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The Milky Way’s Speediest Stars Could Solve a 50-Year-Old Mystery

Stars traveling more than 1,200 kilometers per second hint at a new mechanism behind cosmos-spanning stellar explosions.
The Milky Way's Speediest Stars Could Solve a 50-Year-Old Mystery
A remnant of a nearby Type 1a supernova that flashed in Earth’s skies in November 1572. Today, astronomers use Type 1a supernovae to measure vast cosmological distances.

Ken Shen was racing against the sun. It was 3 A.M. on April 25 and Shen—an astronomer at the University of California, Berkeley—was sitting at his kitchen table in his pajamas. At that precise moment the scientists behind the European Space Agency’s Gaia spacecraft released the mission’s second batch of data. And Shen was on a mission to comb that data to find the Milky Way’s fastest-moving stars, then to verify their identities via independent observations on ground-based telescopes. With 30 minutes to go before the sun’s glare started to hit the west coast, Shen managed to find his first target and send its coordinates along to a collaborator at the Lick Observatory near San Jose.

But he could not crawl back into bed. For the next 24 hours Shen and his colleagues scoured the Gaia data for a handful of candidates to observe with telescopes in South Africa, the Canary Islands, Arizona and California. After a week of careful analysis the researchers were certain they had found three of our galaxy’s fastest stars. Not that they were only interested in speed: such rapid-moving stars were the smoking-gun evidence Shen was seeking for his novel theory of how certain stars explode. On April 30 the team posted their results to the preprint server arXiv and submitted them to The Astrophysical Journal for peer-reviewed publication.

Shen’s theory concerns type Ia supernovae, stellar explosions so consistently and uniformly bright that astronomers have used them as benchmarks to measure the vastness of the cosmos. Whether right here in the Milky Way or in a galaxy on the other side of the observable universe, these cataclysmic explosions always display almost the exact same luminosity, allowing precise calculations of their distances. Such work has demonstrated the universe’s expansion is accelerating—an advance that netted the 2011 Nobel Prize in Physics—and has helped validate the existence of dark energy. Type Ia explosions also create and disperse heavy elements (including the iron in the hemoglobin running through our veins), and so play crucial roles in studies of galactic evolution.

Yet astronomers do not understand the exact process that kick-starts these brilliant explosions. “It’s kind of embarrassing,” says Anthony Piro, an astronomer at Carnegie Observatories who was not involved in the new study. “Type Ia supernovae are super fundamental to a number of different areas in astrophysics.”

Astronomers remain convinced white dwarfs—dense, Earth-size remnants of sunlike stars—are the astrophysical culprits behind Type Ia supernovae. But because these objects are too stable to explode on their own, a companion star—be it another white dwarf, a star like our sun or even a giant star—must push them over the edge. One classic model suggests some Type Ia blasts occur when two white dwarfs spiral toward each other and merge. But for nearly a decade Shen has been honing a slightly different idea: He thinks that in these scenarios the two white dwarfs never actually collide. Instead, as they get closer to each other, one star’s gravity pulls material (specifically hydrogen and helium gas) from the other star. The gas hits its neighbor like a hammer, triggering an explosion on the star’s surface that sends a shock wave deep below, igniting a second—the Type Ia detonation—in and around the star’s core. That blast would be powerful enough to kick the remaining star away at an enormous velocity, upward of a thousand kilometers per second. But for nearly a decade this was just a model. If Shen could find these fast stars, he just might gain observational proof for his hypothesis, and with it an improved understanding of these vital cosmic beacons.

Gaia is uniquely suited for discovering high-speed stars, and thus for providing candidates Shen could investigate. To create its galactic map, the spacecraft tracks the positions, motions and luminosities of more than a billion stars, providing full three-dimensional trajectories for many of them. Such precise tracking helps pin down how fast any particular object is moving relative to our viewpoint in the solar system.

From the Gaia data Shen’s team flagged seven candidate hypervelocity white dwarf stars. Four proved to be rather ordinary, slower-moving stars whose motions were likely inaccurate in the data. But the remaining three were true speedsters. What’s more, they possessed bizarre characteristics thought to be indicative of Shen’s proposed formation mechanism: Each is about 10 times larger than a typical white dwarf, presumably because their outer layers were puffed up by the extra energy they received from the Type Ia explosion. The three stars also lack hydrogen and helium, which would be expected if they had donated those elements to a companion white dwarf to spark the detonation. Finally, Shen and his colleagues traced the stars back in time to find at least one of them seemed to originate from a faint supernova remnant discovered three years ago within the Pegasus constellation.

“This observation is just extremely exciting,” Piro says. “I think this is going to jump-start a lot of really interesting lines of research coming up in the next few years. It’s always exciting when you can pinpoint that seminal moment when you know that a lot of people are going to be jumping into this.” Piro says the three hypervelocity white dwarfs from Gaia’s data boost the odds Shen’s proposed mechanism is correct—but notes more definitive proof is still needed. A Type 1a explosion, for example, would likely spray the surviving white dwarf’s surface with detectable quantities of heavy elements. If astronomers were to spot such stellar ashes on these white dwarfs, that would lend further support to Shen’s hypothesis.

Keith Hawkins, an astronomer at Columbia University who studies hypervelocity stars but did take not part in the research, agrees there is further work to be done. The first step, he says, will be to double-check the data supporting the wild find by Shen’s team. “Whenever you’re looking at an outlier—in this case these things are moving at incredibly high velocities, so they’re outliers—you have to be very careful about whether or not there’s something wrong with the data,” he says. Two of the white dwarfs, for example, only appeared to move across the plane of the sky, with no motion whatsoever toward or away from us—a curious alignment with Gaia’s line of sight that most celestial objects would be unlikely to possess.

For now Shen welcomes further scrutiny of these stars to see whether their origins are truly as bizarre as he suggests. He and his team, he says, were simply thrilled to find any candidates at all; he had estimated the chances of success at around 25 percent, worrying that hypervelocity white dwarfs might prove too dim to detect. But if the evidence grows over the years that these ultraspeedy stars are the bona fide white dwarfs from his hypothesis, the implications would be far-reaching—allowing astronomers to better understand how some Type Ia supernovae explode, thereby providing a better foothold on the measurements that underpin their quest to know the universe’s large-scale structure and evolution.


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