A dying star revealed its heart

Moments before a dying star exploded, it spilled its deepest secrets. 

The celestial object had shed most of its layers, offering the first glimpse at an innermost shell of a massive star, researchers report in the Aug. 21 Nature. The findings hint at a new type of supernova, in which a star strips nearly to its core before bursting. 

In September 2021, a survey at the Palomar Observatory near San Diego spotted an object — later dubbed 2021yfj — brightening about 2.2 billion light-years from Earth. Astrophysicist Steve Schulze thought it might be a supernova, so he and his colleagues used the W.M. Keck Observatory in Hawaii to take a detailed look in a range of visible wavelengths. 

“The next morning, I have an email in my inbox with the spectrum,” says Schulze, of Northwestern University in Evanston, Ill. “It was very clear to us that it’s something we haven’t seen before.”

Data collected over the following months revealed that the emitted energy and light resembled that of supernovas. But “the thing that made supernova 2021yfj so unique is the elements that we saw,” Schulze says, mainly silicon, sulfur and argon — which are “very rare in the universe.” The atoms’ speeds, reaching up to 3,000 kilometers per second, hinted they were part of an expelled star layer. 

Typically, lighter elements such as helium, carbon and oxygen are found around supernovas whose stars shed their outer shells before exploding. Previous work suggests that pressure and heat within massive stars fuses atoms, which creates onionlike layers of increasingly heavier elements — including silicon, sulfur and argon — deeper within until reaching an iron core. 

Schulze and colleagues propose that 2021yfj’s star contained the mass of about 60 suns after purging its outermost hydrogen layer. The loss of internal photons, or particles of light, caused the star to shrink, raise its temperature and violently eject layers every few thousand years. 

“This way the star is able to rip itself apart, so that eventually the star … gets stripped down to its oxygen-silicon layer, and it even ejects some of this material,” Schulze says. When the remaining star exploded, that material may have collided with the expelled shell, triggering the observed brilliant glow. 

2021yfj’s star was stripped to an extent never seen before and may represent a new supernova class. “We need to detect more,” Schulze says, to “give us a better understanding of which physical processes are at play.”