Astronomers have peered directly at a star's inner layers for the first time, thanks to a stellar explosion 2.2 billion light-years away.
The star was stripped of its outer shells, revealing its dense innards to the Keck Observatory in Hawaii just after it exploded in a cataclysmic blast.
The star's demise, reported in the journal Nature, also confirms current theories about stellar evolution.
Study lead author and Northwestern University astronomer Steve Schulze said their team was "awestruck" by the discovery, but there's plenty still to uncover about the supernova dubbed 2021yfj.
"A lot of work remains to be done to fully understand supernova 2021yfj and its implications for the evolution and fate of massive stars."
Stars burn for billions of years, but their deaths can be very abrupt, with supernova explosions over in a matter of hours.
Despite detecting thousands of supernovas, their brevity makes it difficult for astronomers to study them in detail.
Anatomy of a dying star
Generally, stars burn by converting hydrogen into progressively heavier elements via nuclear fusion.
When massive stars run out of hydrogen, they collapse in on themselves, producing a violent supernova explosion.
But before the explosion, astronomers believe a star's heavier elements settle into onion-like layers, with the heaviest — iron — at the core of the biggest stars.
That iron core is surrounded by layers of lighter elements such as argon, sulphur and silicon, which are in turn surrounded by elements that are lighter still, such as oxygen and carbon. Helium and hydrogen form the outermost layers, or the skin of the onion.
Until now, Dr Schulze said, it was nearly impossible to see any of these deeper layers directly.
"The silicon- and sulphur-rich layer is buried under many other materials and is therefore inaccessible [by telescopes] under normal circumstances."
Catching a cosmic moment takes luck
The astronomers were able to see the true weirdness of 2021yfj because they took advantage of a lucky cosmic moment.
In September 2021, a survey of the night sky by the Zwicky Transient Facility in California spotted a bright flash in a star-forming region some 2.2 billion light-years from Earth.
Dr Schulze and their team raced to see if any observatories could collect a spectrum — a record of light from the explosion showing elements present — shortly after the supernova happened.
The Keck Observatory managed to provide one within a day of the explosion.
But rather than seeing the relatively light element shells usually present in a supernova, such as carbon, nitrogen and oxygen, the spectrum was full of heavier elements: silicon, sulphur, and argon.
This means that something pulled those upper layers off the star before it exploded, exposing the layers closer to the core.
Brad Tucker, an astronomer at the Australian National University who wasn't involved in the research, said this find was an "important confirmation" of stellar structure.
"We've always modelled it this way. We've had evidence from our understanding of nuclear physics.
"This is that fundamental evidence that proves that model, and that view, is right."
How did this stripped star form?
The researchers don't know exactly what removed so many outer layers of the star before the supernova.
The most likely theory, they suggest, is a massive star more than 100 times the mass of our Sun fired its outer material off in waves right before exploding.
Dr Schulze said this behaviour had been seen with a few other stars — although only with their lighter upper hydrogen and helium layers, not the heavier elements below.
Dr Tucker agreed this was a likely explanation, and that a star ejecting its lower layers prior to explosion was "the next logical step" after venting the lighter elements.
But there are still puzzles in the data. The researchers also found small amounts of helium in the star's debris, for instance, which they don't yet have a good explanation for.
Dr Schulze said the supernova they observed was "only one among the more than 10,000 supernovas known, making it a very rare event".
Nevertheless, they're confident that astronomers will be able to observe another partially denuded star exploding and learn more about the strange phenomenon.
"We are detecting supernovas at an ever-increasing rate due to advances in monitoring the night sky," Dr Schulze said.
New facilities, like the Rubin Observatory in Chile, allow astronomers to see supernovas in much greater number, according to Dr Tucker.
"We now have surveys that are getting hundreds to thousands of supernovas a year, whereas previously we'd get dozens to maybe 100," he said.
