Free Astronomy Magazine November-December 2025

41 NOVEMBER-DECEMBER 2025 ASTRO PUBLISHING Research in Astrophysics (CIERA). “It shows us how stars are structured and proves that stars can lose a lot of material before they explode. Not only can they lose their outer- most layers, but they can be com- pletely stripped all the way down to the core and still produce a brilliant explosion that we can observe from very, very far distances.” When massive stars explode, astro- physicists typically find strong sig- natures of light elements, such as hydrogen and helium. But the newly discovered super- nova, dubbed SN 2021yfj, displayed a startling different chemical signa- ture containing silicon, sulfur and argon, suggesting it somehow lost its outer hydrogen, helium, carbon, oxygen, neon, and magnesium lay- ers — exposing the inner silicon and sulfur-rich layers — immediately be- fore exploding. Astronomers have long proposed that massive stars possess an onion- like layered structure, with the lightest elements in the outermost shells and progressively heavier ele- ments toward the center. The dis- covery of SN 2021yfj provides direct evidence of long-hypothesized in- ternal layering in giant stars. It of- fers an unprecedented glimpse inside a stellar giant’s interior — captured just moments before its explosive death. “This event quite literally looks like nothing anyone has ever seen be- fore,” added Adam Miller, an assis- tant professor of physics and as- tronomy at Northwestern and sen- ior author on the study. “This star is telling us that our ideas and theo- ries for how stars evolve are too narrow. It’s not that our textbooks are incorrect, but they clearly do not fully capture everything pro- duced in nature. There must be more exotic pathways for a massive star to end its life that we hadn’t considered.” T his video illustrates the supernova explosion SN 2021yfj and the formation of its remnant. [Keck Obs./Adam Makarenko]