A neutron star is the very dense afterlife stage following the death of a star before it could potentially become a black hole. “Imagine taking the entire sun and all of its mass, and smashing it down into the size of a city,” Kenyon student Joe Lucaccioni ’21, who contributed analysis that was featured in the recent LSC paper about the 2019 collision, said. Wade further explained that the tremendous pressure involved in “smashing” such a massive object into a small space means that “you have these nuclear reactions where protons and electrons are essentially morphing together into neutrons,” which hold the star together with a force called neutron degeneracy pressure until it collapses into a black hole with even greater mass than the neutron star itself.
Observing neutron star collisions like GW190425 will help physicists understand the formation of black holes, which aren’t uncommon, but haven’t previously been observed due to low instrument sensitivity. “The total mass involved was high enough that we believe [the heavier star in the 2019 collision] collapsed directly into a black hole,” Lucaccioni said. “Currently, we do not know how much mass a neutron star can have before it collapses into a black hole, so this mass limit is something we are interested in.” Data and theoretical predictions show that neutron stars can be supported up to a maximum mass — around the 2 solar mass range. There is then a gap between the highest mass a neutron star can have and the lowest mass of a black hole. The heavier of the two neutron stars in the GW190425 collision inhabits this gap, meaning it could reshape physicists’ ideas of neutron star mass and the boundary between neutron stars and black holes.
The 2019 collision is far from the end of research for the Wades, Lucaccioni and the rest of the scientists collaborating globally on the LIGO project. Lucaccioni says they are all striving to “probe the workings of the universe through a new means other than light.” Researchers more broadly in LIGO are focusing on gravitational waves as an alternative to light-based observation.
“Gravitational waves are funky,” Wade said. “They’re stretches or compressions in spacetime and they manifest themselves in the change in distances between two objects.” Spacetime is part of the theory of general relativity proposed by Albert Einstein which links space and time to create a fabric of reality. Objects with large mass warp this fabric (the higher the mass, the larger the dent), and when they move, ripples are created. Spacetime is often thought of as a blanket held off the ground between two people, and a neutron star or other massive object as a ball rolling on the blanket, creating ripples as it moves.
Gravitational waves emitted leading up to a collision can be used to understand the internal workings of neutron stars using a mathematical relationship called the neutron star equation of state, which is central to Wade’s work. “We don’t know that much about it yet—we’re trying to constrain it through observation. [The 2017 collision] was the first gravitational wave constraint,” Wade commented. “We were able to rule out some more exotic theoretical predictions based on that measurement.”
By constraining the equation with observations, Wade and fellow researchers, including Kenyon students, are able to reduce down theoretical values until the equation accurately models the real world, enabling them to more precisely understand the internal activity of neutron stars.
In the coming years, the LIGO collaboration will work towards longer observation runs while increasing the sensitivity of their instruments. Wade believes that these steps will even allow researchers to detect gravitational waves from isolated neutron stars or supernovas.