What does a star do when it explodes with nothing to conceal? For SN 2021yfj, the answer is a supernova that has revealed stripped back to expose innermost layers of a massive star to give an unprecedented look at where silicon and sulfur are fabricated in the universe.
These enormous stars burn high-temperature, high-luminosity lives, fusing hydrogen into helium and then progressing through increasingly heavy fuel in the onion-like layers of the shells. At later stages, fusion in the innermost parts creates oxygen, silicon, and sulfur, which are subsequently transmuted into iron-group elements in the instant before the star’s core collapses. These inner layers have been the sole domain of theory and simulation, as the majority of supernovae merely lose the outer hydrogen, helium, or carbon–oxygen layers, and the silicon–sulfur region is forever hidden.
SN 2021yfj disrupted that trend. Found in September 2021 by the Zwicky Transient Facility’s 48‑inch Samuel Oschin Telescope at Palomar Observatory, the transient was immediately targeted for follow-up. Despite early setbacks from weather and scheduling, a fortuitous observation window at the W. M. Keck Observatory in Hawai‘i yielded spectra that startled astronomers. Rather than helium or carbon signatures characteristic of stripped-envelope Type Ib or Ic supernovae, the observations registered powerful absorption and emission lines of silicon, sulfur, and even argon elements that are produced in the hot fusion regions just above the iron core.
The profiles of the emission lines, which were taken at high resolution, showed expansion speeds and densities characteristic of a large newly ejected shell. This implied that the progenitor star had shed not just its hydrogen and helium, but even its carbon–oxygen mantle, to expose the O/Si/S-rich layer. Such severe stripping is unusual and suggests unusual pre-supernova mass-loss mechanisms. One scenario for such processes is based on pulsational pair-instability, wherein runaway thermonuclear burning in the core produces recurring, violent outbursts that strip off successive shells. A different scenario is late-binary interaction, in which a close stellar companion steals the outer layers by gravitational stripping over the few years leading up to collapse.
The rarity of this configuration makes its detection of scientific significance. Direct observation of silicon- and sulfur-enriched material in the circumstellar regime provides an important test of stellar evolution and nucleosynthesis models. As Schulze et al. described, “This supernova originated from a star stripped down to its O/Si/S-rich inner shell, an unprecedented glimpse into advanced stellar evolution stages.” Verification that such layers can be stripped and ejected before core collapse constrains predictions of elemental yields that underlie galactic chemical evolution models.
The consequences reach beyond the inside of stars. Supernovae are major sources of heavy elements in galaxies, and the fact that intermediate-mass elements such as silicon and sulfur can be propelled into space ahead of time in dense shells prior to the main explosion shifts the way astrophysicists simulate their distribution. These elements contribute to the formation of rocky planets and indirectly to the conditions for life.
Observation of the fleeting and unusual phenomenon required a concerted, multi-instrument initiative. The survey capability of ZTF at wide field enabled rapid identification, and the Keck Observatory spectrographs yielded the wavelength coverage and resolution necessary to identify individual silicon and sulfur ionization states. Follow-up photometry from observatories like the Lick Observatory, ALFOSC instrument on the Nordic Optical Telescope, and the Liverpool Telescope obtained photometric and spectroscopic measurements in various bands, limiting the explosion parameters and light curve.
The discovery also underlines the need for untargeted surveys and quick-response facilities. Facilities such as ZTF can observe thousands of square degrees in a night, but without being able to rapidly slew large telescopes for follow-up, the narrow window of observation for such transients would close. In SN 2021yfj, the decisive Keck spectra were obtained because of an unplanned observing time that was scheduled by a UC Berkeley collaborator, highlighting the importance of human networks over hardware.
As newer-generation facilities such as the Vera C. Rubin Observatory come online, the detection rate for rare stripped-core supernovae is expected to soar. Combined with further refinements in radiative transfer modeling and hydrodynamical simulations, these observations will help untangle the physical drivers of extreme mass loss and work out how often stars perish with their silicon–sulfur hearts torn asunder. For now, SN 2021yfj is a lone event a cosmic autopsy that has, in a one-time way, exposed astronomers to the inner workshop where two of the universe’s most important ingredients are manufactured.
