On August 6, 1181, Chinese astronomers spotted a new star in the constellation Cassiopeia. It shone for 185 days, slowly fading, and then vanished, leaving only a record in the chronicles. Today, at its location, there is a hot white dwarf — a compressed stellar core with no nuclear reactions, heated to tens of thousands of degrees. This is the remnant of a faint supernova of the rare type Iax. The explosion could have been fueled by material from a neighboring star. But all searches have shown one thing: the companion is gone.

Among supernovae of type Ia, which serve as cosmic rulers, the faint Iax cousins have long remained in the shadows. The mystery of their origin traces back to the pioneering work of Subrahmanyan Chandrasekhar, who calculated the maximum mass of a white dwarf, and Fritz Zwicky, who predicted supernovae. The birth scenario was envisioned two ways: either a white dwarf pulls helium from an inflated companion star in a binary system, triggering incomplete burning (deflagration, a slow fire instead of a detonation) or two white dwarfs merge into one. In the first case, the donor star survives. In the second, the partner vanishes forever. Unraveling SN 1181 would shed light on the entire family of these strange explosions.

To find the missing partner, astronomers combed the vicinity of the remnant within a radius of 30 arcseconds — at a distance of 2.5 kiloparsecs, this corresponds to a search area about 0.3 parsecs across (nearly a light-year). Data from two instruments were used: the Gaia space telescope, which measures parallaxes (shifts of stars due to Earth's motion) and proper motions, and the ground-based Pan-STARRS1 survey, capable of detecting faint objects down to 8th magnitude. Sixteen candidates found by Gaia were ruled out: their distances didn't match, and their trajectories didn't point to the explosion site. Another six sources, visible only in deep Pan-STARRS1 images, were compared with spectral models of hot helium atmospheres at 20 000 K. Alas, their colors (measured through color filters) were too red — no amount of interstellar extinction by dust could explain that redness. Parallel numerical simulations of the evolution of binary systems showed that a helium donor in such a pair cannot be lighter than 0.3 solar masses, so it should shine brighter than absolute magnitude +6.5. Pan-STARRS1 sensitivity amply covers this threshold, but no suitable object was found. It was never there.

This missing partner makes SN 1181 the direct opposite of another faint supernova — SN 2012Z, where a helium companion star was actually seen in pre-explosion images. The contrast proves that supernovae Iax have no single scenario. Bright ones are born alongside a helium star, while faint ones, like SN 1181, come from the merger of two white dwarfs. This split in evolution channels poses a nontrivial challenge for cosmology: these 'nonstandard candles' could masquerade as ordinary supernovae Ia, distorting distance measurements. Future surveys, the Nancy Grace Roman Telescope and the Vera Rubin Observatory, will find dozens of similar events, allowing us to calibrate their contribution. For now, the 1181 remnant drifts in solitude, reminding us that not all cosmic dances end in a duet. Sometimes a partner merges with you forever, and only a silent white dwarf holds the memory of the consumed companion.