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.