The SN 2021yfj supernova loses its outer layers long before, leaving only an oxygen and silicon core. The remaining star undergoes extreme mass loss stages, ejecting material rich in silicon (gray), sulfur (yellow), and argon (purple). /Courtesy of W.M. Keck Observatory/Adam Makarenko

Through the TV program 'Cosmos', astronomer Carl Edward Sagan (1934-1996) led the popularization of science. He said that our bodies contain remnants of stars. Stars generate energy by fusing light atoms such as hydrogen and helium under immense temperature and pressure. The abundant oxygen and carbon in the human body were produced during nuclear fusion and released into space when a star dies and explodes, becoming a supernova. The fragments of the distant universe have now entered our bodies.

American scientists have for the first time observed a star shedding its outer layer like an onion just before its death, revealing the heavy atoms mixed in the innermost layer. This is the first visual confirmation of the theory of stellar evolution. The star revealed its inner self before meeting its fate while also showing light atoms that it thought couldn't coexist. It provided secrets to scientists while leaving them with problems to solve.

The ancestor star of SN 2021yfj already sheds its oxygen/silicon core long before the supernova occurs. Near the end of its life, an instability phenomenon occurs twice, ejecting a massive shell rich in silicon, sulfur, and argon into space. Later, the energy released during the explosion collides with these shells, creating a brilliant supernova visible even from 2.2 billion light-years away. /Courtesy of Adam Makarenko/Keck Observatory

◇First observation of the deep interior just before a star's death

An international joint research team led by Dr. Steve Schulze from Northwestern University in the U.S. announced on the 21st in the international journal 'Nature' that they discovered a new type of supernova, SN2021yfj, which is rich in heavy atoms such as silicon, sulfur, and argon, through data collected at the Palomar Observatory in California. The Keck Observatory in Hawaii helped analyze the light and emitted atoms generated by the explosion through additional observations.

A supernova is the phenomenon of a massive star emitting an enormous amount of light at the moment of its death. Light atoms such as hydrogen and helium are primarily found outside at this time. The SN2021yfj discovered 2 billion light-years (a light year is the distance light travels in one year, approximately 9.46 trillion kilometers) away from Earth showed much heavier atoms.

For a long time, astronomers thought that massive stars had an onion-like structure. The outermost layers are composed of the lightest atoms, hydrogen and helium, ranked 1 and 2 by atomic mass in the periodic table, while moving inward, the atomic mass gradually increases, leading to layers of carbon, oxygen, magnesium, silicon, sulfur, argon, and finally, the iron core.

Stars have an onion structure thanks to nuclear fusion. Massive stars that are 10 to 100 times heavier than our sun generate heavier atoms by fusing lighter atoms under immense pressure and temperature in their cores. An enormous amount of energy is produced as mass decreases in this process. This is why stars shine. Nuclear fusion power generation is a way to mimic this process on Earth.

The observation results of SN2021yfj suggest that the outer hydrogen, helium, and carbon layers of the massive star were lost, exposing the inner layers rich in silicon, sulfur, and argon. Dr. Schulze said, "This discovery is the first observation of a star with its outer layers stripped down to just its bones, providing direct evidence of the internal layer structure of massive stars, which has long been known only through theory."

SN 2021yfj supernova. a) The star's nuclear fusion creates a layered structure with heavy elements at the center and light elements at the outer edge. b) The silicon-sulfur layer observed by the researchers is expelled near the end of the star's life, leaving an iron core surrounded by a silicon-rich layer. c) When the star reaches the end of its life, the core collapses into a high-density neutron star or black hole, releasing an enormous amount of energy that explodes into the supernova known as 2021yfj. This collides with the silicon-sulfur layer, emitting the light observed this time. /Courtesy of W.M. Keck Observatory/Adam Makarenko

◇Cannot be explained by existing theories, classified as a new type of supernova

At the very core of a star lies a heavy iron core. When a star reaches the end of its life, the central iron core compresses inward under its own gravity, becoming hotter and denser. Extreme heat and temperature trigger nuclear fusion again, and during this process, an enormous explosion occurs, pushing the outer layers of the star outward.

Dr. Schulze's research team estimated that this star has a mass about 60 times that of our sun. Typically, when a star dies, the atoms inside are observed scattering chaotically into space. However, in this case, the star exhibited a phenomenon of shedding its layers in an onion-like manner thousands of years before exploding as a supernova. The research team observed the moment the star's iron core exploded, and as a result of this explosion, the previously emitted layers of the star shone brightly, particularly the layers rich in silicon, sulfur, and argon.

The reason why the star shed its layers in succession is unclear. Dr. Schulze's research team speculated that pulses, like electromagnetic waves or pressure waves, caused by a rapid release of energy from within ripped apart the star's material in sequence.

However, this theory does not explain the observation of helium. Helium is a light element that should have been emitted first from the star. Dr. Anya Nugent from the Harvard-Smithsonian Center for Astrophysics noted in a commentary paper published in 'Nature' that "helium should have disappeared thousands of years ago."

One possibility is that a companion star orbiting this star may have injected helium inward. Alternatively, there could have been a jet emitted from a primordial star. However, Dr. Nugent stated that such a jet was not observed.

Supernovae are generally classified into two main types: Type I, which does not include hydrogen, and Type II, which includes hydrogen. SN2021yfj is a new type, and the research team named it Type 1en. Astronomers expect to discover more Type 1en supernovae in the future using telescopes like the Vera C. Rubin Observatory in Chile. The Rubin Observatory is set to begin a 10-year space survey starting at the end of this year.

References

Nature(2025), DOI: https://doi.org/10.1038/s41586-025-09375-3

Nature(2025), DOI: https://doi.org/10.1038/d41586-025-02425-w

※ This article has been translated by AI. Share your feedback here.