This Strange Nebula Looks Inside Out and Born-Again

A color composite image of planetary nebula HuBi1 reveals a low-excitation inner shell, and high-excitation outer shell, effectively appearing to be inside out.
(Image credit: Nature Astronomy (2018)/M. Guerrero et al.)

The nebula of a dying star has a "inside-out" appearance, scientists announced in a new study — and this may be our sun's ultimate fate.

Four years ago, astronomer Martín Guerrero from the Astrophysics Institute of Andalucía in Granada, Spain, discovered something puzzling: The shell-shaped remains of a stellar explosion — known as a planetary nebula — looked like it was inside out and seemingly born-again in a rare cosmic occurrence.

"I was at a telescope in La Palma, a beautiful island here in Spain, and I was repeatedly making images of planetary nebulas," Guerrero told [Glowing Nebula Decorates Space in Hubble Image]

Planetary nebulas have rounded, shell-like shapes. An example is NGC 6818, also known as the Little Gem Nebula, seen in this image.
(Image credit: ESA/Hubble & NASA)

When Guerrero first looked at the planetary nebula HuBi1, things seemed normal.

"I made the image in the high-ionization filter; I saw some structures, nothing very impressive," he said. But then things got surprising. "The image in the low-ionization stage was something completely crazy. So my first idea was that I'm sleepy and I used the wrong filter!"

Ionization occurs in all planetary nebulas; it's a phenomena by which the radiation of an energetic central star strips electrons off atoms in the clouds of dust and gas. "In almost all cases, atoms closer to the intense light source are likely to have more of their electrons stripped than do atoms further away," Bruce Balick, an astronomer at the University of Washington and a close collaborator on other projects with Guerrero, told in an email.

But HuBi1 showed unusual high ionization at the outside edge of the shell shape, and low ionization near the center.

After more scientists came on board to perform other tests, like stellar evolutionary modelling, Guerrero and his team put the pieces of the HuBi1 puzzle together.

And to explain, Guerrero used a fruit.

The Ring Nebula.
(Image credit: AURA/STScI/NASA)

"Imagine the center of a star of a planetary nebula is something like an apple. So you have most of the apple is the flesh, and there is a very thin skin. Imagine that the star, this apple, is mostly carbon and oxygen," he said.

"This inner material is never going to get to the pressure and to the temperature where the carbon and oxygen are going to start fusing ... so this is very stable." [Ghost of a Dying Star Hints at Sun's Future]

Then, Guerrero discusses the skin of this astronomical apple.

"The skin is hydrogen and helium. In 80 to 90 percent of the cases, nothing is going to happen: The star at this stage is getting its energy from burning hydrogen on the surface of the star into helium. But, in about 10 or 15 percent of the cases, this helium layer is going to get to a critical mass.

"At this critical mass," Guerrero said, "the helium is going to start burning, it's going to produce carbon and oxygen atoms by fusing three or four helium atoms. The funny thing here is that because this happening on the surface of the star, this is a very explosive event. So, as soon as the helium starts producing carbon, the temperature and the pressure are going to increase and these tensions are going to explode into space."

It's this wave of carbon material, now ejected into space, that is shock-heating the atoms at the nebula's outer rim, giving it a born-again appearance, with fresh recombining atoms.

The middle of the apple doesn't realize the skin is coming apart, in Guerrero's example. And the team proposes that, in fact, the central star is fading rapidly: it's "optical brightness has declined continuously by 10 magnitudes in a period of 46 years," the study authors said. Balick called this "amazing!!"

The researchers determined that the star was once about 1.1 solar masses, and deduced that HuBi1 could be an example of what will happen when the sun dies, in 5 billion years.

The new work was published on Aug. 6 in the journal Nature Astronomy.

Long-term spectro-photometric evolution of IRAS 17514, the central star of HuBi1. This graph is derived from observations made from July 1996 to May 2017, and shows the star's rapidly diminishing brightness.
(Image credit: Nature Astronomy (2018)/M. Guerrero et al.)

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