Korea, March 11, 1437. Back then, it was a united country under the rule of the Joseon Dynasty, and paper currency had just been invented on the peninsula. On that very eve, though, Korean stargazers witnessed an explosion of light that they suspected may have been the birth of a new star within our galaxy. For 14 days, this new light adorned the night sky, until it vanished without a trace.
Documented in philosophical texts, the detailed description of the event made it seem like more of a genuine astronomical event rather than anything more mythical. Now, as reported in Nature, contemporary astronomers led by the American Museum of Natural History have found the source of the light: a nova, located in the Constellation of Scorpius.
So what exactly is a nova? Despite the name, it’s not really linked to supernovae, those far more energetic, incandescent, glorious stellar deaths that take on several forms. The word “nova” may derive from a phrase meaning “new star”, but ironically, they involve white dwarfs – the core remnants of extremely old, dead stars.
Novae generally form in binary systems containing a low-mass star and a white dwarf. If they orbit particularly closely, then the white dwarf is able to steal gas from the companion star’s outer envelope over time. Eventually, all this hydrogen gas accumulating on the white dwarf gets to a point wherein it becomes incredibly compressed by the forces of gravity – and it undergoes uncontrollable fusion.
This generates a remarkable amount of energy, which can be seen across vast distances. They’re occasionally seen from Earth, and it appears this type of celestial firework show was seen by Korean astronomers back in the year 1437.
The source was Nova Scorpii 1437. This “non-destructive nuclear explosion” wasn’t exactly easy to find, especially as the event was witnessed several centuries ago. By now, the fires have all burned out.
Fortunately, novae tend to leave a signature remnant behind long after the conflagration has ceased: the cooled ashes and embers of the original explosion. Using historical information and very old photographic plates, the team successfully located the nova’s ghost in Scorpius' tail.
After having a closer look at the system responsible, the team found that in the 1930s and 1940s, fires were still flaring up. Known as dwarf novae, these form when the accretion of gas by the white dwarf is still occurring quite rapidly post-nova.
They were thought to be fairly rare features, but the authors of this study conclude that “many old novae become dwarf novae... between successive nova eruptions.”
The world changed greatly between the height of the Joseon dynasty and the advent of the Second World War, but clearly, in space, some things never change.