Astronomers found how the string of pearls appeared in our sky after a supernova explosion 850 years ago. The remnant, Pa 30, contains a rare zombie star at its core. "If the explosion itself was perfectly symmetric, the ejecta should also be symmetric," said team member Ilaria Caiazzo about the discovery.

Astronomers are unraveling the secrets of a unique celestial relic, the string of pearls, left by a supernova explosion witnessed by humanity around 850 years ago. The discovery is based on detailed observations of Pa 30, the dandelion-shaped chains of gas and supernova debris. It is the only remnant of its kind observed in our galaxy, the Milky Way. What makes Pa 30 even more special is that it hosts a rare zombie star at its core.

The one that created Pa 30 was a Type Ia supernova, which occurs when a white dwarf star consumes too much material from its companion star.

This leads to a runaway nuclear explosion that usually destroys the white dwarf.

However, in rare cases, the supernova doesn't fully destroy the star and leaves behind a shattered stellar body known as a zombie star.

These events are called Type Iax supernovas.

First identified in 2013 by a citizen scientist, Pa 30 was soon linked to a cosmic explosion independently recorded in 1181 by Japanese, Chinese, and Arabic astronomers.

This makes it one of only five supernovas observed before the invention of the telescope.

Reliable historical records of stellar explosions extend back only about a thousand years, said team member Ilaria Caiazzo, assistant professor at the Institute of Science and Technology Austria (ISTA).

Pa 30 is a perfect laboratory for studying supernovas because it is relatively close to Earth on cosmic scales, at around 7,500 light-years away.

The team used the Gemini North telescope to observe Pa 30 and found knots along the strands of gas radiating out from the site of the cosmic explosion.

These knots are each wide enough to encompass Neptune's orbit 10 times over.

The zombie star sits almost perfectly at the heart of Pa 30.

This suggests that when this cosmic explosion happened, it was roughly symmetrical and the kick was relatively light.

Explosions conserve momentum. If the explosion itself was perfectly symmetric, the ejecta should also be symmetric, and the surviving star should not receive a substantial kick, Caiazzo said.

The research team hopes that the unique properties of Pa 30 will help them detect the remnants of other Type Iax supernovas in our galaxy.

While we can see Type Iax supernovas in distant galaxies as point sources of light, resolving the remnants of such explosions is only possible in our own galaxy and in very few nearby ones, Caiazzo concluded.

The team's work was published in The Astrophysical Journal.