Astronomers recover a lost nova

First observed 600 years ago, this binary system is providing vital clues to how novae work.
By | Published: August 30, 2017
The shell of hot hydrogen left over from a nova explosion seen in 1437 appears in this image, taken with the Carnegie SWOPE 1-meter telescope in Chile. The star highlighted with red marks is responsible for the explosion; the small red plus symbol denotes where the star would have appeared 580 years ago, based on its motion. That position lines up nicely with the center of the shell: the green plus sign.
© K. Ilkiewicz and J. Mikolajewska
In the tail of Scorpius the Scorpion, a new star appeared briefly nearly 600 years ago. The year was 1437, and the previously unseen star, which flared bright and lasted 14 days before disappearing, was recorded by Korean astrologers at the time. Lost to astronomers hunting for its source since that date, the binary star system that underwent that nova has now been found.

The work, published today in Nature, describes “the first nova that’s ever been recovered with certainty based on the Chinese, Korean, and Japanese records of almost 2,500 years,” according to lead author Michael Shara, a curator in the American Museum of Natural History’s Department of Astrophysics, in a press release. Rediscovering this nova, which now appears to undergo periodic smaller-scale nova explosions, provides key support for the theory that novae have long life cycles, and can go dormant before ramping up again.

A nova differs from a supernova in that the former is not a catastrophic event that destroys its progenitor star. In a nova system, a white dwarf (the remnant of a star like our Sun) pulls material off a companion star, which is still in the hydrogen-burning phase of its life. This material, mostly hydrogen, piles up slowly over the course of something like 100,000 years, until it reaches a critical point. At that point, the hydrogen envelope suddenly fuses into helium, releasing a huge amount of energy that’s seen as a nova. It’s essentially a giant hydrogen bomb, and the white dwarf can shine several hundred thousand times brighter for days or even months afterward.

In a type Ia supernova, which occurs in similar systems, it’s thought that the white dwarf pulls matter off its companion much more quickly, and reaches a critical mass point that triggers a larger explosion, which does tear the white dwarf apart. Thus, supernovae destroy the stellar remnant that creates them; novae do not, allowing the process of hydrogen buildup to occur again and possibly trigger another nova in the future.

The nova of 1437 has been elusive, with several astronomers including Shara, Richard Stephenson of Durham University, and Mike Bode of Liverpool John Moores University searching for the binary system that caused it.

But recently, after expanding their initial search area, they came across a shell of material: the sign of a classical nova explosion. They even found a bright star near — but not in — the center of the shell.

The team cross-referenced their find with an image of the area taken at the Harvard Observatory station in Peru in 1923, and were able to accurately chart the star’s motion across the sky over time. Based on their calculations, “… we traced it back six centuries, and bingo, there it was, right at the center of our shell. That’s the clock, that’s what convinced us that it had to be right,” said Shara.

That photographic plate is now part of the Digitizing a Sky Century at Harvard (DASCH) project. And with the addition of other plates from the DASCH project taken in the 1940s, Shara and his team discovered that the nova is now something called a “dwarf nova,” which undergoes periodic small outbursts that are likely due to instabilities in the material as it forms a disk around and accretes onto the white dwarf, rather than the huge explosion of a “classical nova,” like the event it underwent nearly 600 years ago.

This behavior helps to cement the idea that classical novae, dwarf novae, and other objects that show novae-like variability are all different stages of the same type of system. “In the same way that an egg, a caterpillar, a pupa, and a butterfly are all life stages of the same organism, we now have strong support for the idea that these binaries are all the same thing seen in different phases of their lives,” Shara explained.

However, because these systems can evolve and change over hundreds of thousands of years, showing different types of behavior at different times, it’s been difficult to link them together. “We simply haven’t been around long enough to see a single complete cycle,” said Shara.

But now, the rediscovery of the 1437 eruption and the ability to trace its evolution over time, astronomers can take a closer, more complete look at that life cycle to approach a better understanding of how systems that may seem different are really all one and the same.