The fate of our Sun, along with 97 percent of the universe’s stars, is to become a white dwarf. These stellar relics are what is left when an aging star sheds most of its mass, ending up as a dense, shrunken, Earth-size core composed of carbon and oxygen. Although scientists know white dwarfs in these broad strokes, many critical details of the objects remain poorly understood. Nailing down their properties is fundamental not only for understanding the lives and deaths of stars, but also for gauging the evolution of the entire cosmos. See also: Cosmology; Star; Stellar evolution; Sun; Universe; White dwarf star

A new study has now mapped the interior of a white dwarf as never before, uncovering some significant surprises. The study finds that the deep core of an examined white dwarf possesses nearly half the mass of the Sun and is made of fully 86 percent oxygen—a whopping 40 percent and 15 percent higher, respectively, than theorists’ best models had anticipated. See also: Mass; Oxygen

An artist’s impression of a white dwarf star, cutaway to show its inner core that has turned out to be both more massive and more oxygen-loaded than theoretical models have suggested. (Credit: Stéphane Charpinet)

This internal mapping became possible thanks to Kepler, a spacecraft that stared unflinchingly at more than one hundred thousand stars for a few years straight. The vigil’s primary goal was to discover exoplanets by registering the tiny dips in starlight they induce when transiting, or crossing, the face of their host star. In the process, however, Kepler also recorded natural brightness fluctuations in its target stars due to vibrations created by the internal flow of matter. Those vibrations travel throughout the star according to its internal composition and structure. Thus, the brightness changes in a star’s surface are helpful in revealing its hidden innards. The technique is just like that used by seismologist to study the Earth’s interior via the vibrations spawned by earthquakes—and hence the name of the astronomical equivalent, asteroseismology. See also: Earth; Earthquake; Exoplanets; Helioseismology; Kepler mission; Seismology; Space probe; Transit (astronomy)

For the study, Kepler gathered asteroseismological data from a white dwarf star,designated KIC 08626021, located 1,375 light-years away near the constellations of Cygnus and Lyra. The research team then ran simulations of the star’s inferred interior to find a match for the Kepler observations, arriving at the makeup that clashes with current models. See also: Constellation; Simulation

Should other white dwarfs turn out to have this unexpected architecture, it will force a rethink on otherwise well-established stellar evolution theories, as well as the underlying physics that comprise them. The outcome could have major implications for measuring the expansion of the universe. Cosmologists rely on a particular kind of supernova involving white dwarfs as a “standard candle” for gauging cosmic distances. These supernovae have uniform luminosities, so their apparent brightness can be used to decipher their absolute, true distances. However, in recent years, variations in the supernovae have cosmologists wondering if their candles are not so standard after all. It could be that white dwarfs have an unappreciated range of compositions, which could lead to differing explosions. If so, calculations of the influence of dark energy, a mysterious force that seems to be accelerating the universe’s expansion, might be off. Predicting the universe’s destiny, then, could boil down to better grasping the ultimate fate of stars like our Sun. See also: Dark energy; Hubble constant; Supernova