When a certain kind of dying star, called a type 1a supernova, explodes, they only shine for a few weeks. They are bright and consistent, but they eventually fade away.
Type 1a supernovae are important, despite their brief glimmer. Their existence, after all, was used by astronomers to prove that the universe wasn’t just expanding, but that its expansion was actually speeding up. That group of astronomers earned the Nobel Prize in Physics in 2011.
But type 1a supernovae remain mysterious. Why do these supernovae explode in the first place?
There’s more than one theory for how type 1a supernova occur, but all have a few things in common: You start with two stars orbiting each other. One of them eventually gets big enough, either by stealing mass from the other or by merging with it completely, to become unstable, and it explodes in a supernova.
One of the leading theories hypothesizes that when one star explodes in a supernova, the other would be ejected at thousands of miles per hour and travel across the galaxy. But whether that was happening or not was almost impossible to prove—until the early morning of April 25.
While most of the U.S. was sound asleep in the wee hours of that morning, a group of astronomers around the world were wide awake and eagerly anticipating data they knew could give them those runaway stars—and, more importantly, the proof they needed for their theory of how stars die.
“We thought if these stars did exist, if our model was right, they’d be so obvious in the data that a lot of people would see them, and be very interested,” Ken Shen at UC Berkeley told the Daily Beast. “We wanted to be really ready to jump on it that night and do those observations as soon as we could.”
Shen was leading the international team of astronomers watching for information from European satellite Gaia, which was about to release data on millions of stars in the Milky Way. It was the satellite’s second data release, but the first that would be detailed enough to include the super fast, runaway stars the group needed.
In the following hours astronomers at the Lick Observatory near San Jose, California, the Steward Observatory in Arizona, the Nordic Optical Telescope in the Canary Islands and the South African Large Telescope had used the Gaia data to track down several of these apparent runaway stars, three of which were later confirmed—less than 24 hours after their frantic search had begun.
“It was so exciting that [staying up all night] wasn’t too bad,” Shen said.”It was in the later days that the tiredness really hit.”
What Shen and his team had seen that night were three stars that appear to have been thrown out of a binary system after their companion exploded in a supernova. The white dwarfs—dense but dim stellar cores left over from old stars—were the missing piece of evidence for the team’s theory about how type 1a supernova explode. They were even able to match one of the three back to the remnant of a specific supernova, making the link “basically irrefutable” in that case, says Shen. They’ve since published their results on pre-print server arxiv.
The team's theory for how type 1a supernova occur is fairly similar to their hypothesis: You start with two white dwarf stars orbiting each other. One star takes some helium from the other. Eventually this triggers carbon in the star to detonate, and it’s enough to make the whole thing go up in a supernova.
But the two white dwarf stars don’t merge. After one explodes, the other is let loose, running away at thousands of miles per hour. Unlike other possible supernova formation routes, this leaves behind a smoking gun—the runaway star.
Mathew Smith at the University of Southampton in the United Kingdom, called the work “a significant step” toward working out why these stars explode. “This study has discovered the first companion white dwarfs to type Ia supernovae, allowing us to directly study the debris from these cosmic explosions, and confirm their origin,” he said. “It’s highly probable that these are just the first of many companions to be confirmed.”
One wrinkle that still needs ironing out is the fact the stars Shen and the team saw were not typical white dwarfs. A normal white dwarf is roughly the size of Earth. The three runaways are at least ten times bigger and ten times brighter. “They’re somewhere between a white dwarf and a star like the sun,” says Shen.
The team expected the runaway stars to be altered by the supernova explosion, but exactly how and if they return to normal afterwards is still up for debate. “Supernovae are the most powerful explosions we know about, so in principle they might impart a lot of energy,” says Jay Farihi at University College London. “But how that might be converted into the internal energy of a compact star is unclear.”
There’s a simple explanation for why all three of those found so far seem to be bigger and brighter than usual, though: Brighter stars are just that much easier to see. Shen is confident that with a little more painstaking analysis of the data, there could be even more, dimmer stars left over from supernovae to uncover.
“These three really stuck out as being the most obvious possible candidates,” he said. “But there are likely a lot of other ones [in the Gaia data] that are harder to pick out.”