Astronomers have discovered what may be a massive star exploding while trying to swallow a black hole companion, offering an explanation for one of the strangest stellar explosions ever seen.
The discovery was made by a team led by the Center for Astrophysics | Harvard & Smithsonian (CfA) and the Massachusetts Institute of Technology (MIT) as part of the Young Supernova Experiment.
The blast, named SN 2023zkd, was first discovered in July 2023 by the Zwicky Transient Facility. A new artificial intelligence algorithm designed to scan for unusual explosions in real time first detected the explosion, and that early alert allowed astronomers to begin follow-up observations immediately — an essential step in capturing the full story of the explosion. By the time the explosion was over, it had been observed by a large set of telescopes, both on the ground and from space.
The scientists think the most likely interpretation is that the massive star was locked in a deadly orbit with the black hole. As energy was lost from the orbit, their separation decreased until the supernova was triggered by the star’s gravitational stress as it partially swallowed the black hole.
“Our analysis shows that the blast was sparked by a catastrophic encounter with a black hole companion, and is the strongest evidence to date that such close interactions can actually detonate a star,” said Alexander Gagliano, lead author of the study and fellow at the NSF Institute for Artificial Intelligence and Fundamental Interactions. “Our machine learning system flagged SN?2023zkd months before its most unusual behavior, which gave us ample time to secure the critical observations needed to unravel this extraordinary explosion.”
An alternative interpretation considered by the team is that the black hole completely tore the star apart before it could explode on its own. In that case, the black hole quickly pulled in the star’s debris and supernova emission was generated when the debris crashed into the gas surrounding it. In both cases, a single, heavier black hole is left behind.
Located about 730 million light-years from Earth, SN 2023zkd initially looked like a typical supernova, with a single burst of light. But as the scientists tracked its decline over several months, it did something unexpected: it brightened again. To understand this unusual behavior, the scientists analyzed archival data, which showed something even more unusual: the system had been slowly brightening for more than four years before the explosion. That kind of long-term activity before the explosion is rarely seen in supernovae.
Detailed analysis revealed that the explosion’s light was shaped by material the star had shed in the years before it died. The early brightening came from the supernova’s blast wave hitting low-density gas. The second, delayed peak was caused by a slower but sustained collision with a thick, disk-like cloud. This structure—and the star’s erratic pre-explosion behavior—suggest that the dying star was under extreme gravitational stress, likely from a nearby, compact companion such as a black hole.
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