Everything else is gone.
When relatively small stars – those with less than 10 times the mass of our Sun – get close to the end of their lifespan, they shed their outer layers and become what are called white dwarfs. Under high gravity, the heaviest elements descend into the star's dense core, while lighter elements such as hydrogen and helium rise to the surface.
At least, that's what usually happens. This star, dubbed SDSS J124043.01+671034.68, bucks the trend, with astronomers discovering its outer atmosphere is essentially greater than 99.99 percent oxygen. Only traces of other elements have been detected, including neon, magnesium, and silicon, but as for the hydrogen and helium you'd expect to find dominating the surface, there's no sign.
It's a puzzle for the team that found it, led by Souza Oliveira Kepler from the Federal University of Rio Grande do Sul in Brazil. "What happened to all these light elements?" he told William Herkewitz at Popular Mechanics. "How did they all get stripped away?"
The answer isn't entirely clear, but astronomers have long speculated that elements being stripped from a star's surface over time could be a possibility. If that's what's happened here, SDSS J124043.01+671034.68 (or 'Dox' as the researchers have nicknamed it) is the first evidence of the phenomenon taking place.
In any case, this oxygen-dominated star is a true one-of-a-kind in terms of the solar bodies we know about, being the only star among some 32,000 white dwarfs with such a pristine oxygen atmosphere.
"This white dwarf was incredibly unexpected," Kepler says. "And because we had no idea anything like it could even exist, that made it all the more difficult to find."
But how did the other elements get stripped away? Nobody knows for sure, but Kepler and his colleagues have a couple of ideas. It's possible that Dox could be part of a binary star system, and that interactions with another star in the system somehow peeled away its other atmospheric elements, exposing an oxygen envelope underneath.
Alternatively, something within the star, such as a massive pulse of burning carbon at Dox's core, might have flared outwards, eliminating lighter elements on the surface.
We won't understand until we learn more about these kinds of rare stars, but the great thing about the discovery of Dox is it's already reshaping what we know about stellar evolution – particularly with a view to how binary systems could impact the development of the stars moving within them.
"I think the main problem is that we have dedicated the last 50 years to [calculating] the evolution of stars that are not interacting with each other," said Kepler, "when at least 30 percent of stars interact with a binary companion."
The findings are reported in Science.