THIS WHITE DWARF WAS INCREDIBLY UNEXPECTED.
“This white dwarf was incredibly unexpected,” says Kepler, “And because we had no idea anything like it could even exist, that made it all the more difficult to find.”
Here’s a quick refresher: White dwarfs like Dox are the antiques of the cosmos. They’re the hyper-dense husks left over when stars largely sputter out of hydrogen and helium fuel. All but the largest 3 percent of stars end up as white dwarfs. Although Dox is only slightly bigger than our home planet, it’s 60 percent the mass of our sun.
Boris Gänsicke, an astronomer at the University of Warwick, in the UK, who was not involved in Dox’s discovery, confirms that the “exotic white dwarf… has an almost pure oxygen atmosphere, diluted only by traces of neon, magnesium, and silicon,” he writes in an essay accompanying the Science paper. “This chemical composition is unique among known [white dwarfs] and must arise from an extremely rare process.”
So, what makes Dox’s oxygen rich atmosphere so unexpected? Kepler explains that Dox presents more than a couple mysteries. For one, almost all other white dwarfs in the sky have an atmosphere thick with light elements like hydrogen and helium. These light elements are the final dregs of the star’s elemental fusion fuel that survived the star’s earlier life-cycle. Simply because of their weight, these light elements naturally float to the top of white dwarfs.
“What happened to all these light elements?” asks Kepler “How did they all get stripped away?”
Kepler also explains that although traces of heavier elements like carbon and oxygen can be detected in about one out of every five white dwarfs, it’s never quite like this. A white dwarf’s atmosphere is never purely one element, and is often diluted in a pool of lighter elements. Perhaps most perplexing, when oxygen atoms are found, they’re spied in far heavier white dwarfs. Smaller white dwarfs evolve from smaller stars, which don’t fuse together atoms into oxygen as they collapse. By all calculations, Dox would have had to be roughly double its weight to have even forged oxygen atoms in its earlier life. “You have to wonder where this oxygen even came from,” says Kepler.
In short, by simply being so weird, Dox completely defies our general, scientific understanding of how stars evolve and eventually form into white dwarfs. But Kepler suggests that maybe this shouldn’t be all that surprising. That’s because, he argues, scientists have often ignored the wacky results that can come about when stars grow and evolve while locked in a binary dance with other stars—rather than alone.
“I think the main problem is that we [astronomers] have dedicated the last 50 years to calculate the evolution of stars that are not interacting with each other, when at least 30 percent of stars interact with a binary companion,” he says.
Kepler believes Dox looks so strange because of an unlikely binary origin-story. His rough theory goes like this:
At some point Dox may have been a larger white dwarf, locked in a twirling ballet with another star much like our own Sun. These two stars were about the same distance apart as the Sun and Venus are. As Dox’s dance partner started to sputter out of Hydrogen fuel, it formed what’s called a red giant. It expanded rapidly—becoming so big that it actually engulfed the white dwarf in its outermost layer of gas. Kepler believes Dox would have started siphoning off the red giant’s gas onto itself. At some point during that siphoning process, “when it reached a few million degrees, it exploded. That explosion threw all types of matter out. That’s when [Dox] might have lost all its hydrogen and helium. This type of situation is known to have happened with other stars, although it’s never been seen to leave just oxygen,” he says.
The World’s Most Boring Job
Dox was discovered in a data mountain of 4.5 million individual star observations, collected over the last 15 years by a New Mexico observatory in a project called the Sloan Digital Sky Survey. It was found by way of a process so grueling that its initial discoverer—one of Kepler’s undergraduate students Gustavo Ourique—deserves a mention.
Ourique was looking for strange, new types of white dwarfs in a data pile of 300,000 possible observations. These observations are simple graphs about what colors of light came from each pinpoint source (called a spectral graph). Because a computer isn’t easily programmed with such a vague task as “find something weird and cool,” Ourique was challenged with the grunt-work task of physically looking at printed out pages of all 300,000 graphs.
“After a few months he could filter a one or two thousand each day, like reading a book” says Kepler. Yeah, but what a heartbreakingly boring book. That is, at least until it gets thrilling, because after half a year of scanning, and toward the end of the 300,000 graphs, Ourique came across Dox. Because of it’s oxygen atmosphere, Dox’s spectral graph looked truly unique, and he brought it to Kepler.
Ourique, man, you are a hero.