For a fortnight in March 1437, a new star appeared in Scorpius, burning bright. It brightened gradually, then disappeared into the night, seemingly forever. Korean astronomers plotted its position exactly, preserving it in the history books for future astronomers to find.
Modern astronomers know now that the object is a classical nova, which occurs when a dead star husk called a white dwarf steals enough gas from a neighboring star to blow up like a stellar atomic bomb before quieting back down. Of all the events recorded in East Asian astronomy, this was one of the best recorded: It had a time frame, a visual breakdown, and a precise location. But in the six centuries since then, astronomers have since been trying to figure out which stars caused the explosion.
Now they have. In a paper published today in , there seems to be something of an answer. The team led by Michael Shara of the American Museum of National History used nearly 100 years' worth of observations to track down the objects at the center of this big boom.
Three in One
Back in 1986, Shara first put forth the idea that three seemingly different objects—called classical nova, nova-like binaries, and dwarf —were actually stages of the same events at different time periods, "in the same way as butterflies and caterpillars are all the same," Shira says.
A nova-like binary is a red dwarf star orbiting a white dwarf, which is the remnant of a larger star like our sun expelling all of its hydrogen and becoming an Earth-sized ball of ultra-dense matter. A classical nova occurs when the white dwarf grabs enough hydrogen to make a natural nuclear "bomb" that can brighten to 10 times brighter than the sun. A classical nova like the 1437 nova leave behind a ghostly shell — which is what the team were looking for. After this event, you're left with the third stage: dwarf nova. The white dwarf still syphons off gasses and makes mini-explosions, but most aren't visible without a telescope.
(These events are all vastly different from a supernova, by the way, which is when a very large star explodes after its core collapses into a neutron star or a black hole, or when a white dwarf is syphoning gasses off of a larger star at such a rate that it explodes so powerfully that it destroys both objects.)
Tracking It Down
The 1437 nova was well studied by Korean astronomers. They provided a pretty good location of the "new star" between two well known stars in the constellation. "They told us between which two stars in the tail of scorpion it appeared, and that was terrific," Shira says. "Lots of historical records say 'a new star appeared in the east,'" while this made the event much easier to nail down. But there was a problem: They didn't quite map out which of the handful of dim stars the event actually occurred in.
The team started with a 1985 observation from the United Kingdom Schmidt Telescope, located in Edinburgh. It had a large volume of stars in several wavelengths focused in on Scorpius, and modern astronomers know what to look for after most classical nova events. They tend to become dwarf novae, which are essentially a reverted form of a classical nova. The object still syphons gas off its companion (typically a low mass star) and gathers up mass over tens of thousands of years before shredding this again, repeating the explosion.
"The challenge in understanding the evolution of these binary stars was that the lifecycle of a nova is hundreds of thousands of years"—so says a statement sent to Seniorhelpline by co-author Joanna Mikolajewska of the Nicolaus Copernicus Astronomical Center in Warsaw, Poland. "We simply have not been around long enough to see a single complete cycle."
The team seemed to have their culprit with a shell around a nova event. The expansion rate of the gases pushing outward slows over time, but astronomers have a rough idea of how much it does. The gas shell expansion stops when the gasses hit a thread of matter called the interstellar medium, while the nova stars move away from this remnant.
The way Shira puts it: "You're in a convertible, you're driving in a highway, the air would like to slow you down, but you have too much mass," he says. But you, the nova pair / convertible, throw some confetti out of the convertible, which then falls in place somewhere on the side of the highway after a little scattering.
For Shira, it's the culmination of 31 years of work, and confirms that three types of objects are just part of the evolution of one event. This will help in understanding these star explosions and how they change over thousands of years.
"This makes sense of it all," Shira says. "It ties the evolution altogether."