Free Astronomy Magazine November-December 2025

42 NOVEMBER-DECEMBER 2025 ASTRO PUBLISHING A rtist’s illustration of the super- nova’s aftermath. Even after the star was stripped down to its core, it continued to experience vio- lent mass-loss episodes, ejecting shells of material rich in silicon (grey), sulfur (yellow), and argon (purple). The catastrophic collision of these massive shells, as depicted in this illustration, generated a bril- liant supernova explosion visible across 2.2 billion light-years of space. [Keck Obs./Adam Makarenko] Weighing in at 10 to 100 times heavier than our sun, massive stars are powered by nuclear fusion. In that process, intense pressure and extreme heat in the stellar core cause lighter elements to fuse to- gether, generating heavier ele- ments. As the star evolves over time, successively heavier elements are burned in the core, while lighter elements are burned in a series of shells around the core. This process continues, eventually leading to a core of iron. When the iron core col- lapses, it triggers a supernova or forms a black hole. Although massive stars typically shed layers before exploding, SN 2021yfj ejected far more material than scientists had ever previously detected. Other observations of “stripped stars” have revealed inner layers of helium or carbon and oxy- gen — exposed after the outer hy- drogen envelope was lost. But astrophysicists had never glimpsed anything deeper than that. “Stars experience very strong insta- bilities,” Schulze said. “These insta- bilities are so violent that they can cause the star to contract. Then, it suddenly liberates so much energy that it sheds its outermost layers. It can do this multiple times.” Schulze and their team discovered SN 2021yfj in September 2021, using the wide-field camera on the Zwicky Transient Facility (ZTF) based on Palomar Mountain in Southern Cal- ifornia. After looking through ZTF data, Schulze spotted an extremely luminous object in a star-forming region located 2.2 billion light-years from Earth. Without any idea what it was, but recognizing they had never seen it before, Schulze and Miller sought to obtain the object’s spectrum to determine which elements were present in the explosion. Schulze contacted Yi Yang, then a postdoctoral scholar in Alex Filip- penko’s group and now an assistant professor at Tsinghua University, China. Filippenko, a distinguished professor of astronomy at the Uni-