A team of astronomers has identified two massive stars at the center of a planetary nebula that are bound to explode into supernova about 700 million years from now.

Using the facilities of the European Southern Observatory (ESO) and the telescopes in the Canary Islands, Miguel Santander-García, from Spain's National Astronomy Observatory, and colleagues have found a pair of white dwarf stars while conducting an investigation on how some stars produce odd-shaped and asymmetric nebulae, interstellar clouds of gas and dust, late in their lives.

The newly discovered pair of white dwarf stars has a total mass equivalent to 1.8 times that of the Solar System's sun making the pair the most massive yet to have been discovered. A white dwarf is a very dense star with size comparable to that of a planet. They are essentially the remnants of a low-mass star that slowly cool and fade away.

The presence of the two stars has explained the odd shape of the nebula dubbed Henize 2-428, one of the objects that the team studied, and backed up the theory of double stars behind the odd shapes of some nebulae.

The researchers likewise found that individually, the newly discovered pair of stars has mass slightly less than that of our sun and orbit each other every four hours.

The two stars are also close enough to each other that based on Einstein's general theory of relativity, they will get closer to each other, losing energy as gravitational waves and will eventually merge in a single star anytime in the next 700 million years.

The new star will also be very massive that it will inevitably collapse and eventually explode as a supernova.

"Here we report the case of Henize 2-428, the first double-degenerate binary which has, without any ambiguity, a total mass above the Chandrasekhar limit. According to its short orbital period (4.2 hours) and total mass (∼1.8 M⊙), the system should merge in 700 million years, triggering a Type Ia supernova (SN Ia) event," the researchers wrote [pdf] in their study, which was published in the journal Nature on Feb. 9.

The event, a subset of explosion known as Type Ia, will produce predictable brightness.

"Until now, the formation of supernovae Type Ia by the merging of two white dwarfs was purely theoretical," said study author David Jones, from Instituto de Astrofísica de Canarias. "The pair of stars in Henize 2-428 is the real thing!"

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