White dwarf stars, long thought to have reached the end of their lives and died, can come back to life, re-igniting to explode as supernovas, astronomers say.

Scientists had long assumed that a star of a mass similar to our Sun, as it approached the last period of its life, would blow off the outer layers and see its core slowly shrink until it became a white dwarf, with nothing in its future except a gradual and continual cooling down.

There is a property known as the Chandrasekhar limit that posits a maximum mass that would keep it stable, but is has been theorized such stars might be able to explosively come back to life if it increased its mass by stealing matter away from a companion star, or perhaps colliding with a second white dwarf, but that was just theory -- until now.

White dwarf explosions, known as Type 1a supernova, only happen every few centuries years in any galaxy, but astronomers were fortunate to detect and observe one in January.

In a supernova in the galaxy M82, dubbed SN2014J, scientists detected radioactive nuclei generated by nuclear fusion in the cosmic blast that suggested it had in fact increased its mass by taking in material from an external source.

That extra mass compressed the carbon in the white dwarf's core to the point where under underwent nuclear fusion.

"Fusion happens in a flash," says astronomer Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics. "A thermonuclear flame rips through the white dwarf, fusing carbon into heavier elements with a sudden release of energy that tears the star apart."

It had been theorized that during such a supernova, carbon and oxygen should fuse into radioactive nickel, which should then decay down to radioactive cobalt and finally into a stable element, iron.

Scientists looking at SN2014J reported finding evidence of such cobalt decay from data obtained during 50 to 100 days following the supernova explosion.

However, they said, it was unclear which theoretical process -- companion star or collision with another white dwarf -- is the more likely cause of initiating this particular white dwarf explosion.

"It is perfectly consistent with the simplest scenario, of a single white dwarf with a mass close to the Chandrasekhar limit [stealing new material]," says lead study author Yevgeny Churazov  of the Space Research Institute of the Russian Academy of Sciences. "But we cannot exclude with this data that this event was caused by a merger [of two white dwarfs]."