Carl Sagan once said we are all made of star-stuff. Astronomers have long predicted that some of the heavier elements in the universe that comprise our very being, like carbon and oxygen, are forged inside stars and released when they die and explode in powerful supernovas.
But astronomers had never seen definitive proof of this occurring deep inside stars. Researchers have now discovered a star about two billion light-years from Earth that shed its onionlike layers down to its heaviest material before exploding, hinting at where we came from.
“We know over 10,000 supernovas, but we detected a supernova that is very, very different to anything we’ve observed before,” said Steve Schulze, an astrophysicist at Northwestern University and the lead author of the paper describing the discovery, published Wednesday in Nature. “We had no idea it’s possible to strip a star to this extreme amount.”
Dr. Schulze and his team discovered the supernova, called SN 2021yfj, from data collected by the Zwicky Transient Facility survey run at the Palomar Observatory in California. Additional observations using the Keck Observatory in Hawaii helped pick apart the explosion’s light and the elements that were released.
Dr. Schulze and his team estimated the star to have been about 60 times the mass of our sun — quite a big one. Usually when a star dies, astronomers observe a jumble of elements from its guts mixed together and flung into the cosmos. But for some reason, this one’s layers were stripped away over thousands of years before it exploded.
This is important because it gives a hint of the star’s layered structure, something never seen before. Scientists have long thought stars have onionlike layers. The outer layers, made of lighter elements like hydrogen and helium, give way to increasingly heavier chemicals like carbon, oxygen, magnesium, silicon, sulfur, argon and, finally, iron at the star’s core.
The team observed the moment when the star’s iron core exploded, lighting up a previously expelled stellar layer, which was rich in silicon, sulfur and argon.
Matt Nicholl, an astrophysicist from Queen’s University Belfast in Northern Ireland who was not involved in the study, said that it “confirms this onionskin structure of massive stars that we all expected to see.”
What made the star steadily shed its layers isn’t clear, but Dr. Schulze and his team favor an explanation in which violent internal pulses might have sequentially ripped material from the star.
However, the presence of helium in the star’s final layer is confusing. “It’s a fly in the soup that shouldn’t be there,” Dr. Schulze said. The element should have been one of the first layers ejected, as it is lighter. “The helium should have gone away thousands of years before,” said Anya Nugent, an astrophysicist at the Harvard–Smithsonian Center for Astrophysics and a co-writer of a news and views article published alongside the paper.
One possibility is that a companion star might have torn the star’s layers away and injected helium into the lower layers. Another is that powerful jets might have shot away from the original star and “dredged up material,” Dr. Nugent said, although no such jet was visible.
Supernovas are typically categorized into two main types: Type I, which lack hydrogen, and Type II, which contain hydrogen. SN 2021yfj would be a new class, Dr. Schulze said, which the team is calling Type 1en.
“The properties of this supernova are so unusual,” Dr. Schulze said. “There is no object that is even remotely similar.”
The team estimates that perhaps one in 1,000 supernovas undergoes extreme stripping like this one, but they are hard to spot because they tend to look quite similar to other supernovas unless you really pick apart their light in detail, like SN 2021yfj. “There was a lot of luck involved in making this discovery,” Dr. Schulze said.
Astronomers might detect more Type 1en supernovas in the future using telescopes like the Vera C. Rubin Observatory in Chile, which is set to begin a decade-long survey of the cosmos later this year.
“Rubin is going to detect thousands of supernovae every day,” Dr. Nugent said, perhaps with many more types awaiting discovery.