In these systems, one of the stars is a white dwarf, the burned out but still hot remnant of a star. The powerful gravity of the white dwarf pulls hydrogen away from its companion star and onto its surface. “You accumulate about a Pacific Ocean’s worth of hydrogen,” Dr. Shara said.
Credit Digital Access to a Sky Century @ Harvard
With more and more hydrogen, the pressure builds until it sets off a thermonuclear explosion, a burst of light known as a nova that is up a million times as bright as the sun.
“You get a giant fusion bomb, a hydrogen bomb going off on top of this white dwarf star,” Dr. Shara said.
That is what the Koreans saw on March 11, 1437.
As powerful as nova explosions are, they do not destroy either star (unlike much larger supernovas). The white dwarf fades, and the cycle repeats until the next explosion, which could be up to 100,000 years later.
In recent decades, astronomers have observed the fading of novas over decades. They have also spotted similar binary star systems that appear quite stable, calmly orbiting each other, and others that are belching only small eruptions known as dwarf novas.
Three decades ago, Dr. Shara proposed that all three types — novas, dwarf novas and the quiescent binaries — were variants of the same type of system but at different stages. The idea was hard to test because astronomers have not been observing them for very long.
Thus, historical novas like the one observed in Korea could provide important clues to the life cycle between explosions. For years, he and colleagues looked for the 1437 nova without success. Recently, they expanded the field of search and came across a promising cloud of gas, or nebula, that looked like the carcass of a nova. Other astronomers had noted this previously, but, puzzlingly, the star at the center of the nebula is not a cataclysmic variable.
Source: New York Times