Mysterious Remnant of Historic Supernova Finally Explained

The historical observation of SN 1181

In the year 1181, the Japan was going through a tumultuous period marked by the Genpei War (1180–1185), a series of conflicts that resulted in a transfer of power from traditional aristocratic families to the new military shogunate. This political and social transition disrupted the structure of Japanese society. That said, the records of this period, while primarily focused on political and social events, also contain valuable astronomical observations. Among these observations, the appearance of a strange star stands out. This celestial object, as bright as Saturn, is said to have illuminated the night sky for approximately 180 days. Several historical documents mention this event, including the Japanese diary Azuma Kagami, a key text that chronicles the significant events of the time. This “guest star,” as it was nicknamed, is said to have captured the imagination of observers of the time, whether they were astronomers or ordinary inhabitants. During those six months, it became a celestial point of reference, fueling much speculation and hypotheses among scholars of the time.

A type Iax supernova

In reality, this object was not a star in care but the result of a much more spectacular and rare phenomenon: a Type Iax supernova. In detail, a supernova occurs when a star reaches the end of its life and explodes, releasing an immense amount of energy that can momentarily eclipse an entire galaxy. As for the guest star observed in 1181, scientists determined that it was specifically a Type Iax supernova, a less common type of supernova. Type Iax supernovae essentially result from the merger of two white dwarfs, extremely dense stars composed mainly of carbon and oxygen, which, after exhausting their nuclear fuel, have contracted to a size similar to that of the Earth while maintaining a mass comparable to that of the Sun. When these two white dwarfs collide, they trigger a thermonuclear explosion that can give rise to a supernova. This explosion then hurls vast amounts of material into space and releases intense light that can be seen from Earth thousands of light-years away. The 1181 supernova was particularly intriguing because, unlike many other supernovae, it left behind a remnant star—a rapidly rotating white dwarf—rather than dissipating completely into space. This makes studying this peculiarity all the more complex, and for centuries the exact location of supernova SN 1181 remained a mystery, which brings us to this recent work. Three years ago, astrophysicists were able to locate the remnant of this stellar explosion. Two white dwarfs orbit each other. Credits: Jingchuan Yu/Beijing Planetarium

What do we know about this object?

Locating this supernova remnant required advanced technologies and an interdisciplinary approach. Specifically, the researchers combined computer modeling techniques with modern observational analyses. The first crucial clue came from the precise historical descriptions, which offered information about the guest star’s position in the sky. The researchers then focused their efforts on the constellation Cassiopeia, a region mentioned in ancient texts. Using the historical data as a starting point, the astronomers then exploited modern observations made by powerful telescopes, including X-ray telescopes such as the European Space Agency’s XMM-Newton space telescope and the Subaru telescope in Hawaii. These instruments detected X-ray emissions from the supernova remnant, a key indicator of stellar debris left after the explosion. In particular, the X-ray observations revealed the presence of two distinct shock regions around the remnants of the explosion – an outer region and an inner region. In detail, the shock regions are formed by the shock wave of the stellar explosion compressing and heating the surrounding material, making these areas detectable by X-rays. By analyzing the structure and composition of these regions, the researchers were able to refine their theoretical model of the explosion. These images show the two shock regions of the remnant of SNR 1181. The bright white in the center is the white dwarf. Credits: K. Kashiyama et al./ The Astrophysical Journal