White dwarf’s inner makeup is mapped for the first time

Astronomers have probed the inner life of a dead star. Tiny changes in a white dwarf’s brightness reveal that the stellar corpse has more oxygen in its core than expected, researchers report online January 8 in Nature. The finding could challenge theories of how stars live and die, and may have implications for measuring the expansion of the universe.

By the end of a sunlike star’s life, it has shed most of its gas into space until all that remains is a dense core of carbon and oxygen, the ashes of a lifetime of burning helium (SN: 4/30/16, p. 12). That core, plus a thin shellacking of helium, is called a white dwarf.

But the proportion of those elements relative to one another was uncertain. “From theory, we have a rough idea of how it’s supposed to be, but we have no way to measure it directly,” says astrophysicist Noemi Giammichele, now at the Institute of Research in Astrophysics and Planetology in Toulouse, France.

Luckily, some white dwarfs encode their inner nature on their surface. These stars change their brightness in response to internal vibrations. Astrophysicists can infer a star’s internal structure from the vibrations, similar to how geologists learn about Earth’s interior by measuring seismic waves during an earthquake.

Giammichele and her colleagues used data from NASA’s Kepler space telescope, which watched stars unblinkingly to track periodic changes in their brightness. Kepler’s chief aim was to find exoplanets, the worlds orbiting distant stars (SN Online: 10/31/17). But it also monitored white dwarf KIC 08626021, located 1,375 light-years away in the constellation Cygnus, for 23 months. The observations provided the highest-precision data ever on tiny changes in a white dwarf’s brightness and, indirectly, its vibrations.

Next, Giammichele borrowed a computer simulation technique from her former life as an aeronautical engineer to figure out how the changes in vibrations related to the makeup of the core. The team ran millions of simulations, looking for one that reproduced the exact light changes that Kepler observed. One simulation fit the data perfectly, showing that the white dwarf had the expected carbon and oxygen core with a thin shell of helium.

But the details were surprising. The core was about 86 percent oxygen, 15 percent greater than physicists had previously calculated. That suggests that something about the processes that convert helium to carbon and oxygen or mix elements in the star’s core during its active lifetime must boost the amount of oxygen.

Four other white dwarfs show a similar trend, says study coauthor Gilles Fontaine, an astrophysicist at the University of Montreal.  “We certainly will go ahead and analyze many more.” If other white dwarfs turn out to be similar, the results will send theorists who study stellar evolution back to the drawing board, he says.

White dwarfs are also thought to be the precursors of type 1a supernovas. These catastrophic stellar explosions were once thought to have the same intrinsic brightness, meaning they appeared brighter or dimmer depending only on their distance from Earth. Measuring their actual distances led to the discovery that the universe is expanding at an accelerating rate (SN: 8/6/16, p. 10), which physicists explain by invoking a mysterious substance called dark energy.

More recent observations suggest that these so-called standard candles may not be so standard after all. If the white dwarfs that help create supernovas have varying oxygen contents, that may help explain some of the differences, Fontaine says.

Accounting for that difference may someday help reveal details of what dark energy is made of, says astrophysicist Alexei Filippenko of the University of California, Berkeley. But those implications are a long way off. “Just how much bearing it will have on cosmology remains to be seen,” he says.

Editor’s note: This story was updated January 19, 2018, to clarify the types of stars that become white dwarfs.