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(English) Star snapped before and after nova explosion

Astronomers have captured rare images of a tiny star before, during and after it exploded as a “classical nova”.

In this type of binary system, a white dwarf sucks gas from a much bigger partner star until it blows up – about every 10,000 to one million years.

Now, a Polish team has caught one in the act using a telescope in Chile.

The observations, reported in Nature, were made as part of a long-running sky survey that was originally aimed at detecting dark matter.

The consistent stream of images snapped for that project, the Optical Gravitational Lensing Experiment, allowed the researchers to go back and see what the star system looked like before the explosion brought it to their attention in May 2009.

Even though it is 20,000 light-years away – a terribly faint pinprick of light barely visible among brighter stars, even in magnified images – this was a rare opportunity to study the build-up and aftermath of a classical nova.

These violent but poorly understood events begin with a white dwarf, the dead remnant of an average star like our Sun, is locked in tight orbit with a regular, active star.

“The distance between those two stars is very small – actually one solar radius,” Mr Mróz said. “Imagine that inside the Sun, you have two stars that are orbiting each other.”

So tight is their orbit, which in this case takes just five hours, that the dwarf steadily steals gas from its larger companion.

That extra matter builds up on the surface of the white dwarf until it kicks off a runaway, explosive thermonuclear reaction. Crucially, however, this blast only rips off the extra material; the white dwarf is left behind.

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