What spectral dominance indicated SN 2021yfj was stripped bare?

Heavy fusion products like silicon, sulfur, and argon.

SN 2021yfj provided direct observational evidence regarding the internal structure of massive stars that undergo significant pre-supernova mass loss. Standard stripped-envelope supernovae often show spectra dominated by hydrogen or helium if outer layers remain, or carbon if only inner layers are exposed. However, the spectrum of SN 2021yfj was overwhelmingly characterized by the presence of heavier elements forged deep within the star, specifically silicon, sulfur, and argon. This composition strongly suggested that the star had been entirely 'stripped to the bone,' meaning all its lighter outer envelopes were lost prior to detonation. This exposed the inner, chemically processed layers, providing unprecedented insight into the layered structure resulting from processes potentially related to pair-instability or extreme mass loss episodes.