In October 2019, when astronomers saw a powerful gamma-ray burst (GRB), the most likely explanation was that it was caused by a massive dying star in a faraway galaxy going supernova. According to a new paper that was published in the journal Nature Astronomy, however, data from subsequent observations showed that the burst originated with the collision of stars (or their remnants) in a densely packed area close to the supermassive black hole of an ancient galaxy. This is the first observational evidence for such a rare event, despite the fact that it has been hypothesized.
Gamma-ray bursts are extremely high-energy explosions that occur in faraway galaxies and last anywhere from a few milliseconds to several hours, as previously mentioned. Gamma-ray bursts are classified into two groups. The majority, or 70%, are prolonged bursts that last longer than two seconds and frequently leave a bright afterglow. Usually, these are connected to galaxies with a lot of young stars. Long bursts are thought to be caused by massive stars collapsing into neutron stars or black holes (or, alternatively, a newly formed magnetar), according to astronomers. X-rays and gamma rays would be released by the baby black hole’s highly energetic jets, which would travel close to the speed of light and be powerful enough to cut through the remnants of the parent star.
Short bursts are those gamma-ray bursts that last less than two seconds (about 30%) and typically emanate from regions with very little star formation. These gamma-ray bursts, which are known as “kilonovas,” are thought to be the result of mergers between two neutron stars or a neutron star and a black hole.
In 2019, NASA’s Neil Gehrels Swift Observatory found a gamma-ray burst that fell into the long category. However, there was no evidence of a corresponding supernova, so astronomers were baffled. According to co-author and Northwestern University astrophysicist Wen-fai Fong, “there is at least one oddball that throws us for a loop for every hundred events that fit into the traditional classification scheme of gamma-ray bursts.” However, these oddballs provide the most information about the awe-inspiring variety of explosions that the universe is capable of.
Using data from the Nordic Optical Telescopes and the Hubble Space Telescope, Fong and his co-authors used the International Gemini Observatory to follow the burst’s waning afterglow. They were able to pinpoint the location of the GRB using the afterglow, which placed it just 100 light-years from the nucleus of an ancient galaxy—i.e., close to the massive black hole at its heart. They came to the conclusion that the collision of two stars or stellar remnants was the cause of the burst.
This is significant because, depending on a star’s mass, there are three well-known processes by which it dies. In a supernova, massive stars explode, whereas a star with the mass of our Sun will eventually shed its outer layers and fade into a white dwarf. Neutron stars or black holes, the remnants of supernovae, can form binary systems and eventually collide.
There is now a fourth choice: In active galaxies, where stars aren’t as dense, collisions between stars in densely packed regions are uncommon. A million stars could be packed into a space a few light-years across in an ancient galaxy. Additionally, in this instance, the motions of those stars would have been altered to a random degree by the gravitational effects of being so close to a supermassive black hole. At some point, there would undoubtedly be a collision.
In point of fact, the authors suggest that collisions of this kind may not even be that uncommon; Because of all the dust and gas that is blocking our view of the centers of ancient galaxies, we simply cannot detect the recognizable GRBs and afterglows. Astronomers may learn more about this kind of stellar death in the future if they can detect a gravitational wave signature in conjunction with a GRB.
According to co-author and Radboud University astronomer Andrew Levan, “These new results show that stars can meet their demise in some of the densest regions of the Universe where they can be driven to collide.” This is exciting for figuring out how stars die and answering other questions, like what unidentified sources could produce gravitational waves that could be detected on Earth.