Abstract
Stars are initially powered by the fusion of hydrogen to helium. These ashes serve as fuel in a series of stages1,2,3, transforming massive stars into a structure of shells. These are composed of natal hydrogen on the outside and consecutively heavier compositions inside, predicted to be dominated by He, C/O, O/Ne/Mg and O/Si/S (refs. 4,5). Silicon and sulfur are fused into iron, leading to the collapse of the core and either a supernova explosion or the formation of a black hole6,7,8,9. Stripped stars, in which the outer hydrogen layer has been removed and the internal He-rich or even the C/O layer below it is exposed10, provide evidence for this shell structure and the cosmic element production mechanism it reflects. The supernova types that arise from stripped stars embedded in shells of circumstellar material (CSM) confirm this scenario11,12,13,14,15. However, direct evidence for the most interior shells, which are responsible for producing elements heavier than oxygen, is lacking. Here we report the discovery of the supernova (SN) 2021yfj resulting from a star stripped to its O/Si/S-rich layer. We directly observe a thick, massive Si/S-rich shell, expelled by the progenitor shortly before the supernova explosion. Exposing such an inner stellar layer is theoretically challenging and probably requires a rarely observed mass-loss mechanism. This rare supernova event reveals advanced stages of stellar evolution, forming heavier elements, including silicon, sulfur and argon, than those detected on the surface of any known class of massive stars.
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Data availability
The reduced spectra and photometry of SN 2021yfj are available on the WISeREP archive (https://www.wiserep.org/object/19115). The raw data of the observations acquired with the European Southern Observatory (X-shooter; programme IDs: 105.20PN, 105.20KC), the W. M. Keck Observatory (LRIS), the Lick Observatory (Nickel and Kast), the Liverpool Telescope (IO:O; programme IDs: JL21A14, JZ21B01), the Neil Gehrels Swift Observatory (UVOT, XRT; object ID: 00014807), the Nordic Optical Telescope (ALFOSC; programme IDs: 61-501, 64-501) and the Zwicky Transient Facility (P48) can be retrieved from their designated public data repositories.
Code availability
Much analysis for this paper has been performed with publicly available software packages. The details required to reproduce the analysis are contained in the manuscript.
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Acknowledgements
M.W.C. acknowledges support from the U.S. National Science Foundation (NSF) with grants PHY-2308862 and PHY-2117997. A.V.F.’s group at UC Berkeley is grateful for financial assistance from the Christopher R. Redlich Fund, Gary and Cynthia Bengier, Clark and Sharon Winslow, Alan Eustace (W.Z. is a Bengier–Winslow–Eustace Specialist in Astronomy), William Draper, Timothy and Melissa Draper, Briggs and Kathleen Wood, Sanford Robertson (T.G.B. is a Draper–Wood–Robertson Specialist in Astronomy) and many other donors. A.G.-Y.’s research is supported by the ISF GW excellence centre, an IMOS space infrastructure grant and BSF/Transformative and GIF grants, as well as the André Deloro Institute for Space and Optics Research, the Center for Experimental Physics, a WIS-MIT Sagol grant, the Norman E. Alexander Family M Foundation ULTRASAT Data Center Fund and Yeda-Sela; A.G.-Y. is the incumbent of the Arlyn Imberman Professorial Chair. N.K. was supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (MST-DIRS) through contract no. 451-03-66/2024-03/200002 made with the Astronomical Observatory Belgrade and contract no. 451-03-66/2024-03/200104 made with the Faculty of Mathematics at the University of Belgrade. R.L. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon Europe research and innovation programme (grant agreement 1010422). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. K. Maeda acknowledges support from the JSPS KAKENHI grants JP20H00174 and JP24H01810. A.A.M. and S.S. are partially supported by LBNL subcontract no. 7707915. N.S. acknowledges support from the Knut and Alice Wallenberg Foundation through the ‘Gravity Meets Light’ project. Y.T. acknowledges support from the JSPS KAKENHI grant 23H04900. D.T. is supported by the Sherman Fairchild Postdoctoral Fellowship at the California Institute of Technology. Y. Yang’s research is partially supported by the Tsinghua University Dushi programme and he was a Bengier–Winslow–Robertson Postdoctoral Fellow in Astronomy at UC Berkeley. We appreciate the excellent assistance provided by the staff at the various observatories in which the data were obtained. UC Berkeley undergraduate student E. Liu is thanked for her efforts in obtaining the Lick/Nickel data. Based in part on observations obtained with the 48-inch Samuel Oschin Telescope and the 60-inch Telescope (P60) at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the U.S. NSF under grants AST-1440341 and AST-2034437 and a collaboration including present partners Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Centre at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, University of Warwick, Ruhr University Bochum, Northwestern University and former partners the University of Washington, Los Alamos National Laboratories and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC and UW. ZTF access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The SED Machine on P60 is based on work supported by NSF grant 1106171. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation. Based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme(s) 105.20KC and 105.20PN. Data presented here were obtained in part with ALFOSC, which is provided by the Instituto de Astrofísica de Andaluciá (IAA) under a joint agreement with the University of Copenhagen and NOT. KAIT and its continuing operation at Lick Observatory were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the U.S. NSF, the University of California, the Sylvia & Jim Katzman Foundation and the TABASGO Foundation. A notable upgrade of the Kast spectrograph on the Shane 3-m telescope at Lick Observatory was made possible through generous gifts from William and Marina Kast as well as the Heising-Simons Foundation. Research at Lick Observatory is partially supported by a generous gift from Google. Based on observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council. We acknowledge the use of public data from the Swift data archive.
