Scientists catch white dwarf star in the act of exploding into a nova

It’s not every day you get to see a star go nova. Scientists at Warsaw University Observatory in Poland have managed to catch a binary star system both before and after its explosive flash.

The findings, described in the journal Nature, confirm a long-held theory about novae known as the hibernation hypothesis — and could potentially help scientists better understand when such stellar outbursts occur.

Novae are typically caused by a gravitationally locked pair of stars, called a binary system, consisting of one white dwarf and a companion star. A white dwarf is an ageing star that has already shed much of its mass, leaving behind a small but massive core that siphons off material from its stellar companion — and every so often, the system becomes so unstable that the white dwarf erupts in a cataclysmic explosion that causes it to flare brightly in the night sky.

“The most spectacular eruptions, with a ten-thousandfold increase in brightness, occur in classical novae and are caused by a thermonuclear runaway on the surface of the white dwarf,” the study authors wrote. “Such eruptions are thought to recur on time scales of ten thousand to a million years.”

Around 50 novae go off every year in the Milky Way, but only five to 10 are actually observed because most of them are shrouded by interstellar gas and dust, lead author Przemek Mroz said in an email.

But while novae can be seen once they go off, scientists don’t often get the chance to study them in depth before they explode.

Researchers have long had a theory about the cycle that causes these novae: When the mass transfer is low (less than a billionth of the sun’s mass per year), the accretion grows unstable; every so often, stuff gets dumped onto the white dwarf in what the authors called “dwarf nova outbursts.”