Binary stars have a fatal attraction

U. TEXAS-AUSTIN (US) — A pair of burnt-out stars spiraling into one another at breakneck speeds will merge and possibly explode as a supernova in about 900,000 years.

The two white dwarfs, orbiting each other in just 13 minutes and circling at 370 miles per second—180 times faster than the fastest jet on Earth—will be used by scientists to test Einstein’s theory of relativity and the origin of some peculiar supernovae.

“These stars are whipping around each other so fast they are literally distorting the fabric of space,” says J. J. Hermes, a graduate student working with Don Winget, professor of astronomy at University of Texas-Austin.

“As J. J. and I watched the first data come in, seeing the eclipses and light variations easily in the raw data, we were elated,” Winget says. “The importance of this object was immediately clear to us.

“This system will give us a chance to test the theory of general relativity in two ways: by detecting the gravitational radiation from this system directly using space missions like LISA (Laser Interferometer Space Antenna) and by measuring the rate of the decay of the orbital period as the two objects spiral together.”

This research will be published in the Astrophysical Journal Letters.

The brighter white dwarf contains about a quarter of the Sun’s mass compacted into a Neptune-sized ball, while its companion has more than half the mass of the Sun and is Earth-sized. A penny made of this white dwarf’s material would weigh about 1,000 pounds on Earth.

Their mutual gravitational pull is so strong that it deforms the lower-mass star by three percent. If the Earth bulged by the same amount, there would be tides 120 miles high.

Researchers have been hunting for pairs of white dwarfs using the MMT telescope at Whipple Observatory on Mt. Hopkins, Arizona, and following up with the Otto Struve Telescope at McDonald Observatory.

The star pairs are too close together to distinguish photographically but by looking at the spectra, scientists were able to differentiate the two stars and measure their relative motions. These stars are also oriented such that they eclipse each other every 6 minutes.

“If there were aliens living on a planet around this star system, they would see one of their two suns disappear every 6 minutes—a fantastic light show,” says Mukremin Kilic, an astronomer with the Smithsonian Institution.

These eclipses provide a very accurate clock, which is useful for measuring any changes in the system. General relativity predicts that moving objects will create ripples in the fabric of space-time, called gravitational waves. These waves carry away energy, causing the stars to inch closer together and orbit each other faster and faster.

“Though we have not yet directly measured gravitational waves with modern instruments, we can test their existence by measuring the change in the separation of these two stars,” Hermes says. “Because they don’t seem to be exchanging mass, this system is an exceptionally clean laboratory to perform such a test.”

The team expects to conduct this test in a few months, when the star pair emerges from behind the Sun as seen from Earth.

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