On March 11, 1437, a group of royal astronomers in an observatory tower in Seoul, Korea watched as a brilliant white flash lit up the night sky. One of the five observers made meticulous note of what they saw: “A guest star began to be seen between the second and third stars of Wei [Scorpius]… It lasted for 14 days.”
Though they didn’t know it then, the shimmering “guest star” came from an event more violent than its name suggests: a nova explosion. Now, using the Korean records as well as glass photographic plates from Harvard University, modern astrophysicists have rediscovered the star and reconstructed the life cycle of a binary star system for the first time.
“The real novelty in this study is we have an indisputable and extraordinarily accurate clock,” thanks to the ancient Koreans, says Michael Shara, lead author on the study and an astrophysicist at the American Museum of Natural History. “In astronomy, you almost never measure anything with better than 20 or 30 percent accuracy. Here, we know what happens to the day.”
The research, published Wednesday in the journal Nature, looks at the evolution of a binary system, a celestial pairing in which two stars are close enough that their gravity causes them to orbit each other. Around 70 percent of stars fall into this category, and their relationship is anything but peaceful. The larger of the two stars is a white dwarf, a super dense star with a mass no higher than 8 times the size of our Sun. (In fact, our Sun will likely become a white dwarf in 5 billion years.)
“If you were standing on a white dwarf you’d be flattened to an oil slick,” says Josh Grindlay, a co-author of the study and principal investigator at the Digital Access to a Sky Century @ Harvard project (DASCH).
The white dwarf’s long-term companion, by contrast, is a smaller, hydrogen-burning red dwarf. The white dwarf mercilessly cannibalizes its partner, siphoning off matter and accumulating it into a ring around its atmosphere. That halo of superhot matter is called an “accretion disk.” Occasionally the amount of matter pouring from the smaller star to its hungry partner will increase enough that the white dwarf brightens dramatically, like a flashlight suddenly turning on. This is known as a “cataclysmic variable.”
What Korean astronomers observed nearly 600 years ago, however, was even more dramatic. They witnessed something called a nova, which is when the matter accumulating on the atmosphere of the white dwarf reaches critical mass and causes a chain of nuclear reactions, growing to be as much as 1 million times brighter than the sun. Unlike a supernova, a star undergoing nova doesn’t fully explode; only its atmosphere does. The white dwarf eventually slips back into a “hibernation” state, sipping matter from its partner. In this state, the only remaining trace of its violent eruption is a cloud of ejected matter known as a shell.
“Novae are often described as the third most energetic explosions in the universe—first being the Big Bang, and second being supernovae and gamma ray bursts,” says Jeremy Drake, a senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics.
Before now, researchers weren’t sure if novae and cataclysmic variables occurred in the same system, or if some systems produced novae while others remained cataclysmic variables. “The fact that we can trace back this particular nova event from the Korean observations, and see that this star is now undergoing normal cataclysmic variable behavior, is a missing piece from the puzzle that tells us nova and cataclysmic variables are the same system undergoing cyclic episodes,” Drake says.
For Shara, the revelation is even more gratifying. For 30 years, he has been looking for physical proof for his hypothesis that binary systems exist in a state of evolution, like “butterflies and caterpillars.” After looking in a slightly different location than he expected, he finally found—or rather, rediscovered—this white dwarf, sitting inside its nova shell. And with the glass plates from DASCH—which were used by Harvard astronomers and “computers” to photograph the skies for 100 years—Shara could see the same star going through dwarf-nova outbursts (those moments of flickering brightness) in 1934, 1935 and 1942.
For this awe-inspiring discovery, we owe a debt of gratitude to the royal observers commissioned by Korea’s King Sejong, who ruled from 1418 to 1450 and constructed “one of the finest astronomical observatories in the world,” writes Joseph Needham in The Hall of Heavenly Records: Korean Astronomical Instruments and Clocks. In addition to building multiple observatories and astronomical instruments (including a revolutionary self-striking water clock), the royal astronomers also made accurate enough observation of the moon, sun and five planets to make predictions on their future movements throughout 1442.
The book these observations and predictions are recorded in, Chilijeongsan (Calculations of the Seven Luminaries), is “evidence of Korean astronomy at its highest level in the contemporary world,” writes historian Park Seong-Rae in Science and Technology in Korean History: Excursions, Innovations and Issues. Nor is this the first time modern astronomers have benefited from the meticulous calculations of early star-gazers. Ancient peoples monitoring solar and lunar eclipses across Asia and the Middle East laid the foundation for future scientific advances, reports Maya Wei-Haas for Smithsonian.com.
But even though we’ve unraveled one big question about the life cycle of novae, Shara believes there’s more to be illuminated. “Do any of these system go into the very deepest phases of hibernation, where the mass transfer rate becomes thousands of times less, or might it even drop to zero? Is there a time that the stars don’t interact? That’s an unknown,” he says. All we know for now is that the cycle—nova, hibernation, cataclysmic variable—repeats itself thousands of times over the binary system’s long lifespan.
At the end of that life cycle, the cannibalized hydrogen star eventually loses its star status. “It becomes a brown dwarf, then a planet, then the rocky core of a planet, then it’s probably shredded into an asteroid belt,” Shara says.
While Shara plans to keep observing the sky for more evidence of what comes next for binary systems, he suspects more of his colleagues might go digging into the past to chase down novae. Drake, for one, seems keen to take up the gauntlet. “I don’t know how many are lurking around, but I’m sure there are more examples in archives that can be followed up,” says Drake, who wasn’t affiliated with the study. He adds that, while exo-planets might get the lion’s share of the public’s attention, novae and cataclysmic variables are really where the fun is at.
“Stellar evolution and the physics of how stars interact and explosion dynamics—they really are fascinating systems to explore,” he says.