A historical supernova, observed by astronomers in China and Japan in 1181, remained a mystery for over 800 years until its remnants were rediscovered recently. The new findings reveal surprising features that have caused confusion among scientists. A research team has now conducted the first thorough 3D analysis of the supernova’s structure and its expansion speed.
In 1181, a new star appeared near the constellation Cassiopeia, shining for six months before vanishing. This occurrence, noted as a “guest star” by astronomers in China and Japan nearly a thousand years ago, has intrigued scientists for generations. It’s one of the few supernovae recorded before telescopes existed. Additionally, it remained “orphaned” the longest, meaning no current celestial objects could be definitively linked to it. Referred to now as SN 1181, its remnants were traced back to the nebula Pa 30 in 2021, which was discovered in 2013 by amateur astronomer Dana Patchick while reviewing archival images from the WISE telescope as part of a citizen scientist initiative.
However, this nebula is not your average supernova remnant. Surprisingly, astronomers found a surviving “zombie star” at its center—a remnant within a remnant. The explosion that created the 1181 supernova is believed to have been a thermonuclear event on a dense, dead star known as a white dwarf. Usually, such an explosion would obliterate the white dwarf entirely, but in this case, part of the star endured, leading to the formation of a “zombie star.” This phenomenon is categorized as a Type Iax supernova. Compounding the intrigue, peculiar filaments extended from this zombie star, looking like the petals of a dandelion. Researchers, led by ISTA Assistant Professor Ilaria Caiazzo and Tim Cunningham, a NASA Hubble Fellow at the Center for Astrophysics, Harvard & Smithsonian, achieved an unprecedented close examination of these unusual filaments.
A 3D model of a ballistically expanding explosion
The team utilized Caltech’s Keck Cosmic Web Imager (KCWI) to perform an in-depth study of this peculiar supernova remnant. Situated over 4,000 meters at the W. M. Keck Observatory on the summit of Mauna Kea, the highest peak in Hawaii, KCWI serves as a specialized spectrograph.
As its name implies, KCWI was engineered to identify some of the faintest light sources in the universe collectively referred to as the “cosmic web.” Moreover, KCWI’s advanced design allows it to gather spectral data for every pixel in an image, enabling it to assess the motion of matter in a stellar explosion and effectively create a 3D representation of a supernova. This is achieved by observing how light shifts as it travels towards or away from us, a principle akin to the Doppler shift we notice when an ambulance siren changes pitch as it speeds by.
This capability enabled the researchers to generate a detailed 3D map of the nebula and its unusual filaments, moving beyond the typical static images of supernovae. They discovered that the filaments’ material expanded ballistically at around 1,000 kilometers per second. “This indicates that the expelled material has neither slowed down nor accelerated since the explosion,” explains Cunningham. “Thus, by analyzing the measured velocities, we were able to pinpoint the explosion almost precisely to the year 1181.”
Evidence of an unusual asymmetry
In addition to the dandelion-shaped filaments and their ballistic expansion pattern, the overall form of the supernova is particularly unique. The team revealed that the ejecta—material within the filaments being propelled away from the explosion—is notably asymmetrical, suggesting this asymmetry originated during the explosion itself. The filaments also exhibit a defined inner edge, outlining an inner “gap” surrounding the zombie star. “Our detailed 3D characterization of the velocity and spatial features of this supernova remnant provides insight into a remarkable cosmic event that our ancestors witnessed centuries ago. However, it also raises new questions and presents fresh challenges for astronomers to explore in the future,” concludes Caiazzo. She began this research project as a Burke-Sherman Fairchild Postdoctoral Fellow in theoretical astrophysics at Caltech, USA, before joining ISTA in May of this year.
