While looking for something else entirely, Harvard-Smithsonian astronomers have discovered several double-star systems which are merging, and which may explode as supernovae in the astronomically near future.
While examining hypervelocity stars escaping the Milky Way, the team observed a number of binary stars consisting of two white dwarfs.
"These are weird systems - objects the size of the Earth orbiting each other at a distance less than the radius of the sun," says Smithsonian astronomer Warren Brown.
The ones found by his team are comparatively lightweight, holding only about one-fifth as much mass as the sun. They are made almost entirely of helium, unlike normal white dwarfs, which are made of carbon and oxygen.
"These white dwarfs have gone through a dramatic weight loss program," said Carlos Allende Prieto, an astronomer at the Instituto de Astrofisica de Canarias in Spain. "These stars are in such close orbits that tidal forces, like those swaying the oceans on Earth, led to huge mass losses."
Because they whirl around so close to each other, the white dwarfs actually stir up the space-time continuum, creating expanding ripples known as gravitational waves. These carry away orbital energy, causing the stars to spiral closer together.
Half of the dozen or so new-found systems are expected to merge eventually. The tightest binary, orbiting once every hour, will merge in about 100 million years.
"We have tripled the number of known, merging white-dwarf systems," said Smithsonian astronomer and co-author Mukremin Kilic. "Now, we can begin to understand how these systems form and what they may become in the near future."
When two white dwarfs merge, if their combined mass exceeds a certain point, they detonate and explode as a Type Ia supernova.
Brown and his colleagues suggest that the merging binaries they have discovered might be one source of underluminous supernovae - a rare type of supernova explosion 100 times fainter than a normal Type Ia supernova, which ejects only one-fifth as much matter.
"The rate at which our white dwarfs are merging is the same as the rate of underluminous supernovae - about one every 2,000 years," says Brown. "While we can't know for sure whether our merging white dwarfs will explode as underluminous supernovae, the fact that the rates are the same is highly suggestive."