It should have been physically impossible. Millions of years ago, a white dwarf—the fading cinder of a sunlike star—was locked in a dizzying dance with a bright companion star. The two had circled each other for eons, connected by a bridge of gas that flowed from the companion onto the white dwarf allowing it to grow heavier and heavier until it could no longer support the extra weight. At this point, the white dwarf should have exploded—blowing itself to smithereens and producing a supernova that briefly shone brighter than all the stars in the Milky Way combined.Then once the supernova faded and the white dwarf’s innards were dispersed across the galaxy, there would quite literally be nothing left save for its companion star. But against all odds, the explosion did not fully rupture the white dwarf. Instead, it survived.
The white dwarf, better known as LP 40-365, was first announced in 2017 in Science. “That’s a pretty extraordinary claim,” says JJ Hermes, an astronomer at Boston University. “So we need some extraordinary evidence to back that up.” Now a study posted to the preprint server arXiv and submitted to Monthly Notices of the Royal Astronomical Society just might provide that evidence. Roberto Raddi, an astronomer at the University of Erlangen–Nuremberg in Germany, Hermes and colleagues have detected two more white dwarfs that also appear to have achieved the unthinkable. Not only does the study provide further proof such death-defying stars exist, but it will ultimately shed light on these mysterious explosions.
Raddi’s team made these discoveries after combing through data from the European Space Agency’s Gaia spacecraft, which is particularly well suited for finding high-speed stars—an important characteristic of ones like LP 40-365 (because a supernova explosion has the power to slingshot stars across the galaxy). Two are destined to escape the Milky Way entirely, and one is orbiting “backward” against the usual rotation of stars in our galaxy. Additionally they all boast large radii, presumably because they were puffed up by the extra energy they received from the failed explosion. And yet they possess relatively small masses, likely due to the loss of much of their material during the explosion. But perhaps the most compelling evidence these stars are supernova survivors is that they brim with heavier elements. Whereas typical white dwarfs comprise carbon and oxygen, these stars are mostly composed of neon. “That’s absurd,” Hermes says. “That’s like some barroom beer sign just flying through the galaxy.” The stars’ second-most common element is oxygen, followed by a sprinkling of even heavier elements such as magnesium, sodium and aluminum. “This is about as weird as it gets,” Hermes says.
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Those heavy elements are the by-products of a star’s advanced stages of burning its nuclear fuel, which suggests that even though these white dwarfs did not fully disintegrate, their outer layers did partially burn. And it is not such a wild idea. When astronomers initially detected type Ia supernovae—the first known variety of exploding white dwarfs—theorists attempted to re-create those celestial cataclysms on their computers but struggled to achieve detonations that did not simply fizzle out before consuming the entire star. So astronomers tossed those models aside, convinced exploding white dwarfs were an all-or-nothing catastrophic affair. Then in 2002 observers discovered a new class of supernovae dubbed type Iax. These explosions look like their cousins except they move slower, fade faster and have less energy overall. To explain these sluggish stellar blasts, astronomers resurrected the models they had previously trashed.
Thanks to those early models and a little recent help from theorists, astronomers now think they understand the mechanism behind a half-baked white-dwarf supernova. As a white dwarf siphons matter from a companion star, it grows both heavier and denser. Eventually the density is so high carbon nuclei within the white dwarf fuse together, allowing the star to build heavier elements for roughly a thousand years. But the trick is that this nuclear fusion only takes place within a specific region inside the star—a bubble, if you will. That bubble eventually becomes so hot it surges up from the star’s interior, breaking out at the surface in less than a second. The outburst spews most of the star’s guts out into space, acting much like a rocket, accelerating the element-enriched white dwarf to incredible velocities.
This story explains the three newfound white dwarfs quite nicely, albeit with one major caveat: The stars should host some carbon that did not burn in the conflagration. But against those expectations they appear relatively carbon-free. “That's the last piece of the puzzle that doesn’t quite fit,” says Anthony Piro, an astronomer at Carnegie Observatories who was not involved in the new study. “Nature is always full of surprises. There's always a missing piece of the puzzle that you need to investigate.” Yet astronomers remain fairly convinced these stars have survived Type Iax supernovae. “Certainly, in terms of the gross qualitative picture this definitely does look like a white dwarf that had some kind of explosive fusion on it,” says Saurabh Jha, an astronomer at Rutgers University who was also not involved in the new research.
And that just might open up a number of doors in studying these brilliant explosions. Because supernovae are so rare, astronomers mostly study the blasts that occur in far-off galaxies—forcing them to scrutinize the light before it fades in a matter of months. But these white dwarfs zipping through the Milky Way provide the first stellar remnants that can be studied in detail millions of years later. “It’s like we can see the supernovae—the dinosaurs—in these other galaxies,” Jha says. “But here nearby we—with these kinds of stars—we have the fossils.”
Those fossils could give us a wealth of information about the specifics behind their progenitor explosions. Like their type Ia cousins, Iax supernovae similarly create and disperse heavy elements throughout the universe. But unlike their brighter cousins, which leave little lasting evidence of this process behind, the stars that survive type Iax eruptions are littered with the elements they produced—allowing astronomers to study these cosmic cocktails in detail. Already astronomers suspect the iron that runs through your veins was created in type Ia supernovae, and the gold in your wedding band was created when two neutron stars—the cinders left over when massive stars die in supernova explosions—collided. “It’s exciting to be able to finally get a more complete picture of where all of these different elements are coming from,” Piro says.
Raddi’s team also estimates there are many similar supernova survivors lurking within the galactic neighborhood—perhaps 20 of which should be visible by the time Gaia’s final data set is released. “This isn’t the end of the story,” Piro says. “This is something that is its own industry and it’s just beginning now.” For Jha, that is particularly gratifying. He has studied type Iax supernovae—the so-called weirdos—for roughly 15 years and is excited to see the field, which had long been considered decidedly niche, starting to erupt.