White dwarfs—the hot, burned-out remains of ordinary stars—are very common in the universe, and weird. (Our very own sun will become a white dwarf in a few billion years, too.) Imagine something the size of Earth, but 300,000 times more massive, glowing white-hot and bright enough to be seen far away despite its tiny size. “It’s just a pixel of light,” Noemi Giammichele, an astronomer at the University of Toulouse, told The Daily Beast. “I find it really amazing all the information we can gather just from that one tiny dot.”
Made of pure carbon and oxygen, with only a thin haze of other atoms acting as its atmosphere, white dwarfs certainly aren’t like anything we can make in a lab on Earth. But Giammichele used seismology to measure “dwarfquakes” to not only understand the internal structure of these white dwarfs but also the future expansion rate of our universe.
“It’s a fine-tuning of everything we know about white dwarfs,” Giammichele said. “We can better tune some physical processes that happen way before the white dwarf phase: the sun how it really is right now, and how it will be when it’s a white dwarf.”
The birth of a white dwarf
Stars like the sun shine by nuclear fusion, turning hydrogen atoms into helium deep in their interiors thanks to the crushing pressure and extreme temperature are provided by the star’s big mass. (We can’t do that on Earth easily, so fusion here needs more complicated—and expensive—processes.) Eventually they use up the available hydrogen, and begin fusing helium into carbon and oxygen.
For the vast majority of stars, including the Sun, that’s the end of the road—they aren’t massive enough to build up enough pressure to fuse atoms into heavier elements. At that point, they shed their outer layers, and the core they leave behind becomes a white dwarf (though they can range in color from blue-white to orange, depending on how hot they are). Even though fusion is over, they have enough residual heat to glow for many billions or trillions of years. In fact, the universe isn’t old enough yet for any white dwarfs to burn out.
While their glow fades, they change. Something made of glass can crack or break if it changes temperature quickly, as the atoms inside adjust positions. A white dwarf isn’t solid like glass, but as it cools off, jostling atoms set off off vibrations inside. Even though a white dwarf is denser than any rock on Earth, it’s more like a fluid than a solid, so these “white dwarfquakes” send off waves of higher or lower temperatures that ripple across the surface of the white dwarf. Even though we can’t see those waves directly, even with our most powerful telescopes, the fluctuations in temperature result in flickering in the white dwarf’s light.
Using the Kepler planet-hunting observatory, Giammichele and her colleagues picked up those flickers in a white dwarf named KIC08626021.
“For the first time we can match a model to what we see,” Giammichele said. “We can have information on the core of the white dwarfs, which is really new. We can map the entire star, but the core is important here.”
To put it another way, they reconstructed the entire interior of a white dwarf using the seismic flickers of light from a white dwarfquake visible only as a single pixel in a telescope’s camera. For the first time ever, Giammichele and her collaborators were able to figure out exactly how the insides of a dense, tiny object are put together.
More oxygen, please
“The core of the white dwarf is like the fossil of all the processes that happened before,” Giammichele explained. The seismic data showed that the core was bigger than astronomers expected, which meant the white dwarf had a lot more oxygen than previous theories suggested. Up to 86 percent of the white dwarf is oxygen, unlike the 50/50 split that most astronomers assume when they talk about these things. Giammichele repeated these observations for five other white dwarfs and found the same results: seismic ripples, more oxygen, and a larger core.
That extra oxygen means we need to adjust our theories about exactly how nuclear fusion goes when a star is approaching the end of its life. Somehow, it needs to make more oxygen than carbon, and circulate the material to bring that oxygen to the very middle of the core, which means our current understanding of how some white dwarfs die could be wrong. White dwarfs have a maximum mass, beyond which they become unstable and explode in a predictable supernova.
Shifting the chemical balance of a white dwarf also changes how they blow up, which affects the measurements we make about the expansion of the universe. It doesn’t change the big picture, though: While we’re not going to see a huge difference in the rate of expansion, we’ll need to refine our prediction. When white dwarfs explode, the atoms inside undergo more nuclear fusion, and all those pieces get scattered around the galaxy. From there, they can be made into new stars and planets, so the chemistry of a white dwarf supernova is important for learning the atomic makeup of planets like ours.
Like the seismic vibrations in the white dwarf, these seemingly small changes are reverberating all the way here back to Earth and our understanding of our universe.