Discovery of a weird ‘inside-out’ nebula could show us our sun’s future

An international team of astronomers has found a planetary nebula that appears to be ‘inside-out’, offering insight into the sun’s future.

We can almost take it as a given that there are weird things to be found in deep space, but a new discovery made by an international team of researchers seems to be truly strange.

In a paper published to Nature Astronomy, the team observed the evolution of the central star of a planetary nebula in our Milky Way.

What makes it so unusual is that it appears to be structurally ‘inside-out’, possibly because the central star is undergoing a ‘born-again’ event, ejecting material from its surface and creating a shock that excites the nebular material.

Planetary nebulae are ionised clouds of gas formed by the hydrogen-rich envelopes of low- and intermediate-mass stars ejected at late evolutionary stages.

As the stars get older, they tend to shed their outer layers, resulting in the formation of ‘wind’.

When it starts transitioning to a white dwarf from a red giant, the star becomes significantly hotter, thereby ionising the material in the surrounding wind.

This causes the gaseous material closer to the star to become highly ionised, while the gas material further out is less so.

Planetary nebula HuBi 1 shows a double-shell structure – a hydrogen-rich outer shell and a nitrogen-rich inner shell – after its central star experienced a ‘born-again’ event. Image: Guerrero, Fang, Miller Bertolami et al

Not your typical example

However, planetary nebula HuBi 1 – located 17,000 light years away and nearly 5bn years older than the sun – is not your typical example.

Its central star is actually surprisingly cool and metal-rich, evolved from a low-mass progenitor star with a mass 1.1 times that of our sun.

The researchers believe that the inner nebula was excited by the star ejecting matter unusually late in its evolution. In the absence of ionising photons from the central star, the outer nebula has begun recombining, becoming neutral.

This collection of matter could well explain why the central star’s optical brightness has darkened substantially over the past 50 years.

These findings could give us an indication of what will happen to the sun billions of years from now.

“Our findings suggest that the sun may also experience a ‘born-again’ process while it is dying out in about 5bn years from now,” said Dr Xuan Fang, co-author of the paper.

“But way before that event, our Earth will be engulfed by the sun when it turns into a super-hot red giant, and nothing living will survive.”