In 1181, ancient Chinese and Japanese astronomers recorded the sudden appearance of a bright “guest star” near the Cassiopeia constellation that lingered for six months before fading from view. This rare event puzzled astronomers for centuries as it remained the only known supernova without a confirmed modern remnant. Dubbed SN 1181, this supernova continued to be an enigma until recent advances in technology helped solve parts of the mystery.
The breakthrough came in 2021 when researchers traced the remnants of SN 1181 to an unusual nebula called Pa 30. This nebula had first been identified in 2013 by amateur astronomer Dana Patchick, who discovered it in archived images from the WISE (Wide-field Infrared Survey Explorer) telescope as part of a citizen science initiative. Despite this discovery, Pa 30 turned out to be far from an ordinary supernova remnant. At its core was a remarkable find—a “zombie star,” or a stellar remnant that had survived a catastrophic explosion.
The 1181 supernova is now believed to have been a rare Type Iax supernova. In such events, a thermonuclear explosion occurs on the surface of a white dwarf, a dense remnant of a star that has exhausted its nuclear fuel. Unlike standard Type Ia supernovae, where the white dwarf is completely obliterated, a Type Iax supernova is only partial, leaving behind part of the original star. This surviving core is what astronomers refer to as a “zombie star.”
What further intrigued scientists were the unusual filaments extending outward from this zombie star. These filamentary structures had a shape reminiscent of dandelion petals, which caught the attention of Ilaria Caiazzo, now an assistant professor at the Institute of Science and Technology Austria (ISTA), and Tim Cunningham, a NASA Hubble Fellow at the Center for Astrophysics, Harvard & Smithsonian. Their team conducted a detailed study of these filaments to learn more about this peculiar explosion.
The findings from their research were published in The Astrophysical Journal Letters. The team used the powerful Keck Cosmic Web Imager (KCWI), a sophisticated spectrograph at the W. M. Keck Observatory in Hawaii. KCWI is designed to capture some of the faintest cosmic light sources and can provide detailed spectral information for every pixel in an image. This capability allows astronomers to measure the motion of matter, effectively creating a 3D representation of how material expands following a stellar explosion.
The Keck Cosmic Web Imager’s technology works by observing the Doppler shift of light, similar to how the pitch of a siren changes as an ambulance passes by. By analyzing these light shifts, researchers were able to map the movement of matter in Pa 30’s filaments and create a 3D model that displayed the behavior of the supernova’s ejecta. Cunningham noted that the filaments were traveling ballistically at an impressive speed of about 1,000 kilometers per second. This consistent speed indicated that the material had not decelerated or accelerated since the original explosion.
“From the measured velocities, looking back in time allowed us to pinpoint the explosion to almost exactly the year 1181,” Cunningham explained. This precise dating linked Pa 30 definitively to the supernova observed nearly a millennium ago.
The team also uncovered evidence of asymmetry within the explosion, which was an unexpected revelation. The ejected material from the supernova appeared unevenly distributed, suggesting that the explosion itself had an asymmetric nature. The filaments showed a sharp inner edge, indicating a gap around the zombie star at the center of the nebula. This unique shape further confirmed that SN 1181 was no ordinary supernova.
Caiazzo reflected on the significance of this research, noting that it provided the first detailed 3D characterization of both the velocity and spatial structure of a supernova remnant. “Our study not only gives us insights into a remarkable cosmic event that our ancestors witnessed, but it also raises new questions and presents challenges for future astronomical research,” she said.
Before joining ISTA, Caiazzo was a Burke-Sherman Fairchild Postdoctoral Fellow in theoretical astrophysics at Caltech, where she began her work on this project. The new understanding of SN 1181 and Pa 30 showcases how modern technology, such as KCWI, can illuminate historical celestial events with unprecedented clarity. For Cunningham and Caiazzo, these findings open doors to further study, as they plan to apply their approach to other supernova remnants and continue probing the mysteries of stellar explosions.
This study has reshaped our understanding of SN 1181, offering a glimpse into the powerful processes that can unfold when a dying star refuses to disappear quietly, instead leaving behind a lasting, puzzling legacy in the form of a “zombie star.”