A strange nebula has revealed a stellar facelift Courtesy of Martin A Guerrero/Nordic Optical Telescope

A distant star just underwent an explosive facelift, leaving it looking much younger than it really is. Only a tell-tale shroud of dust and gas gives away its true age, and it is possible that our sun could someday share the same fate.

Martín Guerrero at the Institute of Astrophysics of Andalusia in Spain and his colleagues observed a planetary nebula – a cloud of dust and gas around an ageing star – called HuBi1. Strangely, the cloud’s centre was cooler than the edge that was furthest from the star.

“Imagine that you are close to a bonfire. As you get closer you notice that there is more heat, and as you go farther away it’s colder,” says Guerrero. That is what it is like in a normal planetary nebula, but the opposite was true for HuBi1. When they examined archival data from 1971, they found that the central star’s brightness had dropped by a factor of 10,000 since then.


Born again

The team realised they were witnessing the aftermath of a “born-again event”, in which a star gets hot enough to fuse helium into carbon and expel it in a wave at high speeds. This makes the star look cooler and dimmer, and thus younger.

In this case, that carbon hurtled through HuBi1 in a shock wave, heating up the area in front of it as it went. The researchers observed the nebula after the wave had passed through the innermost area, which is why the outer part of the shell beyond the wave was hotter, Guerrero says. As the carbon streamed away from the star, it cooled down and condensed into dust grains that blocked out the star’s light, which explains the dimming.

This star and its nebula began life as a star with about the same mass as the sun. “We didn’t know before that stars like the sun could end this way,” Guerrero says. He says that there is a 10 to 15 per cent chance the sun could evolve over billions of years to be born again, too.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-018-0551-8