Turning back the clock
Humans have been studying supernovae for thousands of years, though of course it was only recently that we understood what they are. If it’s close enough to Earth and with minimal dust along the line of sight, a supernova can be visible all over the world as a bright new naked-eye star for several months. And you can bet that people noticed — some with fear, some with wonder, some with 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 earliest such supernova record dates to A.D. 185 and was visible for eight months; in modern times, astronomers found the remnant from the explosion, RCW 86, and determined it was created by a type Ia supernova.
The most recent type Ia supernova seen with the naked eye (and the last supernova observed within our Milky Way) was first spotted in October 1604 and named Kepler’s Supernova, after astronomer Johannes Kepler. Kepler was not the first one to discover the supernova, but he took meticulous records of its position and its light curve for over a year and compiled his measurements with those of other astronomers for a book, De 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 it’s consistent with a type Ia supernova. Such historical records are so vital because they have guided modern astronomers to the remnants and allowed them to verify their ages — and such still-fresh remains are our best chance of distinguishing between the SD and DD scenarios.
Four hundred years may sound like a long time, but that’s a blink of an eye, cosmically speaking. “This is still the time where we’re probing what the actual explosion itself made,” explains Holland-Ashford, who is studying the remnant using data from the Japanese Suzaku X-ray telescope. The X-rays we see are still from the material ejected by the explosion itself, known as ejecta — some of which is speeding outward at a whopping 23 million mph (37 million km/h), even centuries later. Holland-Ashford is studying 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 the clues of their progenitors, but they’re not the only potential clue hiding in our galaxy. Shen has proposed a DD scenario where both stars don’t get shredded apart: Instead, back-to-back explosions first end one white dwarf as a type Ia supernova and then fling outward the second white dwarf at a fantastic speed. The surviving white dwarf would travel at thousands of miles a second; such “hypervelocity white dwarfs” would theoretically be all over the galaxy. According to Shen’s idea, if the majority of type Ia supernovae are produced this way, there should be about 30 such hypervelocity white dwarfs within 3,000 light-years of Earth. But do such stars exist?
“We didn’t really know if they’d survive,” recalls Shen, but he and his team have used data from the European Space Agency (ESA) observatory Gaia to find proof that some do. Gaia has obtained precise positional data on approximately 1 billion astronomical objects, and Shen and his team led a search for local hypervelocity white dwarfs. After follow-up observations, they found three hypervelocity white dwarfs that fit the bill, each speeding along at a whopping 2.2 million to 6.7 million mph (3.5 million to 10.7 million km/h). What’s more, the team traced the path each white dwarf has traveled in the past. Two of the candidates show no sign that they originated in a nearby supernova remnant, which is perhaps not surprising, as the remnants could be faint or have dissipated over time. But one traced back to the location of a large, faint supernova remnant called G70.0–21.5, estimated to be from a supernova explosion approximately 90,000 years ago.
It’s not quite a smoking gun — for one thing, Shen’s study fell a bit short on finding the right number of hypervelocity white dwarfs. But there are many reasons Gaia might not have spotted them, Shen says. The white dwarfs the team did see were bright, but because these remnants cool over time, they also fade. Some may have dimmed below Gaia’s ability to see them, Shen says, though future surveys may pick them up.
Going to gravitational waves
The true origin of type Ia supernovae is unlikely to hide forever. One of the ESA’s primary future research missions is a gravitational-wave detector called the Laser Interferometer Space Antenna (LISA), a space-based observatory that will look for ripples in space-time itself. Gravitational-wave studies are still in their infancy — the first detection by the Laser Interferometer Gravitational-wave Observatory (LIGO) happened 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 binary white dwarf pairs in our galaxy with very short periods and glean details such as how long it will take for them to merge and the rate of such events. Perhaps, if we are very lucky, LISA might detect a signal just before a type Ia supernova lights up the sky as a new guest star. Using LISA, astronomers will finally know whether such mergers explain all type Ia explosions or if more than one scenario is at play — and perhaps uncover a bit more about fundamental physics along the way. What’s clear is that in a universe filled with cosmic explosions as exotic as type Ia supernovae, there is still much to uncover.