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Contributors are sorted alphabetically. Observations and data reduction: M.B., R.L., S.S. (X-shooter), T.G.B., A.V.F., Y. Yang and W.Z. (Keck, KAIT), K.H. and D.A.P. (LT), S.S. and J.S. (NOT), Y.S. and T.S. (P200), R.L., D.A.P., Y.S. and Y. Yao (Keck), S.S. (Swift), A.G. (bolometric light curve of SN 2020al) and K.H. and D.A.P. (ZTF BTS catalogue). Discoverer of the Si, S, Ar lines: A.G.-Y. Analysis: P.C., L.D., A.G.-Y., I.I., N.K., A.A.M., D.A.P., N.S., S.S., N.L.S., D.T. and O.Y. Discussion, interpretation and paper review: all authors contributed to discussions, interpretation and paper review. Paper writing: L.D., A.V.F., A.G.-Y., A.A.M., S.S., J.S., N.L.S., D.T. and S.E.W. ZTF Infant SN Programme 2018–2023: R.J.B., M.B., P.C., S.D., A.G.-Y., I.I., S.S., J.S., N.L.S., Y. Yang, O.Y. and E.A.Z.
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Extended data figures and tables
Extended Data Fig. 1 Spectral evolution from days 1 to 49.8 of SN 2021yfj in the UV-optical and NIR.
a, UV-optical. b, NIR. Up to day 11, the spectra are characterized by a black-body shape with superimposed narrow emission and P Cygni lines from silicon, sulfur, argon, carbon and helium. As the photosphere cools, the ionization state of silicon, sulfur and argon decreases. By day 20, the spectrum is dominated by a blue pseudocontinuum with superimposed intermediate-width emission lines from magnesium, silicon, sulfur and helium. The most prominent features of both phases are marked. Regions of high atmospheric absorption are marked (‘⊕’) and a NIR spectrum of the opacity of Earth’s atmosphere is shown as black vertical lines (black = high opacity). Host-galaxy emission lines are clipped. The original spectra are in grey and rebinned versions are in black.
Extended Data Fig. 2 Evolution of the line profiles of selected lines from helium, magnesium, silicon and sulfur.
a, At early times, all lines show well-developed P Cygni profiles. The absorption minima are at about 1,500 km s−1. The blue edge, a proxy of the maximum velocity, extends to about 3,000 km s−1. These velocities are comparable with velocities of winds around some Type Icn supernovae13,14, CSM shells around supernovae (refs. 24,64,110) and stellar winds as seen in Wolf–Rayet stars10 and much slower than supernova ejecta velocities at similar phases (approximately 10,000 km s−1 (ref. 23)). The Si iii and S iv lines are blended with other lines and exhibit complex line profiles. The Mg ii line shows narrow absorption lines from the ISM in the host galaxy. b, Up to day 6, the 5,876-Å feature shows a well-developed P Cygni profile and is dominated by He i. At later phases, this feature transitions into a pure emission line with time-variable contributions from silicon, sulfur and helium. The spectra are rebinned for illustration purposes.
Extended Data Fig. 3 Spectrum obtained 1.6 days after the first ZTF detection with VLT/X-shooter, after subtracting the black-body continuum.
