In 2009, a super dense dead star exploded in a brilliant flash of light — and astronomers were able to watch the entire thing from Earth. The dead star was a white dwarf, the leftover remnant of a star that has used up all its fuel and collapsed. It exploded in an event known as a classical nova, a rarely seen type of star explosion that occurs only in white dwarfs. The astronomers were able to watch this star before and after it exploded, providing the first ever visual confirmation of what triggers this strange star eruption.

A classical nova occurs in a two-star system — where a white dwarf and a companion star orbit closely around each other. The white dwarf is so dense that its extreme gravity pulls hydrogen gas off of the second star. That hydrogen builds up around the white dwarf, and eventually it gets so hot that it erupts in an extremely bright thermonuclear reaction. "This eruption is only on the white dwarf’s surface," says lead study author Przemek Mroz. "What goes off is only this thin layer of gas on the white dwarf."

The astronomers saw a nova just like this in 2009 with the distant binary system V1213 Cen, which they detailed in the journal Nature. The researchers had been watching these two stars well before the explosive event. They’ve monitored V1213 Cen since 2003, watching the build up to the white dwarf’s explosion and what has happened to the star system once the nova was over.

Astronomers rarely get to see the events leading up to a star explosion. "When novae or supernovae go off, they are usually followed up with many telescopes, and therefore we know a great deal about the ‘after’ of these explosions," says Carles Badenes, an astronomer at the University of Pittsburgh, who was not involved in the study. "But it is of course very hard to know...which star is going to do something interesting, so the ‘before’ is very much a mystery."

Because they saw what happened before the explosion, the astronomers have provided first-ever confirmation of how a star system evolves before and after a classical nova.

The Warsaw Telescope dome at the Las Campanas Observatory, which observed the explosion.

In the six years leading up to the classical nova, the astronomers saw the star system periodically brighten. During those small brightening events, a small portion of hydrogen gas was being dumped onto the white dwarf’s surface. The periodic bumps meant the transfer of hydrogen to the white dwarf was unstable and happening at an extremely low rate, according to Mroz.

But once the explosion occurred, the star system was slightly altered. "The whole system survives, so after the eruption we still have those two stars," says Mroz. And hydrogen still gets sucked from the companion star to the white dwarf. But now, the companion star received a big blast of radiation during the explosion, causing it to swell. It’s now slightly bigger than it was before the explosion and the star system is much brighter. The swelling changes the rate at which hydrogen goes from one star to the other, Mroz says. Now, the gas is being transferred much more quickly. Since the explosion, the astronomers have no longer seen the bumps in light, meaning the hydrogen is flowing much more smoothly at a higher rate.

This change in the flow of hydrogen before and after a classical nova has been predicted but never seen directly before.. Scientists think it takes maybe millions of years before a white dwarf accumulates enough hydrogen to explode in a classical nova. "The probability of seeing something like this is very small," says Mroz. "People thought we would have to observe some stars for hundreds of thousands of years. This is a very rare transition."

As for the white dwarf in V1213 Cen, it will likely accumulate enough hydrogen to explode again. But you’ll need some patience if you want to see it: the next classical nova for the system likely won’t occur for another million years.