Astrophysicists solve mystery of last historical supernova from 1181 AD

In 1181 AD, a bright new star appeared near the Cassiopeia constellation and shone in the sky for six months before vanishing. Astronomers in China and Japan recorded this extraordinary event as a "guest star."

For centuries, the nature of this phenomenon puzzled scientists, as it remained one of the few supernovae documented before telescopes were invented, with no visible remnant to explain its origin. Designated SN 1181, this stellar explosion was left unexplained for nearly a millennium, earning the title of an "orphan" supernova. However, in 2021, scientists finally traced its remnant to a nebula called Pa 30, discovered in 2013 by amateur astronomer Dana Patchick.

Pa 30 stands out among other supernova remnants for several reasons. Most notably, at its center lies a surviving remnant of the exploded star—a "zombie star" that still exists despite the destructive supernova that occurred.

Typically, a supernova obliterates the white dwarf star at its core, but in this case, some of the star survived, leading to the formation of a Type Iax supernova. This rare type of explosion results from a thermonuclear event on a white dwarf that doesn't completely destroy the star, leaving behind a partial remnant.

Interestingly, strange filaments emanate from this surviving zombie star, resembling the petals of a dandelion flower. Recent advances in astronomical technology have allowed researchers to study these filaments in greater detail than ever before. Using the Keck Cosmic Web Imager (KCWI), a state-of-the-art spectrograph located at the W. M. Keck Observatory atop Hawaii’s Mauna Kea volcano, scientists were able to capture an unprecedented close-up view of the nebula and its unique features.

The KCWI is designed to detect faint and distant sources of light, such as those found in the "cosmic web," the vast network of gas and galaxies that fills the universe. Its ability to capture spectral information for every pixel in an image allows researchers to create a three-dimensional map of the explosion.

This tool also measures the motion of matter within a supernova by analyzing how light shifts as it moves toward or away from us—a process similar to the Doppler effect experienced with sound waves, like the changing pitch of a passing siren.

Through these observations, astronomers have mapped out the nebula's filaments and shown that they are traveling at approximately 1,000 kilometers per second. Lead researcher Tim Cunningham of the Harvard-Smithsonian Center for Astrophysics explains, "This means that the ejected material has not been slowed down or sped up since the explosion. By measuring these velocities and working backward, we can pinpoint the explosion to almost exactly the year 1181."

The supernova's remnants are also highly asymmetrical, suggesting that this irregularity originated during the explosion itself. The filaments surrounding the zombie star have a sharp inner edge, creating an inner "gap."

Assistant Professor Ilaria Caiazzo from the Institute of Science and Technology Austria (ISTA), who led the research alongside Cunningham, notes that this discovery offers a new perspective on the cosmic event observed nearly a thousand years ago. "Our detailed 3D characterization of the velocity and structure of this supernova remnant not only tells us a lot about this unique event, but it also raises new questions for astronomers to explore," she said.

The guest star observed in 1181 AD is one of only five confirmed Galactic supernovae recorded in human history. Before 2013, SN 1181 was the youngest Galactic supernova without a confidently identified remnant. Patchick’s discovery of Pa 30 changed that. Evidence continues to mount, linking this nebula to SN 1181, with its age and position in the sky matching historical records.

Pa 30's central star is especially unusual, with surface temperatures reaching 200,000 K and wind speeds exceeding 15,000 kilometers per second. The spectra of both the star and the nebula show no signs of hydrogen or helium, which is unusual for supernova remnants. Additionally, X-ray analysis has revealed the presence of carbon-burning ashes, further confirming the nebula’s supernova origin.

Astronomers believe that Pa 30's creation was the result of a failed detonation in a white dwarf star that should have been destroyed by the supernova. Instead, the explosion left behind a subluminous transient, a type of event classified as a Type Iax supernova. These types of supernovae are thought to occur when two white dwarfs merge, but fail to detonate completely, leaving behind a remnant like the zombie star at Pa 30’s center.

Pa 30’s structure is also unique, with its filamentary shape standing in stark contrast to the clumpy internal structures typically observed in supernova remnants. These filaments extend radially from the central star in a symmetrical pattern, resembling cometary tails or the wind-blown tails of certain novae, such as GK Persei and DQ Herculis.

These structures are likely formed by Rayleigh-Taylor instability, which occurs when fluids of different densities interact, and the high-speed wind from the central star could be shaping the filaments by accelerating low-density material outward.

In the infrared spectrum, Pa 30 exhibits a diffuse halo and a smaller, brighter ring that extends about one arcminute from the central star. Researchers have proposed that this nebula emits primarily in infrared wavelengths due to dust and strong emission lines from oxygen and neon atoms, adding yet another layer of complexity to the remnant.

X-ray imaging of Pa 30 has revealed both an outer nebula, which likely marks the location where the supernova's ejecta is interacting with surrounding material, and a much smaller, inner nebula. The origin of this inner nebula is still uncertain, but some scientists suggest it may represent the point where the high-speed winds from the white dwarf collide with the slower-moving material ejected during the supernova.

The strange and enigmatic characteristics of Pa 30 have opened new avenues for studying supernovae, white dwarf stars, and the processes that drive stellar explosions. As astronomers continue to investigate this remnant, they hope to uncover further insights into the forces that shaped this cosmic event and its place in the history of our galaxy.

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