Astronomers have discovered a white dwarf star that has 99.9 percent pure oxygen atmosphere.

The presence of an ancient sun that is able to live for so long, causing its outermost layer to consist of only pure oxygen, was just a theory before the white dwarf star was discovered.

Small stars that have less than 10 times the mass of the sun become white dwarfs when they shed their outer layers, as the end of their lifespan nears. Ideally, due to high gravity, lighter elements of the star rise to the surface, while the heaviest elements descend to its dense core.

But in the case of this white dwarf star, also known as SDSS J124043.01+671034.68, the outer atmosphere is composed of more than 99.9 percent of pure oxygen. The expected surface helium and hydrogen are absent. It only has few traces of magnesium, neon, and silicon.

Astronomers cannot provide an exact answer to how it all happened but they speculated that the missing elements were stripped off the star's surface over time.

Souza Oliviera Kepler of Federal University of Rio Grande do Sul in Brazil said their discovery of the star was unexpected. SDSS J124043.01+671034.68 or "Dox" is the first ever recorded star that underwent such process.

"We had no idea anything like it could even exist, that made it all the more difficult to find," Kepler said.

If it indeed underwent "stripping," the astronomers are still baffled as to how it could have happened. Kepler and his team offer several theories.

They are considering the possibility that Dox is part of a binary star system, and its resultant interaction with another star helped peel away the missing atmospheric elements and leaving behind only the oxygen envelope on its surface.

It could also be due to internal process within the star. Massive pulses of carbon burning at the core could have flared outwards, taking with it the lighter surface elements.

Kepler believes their discovery is crucial in understanding the process of stellar evolution, particularly the binary star systems.

In February 2015, astronomers discovered the first double-degenerate binary star, Henize 2-428, which can merge in 700 million years and trigger a supernova event.

The findings of Kepler and his colleagues were published in Science.

Photo: European Southern Observatory | Flickr