Supernovas have long been used as “cosmic mile markers” to measure the expansion of the universe, but NASA scientists now claim that they have finally discovered what actually sparks the massive stellar explosions.

A team of astronomers led by Marat Gilfanov used NASA’s Chandra X-Ray laboratory to study supernovas in five nearby elliptical galaxies and the central region of the Andromeda galaxy — a spiral galaxy closest to our own, the Milky Way.

They found that most Type 1a supernovas are sparked by the merging of two white dwarf stars, or the collapsed remnant of old stars.

The stars become unstable when they exceed their weight limit which causes a stellar explosion, the Telegraph reported.

“It was a major embarrassment that we did not know how they (supernovas) worked. Now we are beginning to understand what lights the fuse of these explosions, said Gilfanov, an astronomer from the Max Planck Institute for Astrophysics in Germany.

Previously it was thought that Type 1a supernovas were also caused by accretion — when the gravity of a star draws in enough material from a sun-like companion and becomes unstable.

However, Akos Bogdan of the Max Planck Institute said: “If the supernovas were produced by accretion, the galaxies would be roughly 50 times brighter in x-rays than actually observed.”

It is not clear whether merging is also the primary cause of supernovas in spiral galaxies, and further study is required, he said.

Pairs of white dwarfs are extremely difficult to find.

Once the white dwarfs spiral into distances when they are about to merge, it takes just a few tenths of a second for them to explode.

The study is published in the latest edition of the journal Nature.

More In: Science | Sci-Tech