The full spectrum covers the wavelength range from 2,635 to 21,960 Å. The shown wavelength range is limited to 2,720–11,000 Å, at which supernova features are well visible. The top panel shows the same wavelength interval as the discovery spectrum in Fig. 1 obtained 12 h earlier. The evolution between both epochs is gradual at most. As well as the supernova features, the spectrum shows emission lines from star-forming regions in the host galaxy and narrow absorption lines from the host ISM. Strong telluric features are marked with ‘⊕’.
Extended Data Fig. 4 Diagnostic plot for assessing the significance of the supernova features in the X-shooter spectrum from day 1.6 (Extended Data Fig. 3).
The intensity spectrum is shown in black and the 1σ error spectrum is shown as a grey band. The red spectrum shows the corresponding S/N spectrum. The significance of a feature is the product of its average S/N ratio and the square root of the number of spectral bins. To guide the eye, a scale is provided in the upper-right corner of each panel. All labelled features (based on Supplementary Table 1) have a significance between 3 and several 10σ. For clarity, the spectrum was rebinned to 1.3 Å (top and middle panels) and 2.6 Å (bottom panel). Host-galaxy emission lines are indicated by the dotted vertical lines and strong telluric features by the ‘⊕’ marker.
Extended Data Fig. 5 SN 2021yfj in a four-dimensional light-curve feature space, together with 4,032 extragalactic transients from the ZTF BTS (79% Type Ia supernovae, 11% Type II supernovae and 10% other types of core-collapse supernovae and other types of transient).
The panels above the main diagonal show all measurements in different projections of the feature space, the panels below the main diagonal show two-dimensional kernel density estimates and the panels on the main diagonal show one-dimensional kernel density estimates. The locations of SN 2021yfj and Type Ibn/Icn supernovae are highlighted in all two-dimensional projections. The light curve of SN 2021yfj shares similarities with Type Ibn and Icn supernovae: a fast rise and a high peak luminosity. However, it sustains a high luminosity for a much longer period of time, which is uncommon for interaction-powered SESNe but comparable with normal supernovae. The combination of short rise and long duration places SN 2021yfj in a sparsely populated area of the light-curve parameter space.
Extended Data Fig. 6 Fits of the light curve of SN 2021yfj with models of three different powering mechanisms.
a, The CSM model provides an adequate description of the observations with ejecta mass Mej = 5 M⊙, a CSM mass of ≳1 M⊙ but <Mej and an explosion energy of approximately 1.6 × 1051 erg (Methods section ‘Light-curve modelling’). The mismatch between the observed and predicted rise can probably be mitigated with more complex CSM geometries and CSM density profiles than those considered here. The shaded bands of the bolometric light curve indicate the statistical uncertainties at the 1σ confidence level. b, Although CSM interaction is the primary source of energy, a spinning down magnetar or the decay of 56Ni might contribute to the peak luminosity of SN 2021yfj. The fit quality and the unphysical model parameters (magnetar: extremely low opacity; nickel model: 97% nickel mass fraction) rule out a substantial contribution of these powering mechanisms to the peak luminosity (Supplementary Information section ‘Additional light-curve fits’). The vertical dotted line in each panel indicates the date of the last non-detection.
Extended Data Fig. 7 Comparison of the light curves (a and b) and black-body properties (c and d) of SN 2021yfj with those of other interaction-powered SESNe and the Type Ic SN 2020oi.
Compared with examples of interaction-powered SESNe as well as the nickel-powered SN 2020oi, SN 2021yfj has a bright peak luminosity and a fast rise. Its black-body radius and temperature evolve slowly compared with Type Icn supernovae. This gradual evolution is reminiscent of some Type IIn supernovae that are embedded in an optically thick CSM (for example, ref. 67), whereas the rapid evolution of Type Icn events suggests a CSM that is much less optically thick (Methods section ‘Bolometric light curve’). The vertical dotted line in each panel indicates the date of the last non-detection of SN 2021yfj. The statistical uncertainties at the 1σ confidence level are indicated as vertical error bars in the left panel and as bands in all other panels. Non-detections are shown as ‘▾’.
Supplementary information
Supplementary Information
This file provides details about the discovery, observations and data reduction, and additional discussions on SN2021yfj regarding the redshift measurement, limits on pre-cursor activity, X-ray emission, event rate, and host galaxy.
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Schulze, S., Gal-Yam, A., Dessart, L. et al. Extremely stripped supernova reveals a silicon and sulfur formation site. Nature 644, 634–639 (2025). https://doi.org/10.1038/s41586-025-09375-3
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DOI: https://doi.org/10.1038/s41586-025-09375-3