The ultimate cosmic “unsolved mystery” has been a mystery for 843 years. And now, space detectives may have finally solved it. In 1181 AD, during the Genpei War in Japan, a mysterious “guest star” briefly appeared over Asian skies. Astronomers puzzled over the brief event until 2021, when a team of researchers tracked its location in space. But the cause of the event, now named Supernova (SN) 1181, remains a mystery.
It wasn’t until a team of scientists used computer modeling and observational analysis to recreate the event that they discovered that it was a supernova caused by the collision of two “dead stars” – white dwarfs. The remnants of the white dwarf structure and the remnants of the double impact formation were left behind by this rare occurrence of two colliding white dwarfs.
But there’s more. The same team found that high-velocity stellar winds began to blow from the surface of remnant white dwarfs just 20 to 30 years ago. This discovery highlights the power of combining cutting-edge science from modern astronomy with historical records to learn about the universe. More specifically, the new results could help us better understand the diversity of supernovae.
“There are many stories in historical records from Japan, China and Korea about this transient guest star, whose brightness at its peak was similar to that of Saturn,” said Takatoshi Go, team leader at the University of Tokyo’s Department of Astronomy, in a statement. “It was visible to the naked eye for about 180 days, but then gradually faded and became invisible. The remnants of the SN 1181 explosion are now very old, dark and difficult to find.”
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‘Dead stars’ colliding in the sky
White dwarfs are the cooled stellar embers that form when stars with masses similar to the Sun die. As these stars exhaust the hydrogen fuel needed for nuclear fusion in their cores, the outward thrust of radiation pressure that this process creates also ends. This ends the tug-of-war between the inward thrust of the star’s gravity and radiation pressure that has been going on for billions of years.
As gravity wins this cosmic challenge, the cores of these stars undergo gravitational collapse, shedding their outer layers in what is called a red giant phase. Our star, the Sun, will undergo this process in about 5 billion years, expanding as it orbits Mars as a red giant, engulfing the inner planets, including Earth.
Eventually, the outer layers of the dead star drift away, leaving behind a cooling stellar core that is a cosmic ember the size of Earth, called a white dwarf.
While the Sun exists in isolation, about 50% of stars of similar mass exist in binary systems with another star, which can also become dense, compact white dwarfs.
As the binary white dwarfs orbit each other, they release tiny ripples in space and time called “gravitational waves,” which pull the white dwarfs together by stealing their angular momentum. Conventional wisdom would have it that this collision would destroy both white dwarfs, but in rare events called Type Iax supernovas, a single, rapidly spinning white dwarf remains.
The new study suggests that SNR 1181 is just such a supernova remnant. Observations of the remnant suggest that SNR 1181 consists of distinct outer and inner shock regions. Ko and colleagues analyzed X-ray data from SNR 1181 to explain the observations and create a computer model that reproduces the evolution of the event’s structure. For example, they wanted to understand why a colliding dead star leaves behind a white dwarf daughter.
Shortly after forming as a white dwarf, the object should have been emitting a rapid stream of particles called a “stellar wind.” But the team found something that defied their expectations.
“If the winds had started blowing shortly after SNR 1181 formed, we would not have been able to reproduce the observed size of the inner shock region,” Goh explained. “However, by treating the wind onset time as a variable, we were able to accurately describe all the observed features of SNR 1181 and unravel the mysterious properties of these high-speed winds. Furthermore, using numerical calculations, we were able to simultaneously track the temporal evolution of each shock region.”
What surprised the researchers even more was that the stellar wind from the white dwarf resulting from this collision appeared to have started only recently, perhaps as recently as 20 years ago.
This discovery could mean that some form of nuclear fusion is still occurring inside the white dwarf, causing it to “burn” again. This could be the result of a Type Iax supernova that would have been seen by Earth combatants during the Genpei War, which was a struggle for control of Japan. The war, which began in 1180 and ended five years later, was fought between the Taira and Minamoto clans. The war led to the downfall of the Taira clan and the establishment of the Kamakura shogunate.
The research team that made this discovery now plans to confirm their results with further observations of SNR 1181 using the Very Large Array (VLA) radio telescope in New Mexico and the 8.2-meter Subaru telescope on the summit of Mauna Kea, Hawaii.
“The ability to determine the age of a supernova remnant or its brightness at the time of its explosion through an archaeological perspective is a rare and valuable asset to modern astronomy,” said Ko. “This interdisciplinary research is not only exciting, but also highlights the enormous potential of combining different fields to discover new dimensions of astronomical phenomena.”
The study was published on July 5. Astrophysical Journal.
