Scientists may have figured out what caused one of the strangest explosions ever observed in our universe: A massive star exploded while getting ripped apart by a black hole. Astronomers identified this blast as a new flavor of supernova, the powerful bursts that mark the end of a star’s life.
The blast — which occurred about 730 million light-years from Earth — was first spotted in July 2023 with the help of a new artificial intelligence algorithm, which scans the cosmos for unusual explosions in real time. When scientists further studied the event, they concluded an enormous star was locked in a dangerous orbit with a small black hole that had been pulling gas and dust off the star. Before the star could swallow the baby black hole, the black hole’s gravitational forces caused the star to explode.
“That black hole has shaped that star’s entire life” from its evolution to the type of explosion that ended its life, said Ashley Villar, a study author and professor at Harvard University. The discovery was published Wednesday in
The Astrophysical Journal.
It’s not unusual to see these objects in close proximity to one another because most massive stars come in pairs or multiples, Villar said. Presumably, both objects in this scenario were once stars orbiting near each other before one evolved into a small black hole. Now orbiting around a black hole, the remaining star lost energy and was drawn in closer until it exploded into a new type of supernova never observed before.
Most supernovas follow a typical fate: The star’s core quickly collapses and sends a powerful shock wave through the outer layers of the star. The strong wave of energy causes a brief but bright flash of light. Meanwhile, the explosion’s heat and pressure create radioactive elements, which decay and release energy. The energy is reabsorbed by surrounding material, causing it to glow and keeps the supernova bright for weeks to months. Over time, this glow dissipates.
But this particular explosion, given the name SN 2023zkd per the International Astronomical Union, was unusual for several reasons. First, the algorithm caught the supernova as it was dimming, but the light was fading at a much slower rate than expected. Second, scientists looked back at archival data and saw the supernova was actually brightening for about four years before it started to dim, which is an unusually long time for a supernova. Finally, as they continued to monitor the star, it unexpectedly brightened again.
The team gathered ultraviolet to infrared data of the star from ground and space telescopes, capturing the sequence of both brightening events and running simulations in computer models. Based on the model scenarios, they determined the first brightening and subsequent decline was probably not powered by a radioactive decay seen in most supernovas.
Instead, the researchers said the light source probably came from a traffic jam around the exploding star. It started as the distance between the massive star and black hole decreased. The black hole’s strong gravity began unevenly ripping gases and dust from the star — like one side of a bedsheet getting caught in a vacuum cleaner. When the star started to explode, its blast wave collided with the dislodged gases around it and created the first flash. The second, later flash was caused by a slower but persistent crash with the thick, disklike cloud.
“We think that the light source is actually from stuff hitting each other as it’s trying to escape,” Villar said. “That explosion hits that disk, and now we’re seeing all this additional light.”
The team considered another possible explanation for the light source, where the black hole tears apart the star before it has the chance to explode on its own. The black hole pulls in the star’s debris, which crashes into the surrounding gas to generate a supernova-like flash.
In either scenario, the star collapses to some degree.
Scientists have previously theorized that some supernovas can be influenced by another object tugging on it, even capturing some data of supernovas with double brightening events.
But “this is the first time when we have all of those pieces for the same explosion” from beginning to end, said Alex Gagliano, a co-author and researcher at Massachusetts Institute of Technology. He gives a large credit to their algorithm for catching the event early, detecting subtle anomalies about 100 days before scientists saw more obvious unusual activity.
