Go back in time
Humans have been studying supernovae for thousands of years, although of course it’s only recently that we understood what they are. If close enough to Earth and with minimal dust along the line of sight, a supernova can be visible anywhere in the world as a bright new star to the naked eye for several months. And you can bet people noticed – some in fear, some in wonder, some in confusion – which often led early astronomers to write down what they saw. Ancient Chinese astronomers were particularly careful record-keepers, detailing many bright “guest stars” over the centuries, along with their locations. The first such supernova record dates from 185 AD and was visible for eight months; in modern times, astronomers have found the remnant of the explosion, RCW 86, and determined that it was created by a Type Ia supernova.
The most recent Type Ia supernova seen with the naked eye (and the last supernova observed in our Milky Way) was first spotted in October 1604 and named the Kepler Supernova, after astronomer Johannes Kepler. Kepler was not the first to discover the supernova, but he took meticulous readings of its position and light curve for more than a year and compiled his measurements with those of other astronomers for a book, From Stella Nova. The work is so meticulous that not only have modern astronomers identified the location of Kepler’s supernova remnant centuries later (some 20,000 light-years from Earth), they have even reconstructed the light curve to confirm that it corresponds to a type Ia supernova. These historical records are so vital because they have guided modern astronomers to the remains and allowed them to verify their age – and these still-fresh remains are our best shot at distinguishing between SD and DD scenarios. Four hundred years may seem like a long time, but it’s the blink of an eye, cosmically speaking. “It’s always the time when we probe what the explosion itself did,” says Holland-Ashford, who studies the rest using data from Japan’s Suzaku X-ray Telescope. The X-rays we see still come from material ejected by the explosion itself, known as ejecta – some of which accelerates to 37 million km/h, even centuries later. Holland-Ashford studies the elemental composition of this ejecta. Different types of explosions “would have different elements,” he says. So, by conducting the most detailed study of these elements to date, Holland-Ashford aims to find 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 unlock clues to their ancestors, but they aren’t the only potential clue hidden in our galaxy. Shen proposed a DD scenario where the two stars are not shredded: instead, consecutive explosions first terminate a white dwarf as a Type Ia supernova, then fling outward the second white dwarf at a fantastic speed. The surviving white dwarf would travel at thousands of kilometers per second; such “hypervelocity white dwarfs” would theoretically be everywhere in the galaxy. According to Shen’s idea, if the majority of Type Ia supernovae are produced this way, there should be about 30 hypervelocity white dwarfs within 3,000 light-years of Earth. But do these 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’s (ESA) Gaia Observatory to find evidence that some survived. Gaia obtained precise positional data on about 1 billion astronomical objects, and Shen and his team conducted a search for local hypervelocity white dwarfs. After follow-up observations, they found three hypervelocity white dwarfs that fit the bill, each accelerating to a whopping speed of 2.2 to 6.7 million mph (3.5 to 10.7 million km/h). ). Additionally, the team traced the path taken by each white dwarf in the past. Two of the candidates show no signs that they came from a nearby supernova remnant, which is perhaps unsurprising, as the remnants could be faint or have dissipated over time. But one of them traced the location of a large faint supernova remnant called G70.0–21.5, estimated to be from a supernova explosion around 90,000 years ago. This isn’t quite hard evidence – for one thing, Shen’s study failed to find the right number of hypervelocity white dwarfs. But there are many reasons why Gaia may not have spotted them, Shen says. The white dwarfs the team saw were bright, but as these remnants cool over time, they also fade. Some may have dwindled below Gaia’s ability to see them, Shen says, though future surveys may detect them.
Towards gravitational waves
The true origin of Type Ia supernovae is unlikely to be hidden forever. One of ESA’s main future research missions is a gravitational wave detector called the Laser Interferometer Space Antenna (LISA), a space observatory that will search for ripples in spacetime itself. Gravitational wave studies are still in their infancy – the first detection by the Laser Interferometer Gravitational Wave Observatory (LIGO) was in 2016, and LIGO is not sensitive enough to study binary pairs of white dwarfs.
However, when launched in 2037, LISA will be able to detect binary pairs of white dwarfs in our galaxy with very short periods and glean details such as how long it will take them to merge and the timing of these events. . Perhaps, if we’re very lucky, LISA might pick up a signal just before a Type Ia supernova lights up the sky as a new guest star. Using LISA, astronomers will finally know if such mergers explain all Type Ia explosions or if more than one scenario is at play – and perhaps discover a bit more about fundamental physics along the way. What is clear is that in a universe filled with cosmic explosions as exotic as Type Ia supernovae, there is still much to discover.
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