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White dwarf stars orbit one another.

Two orbiting white dwarfs radiate gravitational waves, as seen in an artist's conception.

Illustration courtesy Tod Strohmayer/CXC/NASA and Dana Berry/CXC

Andrew Fazekas

for National Geographic News

Published July 19, 2011

A pair of aging stars discovered 3,000 light-years away is locked in a "dance" of death—a union that will end in their collision and a possible supernova, astronomers say.

The binary star system consists of two white dwarfs—the burnt-out cores of sunlike stars. The white dwarfs are gradually spiraling toward each other at breakneck speeds of 370 miles (595 kilometers) a second, and they're destined to merge in 900,000 years.

But astronomers hope that, before the collision, the spinning stars will help scientists test Einstein's general theory of relativity and even reveal the origins of an entire class of supernovae. (Related: "Einstein's Gravity Confirmed on a Cosmic Scale.")

"What is so incredible is that this exotic pair of Earth- and Neptune-sized stars are orbiting each other at only a third of our Earth-moon distance, circling each other every 12 minutes," said study leader Warren Brown, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

"And because there is no interaction—or star matter streaming between them—we may have a unique stellar laboratory here to look for effects of general relativity and probe for extreme gravity."

(Also see "Superfast Stars Have Five-Minute Orbits.")

Star System Offers Rare Look at Space Gravity

The team found the dancing stars while surveying for white dwarf pairs using the 6.5-meter Multiple Mirror Telescope on Arizona's Mount Hopkins. Astronomers measured the stars' relative motions by looking at the light signatures, or spectra, coming from the stars as they eclipse each other.

White dwarfs have superhigh densities—just a spoonful of matter from one of these objects would weigh as much as a car.

When two such massive objects are whirling around each other, they stir up the space-time fabric, creating ripples like those made by a stone thrown into a pond. As these ripples, called gravitational waves, are produced, the star pair loses some of its energy, causing their orbits to slowly shrink.

(See "Space-Time 'Wrinkles' Igniting Odd Gamma-ray Bursts?")

But the newfound star pair is unique in that they don't swap matter as they spin, providing a "clean clock" to measure the effects of gravitational waves, Brown said.

"There are lots of stellar pairs in the universe, but all are interacting and exchanging matter with one another because they are so close together."

That "complicates their interpretation, because you typically don't see either star, except for the light coming from the matter that is going back and forth between the stars."

With a noninteracting pair, astronomers can precisely measure the change in the orbital period of the stars as they spiral toward each other.

Astrophysicist Gijs Nelemans, of Radboud University in the Netherlands, added that the new white dwarf system is probably a strong gravitational wave source, one that may soon be detectable by a gravity-wave space satellite called LISA, which could be launched around 2020.

"The most exciting aspect is that the change in the orbital period as it emits gravitational waves can actually be measured," said Nelemans, who was not involved in the new research.

"That means we will not only indirectly test general relativity, but also directly measure the predicted gravitational waves, which has never been done before," he said.

"Such a mission would truly open a new way to study the universe."

Newfound Pair May Illuminate Star Evolution, Death

The discovery also could help astronomers understand star evolution and death. It's long been speculated, for instance, that white dwarf collisions may produce Type Ia supernovae, which are thought to be caused when matter from a companion star is dumped onto a white dwarf, increasing the dwarf's mass beyond a set physical limit and igniting a thermonuclear explosion.

When the two newfound stars merge, according to current models, the result may be one supermassive white dwarf or an unusually faint stellar blast called an underluminous supernova. (See supernovae pictures.)

"If this is the progenitor of these rare subclass of supernova, we expect to be finding these exotic pairs at the same frequency as the supernova. We'll have to wait and see what our survey comes up with," study leader Brown explained.

Initial measurements were made in March, but now the binary system has moved almost directly behind the sun from our perspective on Earth, making the pair currently invisible to telescopes.

As a result, Brown and his team will have to wait until fall to measure the expected shortening of the orbital period.

"Stars merging in less than a million years is really a blink of an eye in cosmic time scales," Brown said. "Just the fact that we found something like this—and it exists—is interesting for astronomers."

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