Turn back the clock
Humans have been studying supernovae for thousands of years, although, of course, we only recently understood what they are. If it is close enough to Earth and with minimal line-of-sight dust, the supernova could be visible around the world as a bright new star to the naked eye for several months. And you can bet that people noticed—some frightened, some in amazement, some bewildered—which often led early astronomers to write down what they saw. Ancient Chinese astronomers meticulously kept record-keeping, detailing many bright “guest stars” over the centuries, along with their locations. The oldest known supernova was dated to 185 AD and was visible for about eight months. In modern times, astronomers found the remnant of the explosion RCW 86, and determined that it was caused by a type Ia supernova.
The most recent naked-eye Type Ia supernova (and the last observed inside our Milky Way galaxy) was first observed in October 1604 and named the Kepler supernova, after the astronomer Johannes Kepler. Kepler wasn’t the first to spot a supernova, but it made meticulous records of its location and light curve for more than a year and combined its measurements with those of other astronomers for a book, De Stella Nova. The work was so meticulous that not only did modern astronomers locate the Kepler supernova remnant centuries later (about 20,000 light-years from Earth), but they also reconstructed the light curve to confirm its alignment with a Type Ia supernova. These historical records are vital because they have guided modern astronomers to the remnants and allowed them to verify their ages — and these still fresh remnants are the best chance we have for distinguishing between SD and DD scenarios. Four hundred years might seem like a long time, but that’s the blink of an eye, in cosmological terms. “It is still time for us to investigate the cause of the actual explosion itself,” explains Holland-Ashford, who studies the remnants using data from Japan’s Suzaku X-ray telescope. The X-rays we see are still from the material released by the explosion itself, known as ejecta—some of it still rushing outward at a whopping 23 million miles per hour (37 million km/h), even centuries later. Holland-Ashford studies the elemental composition of these projectiles. He says that different types of explosions “will have different elements”. So, by conducting the most detailed study of these elements yet, Holland-Ashford aims to find out what event led to the “stella nova” that Kepler saw in the sky more than four centuries ago.
Supernova remnants are a promising way to uncover evidence of their ancestors, but they aren’t the only possible clues hiding in our galaxy. Shen proposed a DD scenario in which both stars are not shredded: instead, successive explosions first end in one white dwarf as a type Ia supernova and then eject the second white dwarf at dizzying speed. A surviving white dwarf will travel thousands of miles per second. Such “hypervelocity white dwarfs” would theoretically be found throughout the galaxy. According to Shen’s idea, if the majority of Type Ia supernovae were produced in this way, there should be about 30 supernovae within 3,000 light-years of Earth. But do such stars exist?
“We didn’t really know if they would survive,” Shen recalls, but he and his team used data from the European Space Agency (ESA) Gaia Observatory to find evidence that some do. Gaia has obtained accurate positional data for nearly a billion astronomical objects, and has led Shen and his team in search of local hypervelocity white dwarfs. After follow-up observations, they found three supervelocity white dwarfs fit the bill, each speeding at a whopping 2.2 million to 6.7 million miles per hour (3.5 million to 10.7 million km/h). Furthermore, the team tracked the path each white dwarf had taken in the past. Two of the candidates show no sign of originating in nearby supernova remnants, which may not be surprising, as the remnants could be faint or have dissipated over time. But one of them dates back to the site of the remnants of a large, faint supernova called G70.0-21.5, estimated to have resulted from a supernova explosion some 90,000 years ago. It’s not quite a smokescreen — for one thing, Shin’s study fell a bit short in finding just the right number of hypervelocity white dwarfs. But there are many reasons Gaia might not have discovered, says Shane. The white dwarfs the team saw were bright, but as these remnants cool over time, they also fade. Some may have dimmed Gaia’s ability to see, Shen says, though future surveys may pick it up.
Go to gravitational waves
The true origin of Type Ia supernovae is unlikely to be hidden forever. One of ESA’s primary research missions for the future is a gravitational-wave detector called the Laser Interferometer Space Antenna (LISA), a space observatory that will look for ripples in space-time itself. Gravitational-wave studies are still in their infancy—the first detection by the Laser Gravitational-Wave Observatory (LIGO) occurred in 2016, and LIGO is not sensitive enough to study white-dwarf binary pairs.
However, when it launches in 2037, LISA will be able to detect pairs of binary white dwarfs in our galaxy at very short intervals and piece together details such as how long it will take for them to merge and the rate of such events. Perhaps, if we’re very lucky, LISA might detect a signal before a Type Ia supernova lights up the sky as a new guest star. With LISA, astronomers will finally know if these mergers explain all Type Ia explosions or if more than one scenario is at play—and perhaps reveal a bit about the underlying physics along the way. What is clear is that in a world filled with exotic cosmic explosions such as Type Ia supernovae, there is still much to uncover.
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