Today’s NASA Astronomy Picture of the Day, credited to astrophotographer David Cowland, is a wide, wispy shell of red-glowing gas known as RCW 86. It’s a quiet-looking target, but the story behind it stretches back almost two thousand years and touches on one of the more elegant detective stories in modern astrophysics.

A new star in 185 AD
In the year 185 AD, Chinese court astronomers recorded the sudden appearance of a bright new star in the region of sky we now associate with Alpha and Beta Centauri. It stayed visible for months before fading away. Historians of astronomy consider this the earliest reliably documented supernova sighting on record, predating Tycho’s and Kepler’s famous stars by well over a millennium.

Matching the ancient record to a modern remnant
RCW 86 sits in roughly the right part of the sky, at a distance of about 8,000 light-years, and spans somewhere around 100 light-years across, slightly larger than the full Moon appears from Earth, though far too faint to see without a telescope or long-exposure imaging. Its age, position, and size make it the leading candidate for the remnant of that 185 AD event.

Solving the case with X-rays and infrared
For a long time, RCW 86’s size was a puzzle: it looked too big for its estimated age, which pointed to it expanding unusually fast. The breakthrough came from combining data across wavelengths. X-ray observations revealed an unusually large abundance of iron in the debris and no trace of a neutron star or pulsar at its center, the telltale fingerprint of a Type Ia supernova, the thermonuclear detonation of a white dwarf that has pulled material from a companion star, rather than the core collapse of a massive star. Infrared observations from the Spitzer Space Telescope and WISE then showed that the remnant is expanding into a low-density bubble that the progenitor system likely carved out of the surrounding gas before it exploded. That cavity is what let the shock wave expand so quickly, explaining the remnant’s oversized appearance for its age.

Why it matters
Confirming a wind-blown cavity around a Type Ia progenitor was significant because it had been theorized but never directly observed until RCW 86. It offered rare physical evidence for how the white dwarf’s companion system behaves in the run-up to detonation, the kind of detail that’s normally invisible, since Type Ia supernovae themselves are only ever caught after the fact.

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