Einstein’s light-bending by single far-off star detected

For the first time, astronomers have seen a star outside of the solar system bend the light from another star. The measurement, reported June 7 in Austin, Texas, at a meeting of the American Astronomical Society, vindicates both Einstein’s most famous theory and what goes on in the inner lives of stellar corpses.

Astronomers using the Hubble Space Telescope watched as a white dwarf passed in front of a more distant star. That star seemed to move in a small loop, its apparent position deflected by the white dwarf’s gravity.
More than a century ago, Albert Einstein predicted that the way spacetime bends around a massive object — the sun, say — should shift the apparent position of stars that appear behind that object. The measurement of this effect during a solar eclipse in 1919 confirmed Einstein’s general theory of relativity: Mass warps spacetime and bends the path of light rays (SN: 10/17/15, p. 16).

The New York Times hailed it as “one of the greatest — perhaps the greatest — of achievements in the history of human thought.” But even Einstein doubted the light-bending effect could be detected for more distant stars than the sun.

Now, in a study published in the June 9 issue of Science, Kailash Sahu of the Space Telescope Science Institute in Baltimore and his colleagues have shown that it can.

“This is an elegant outcome,” says Terry Oswalt at Embry-Riddle Aeronautical University in Daytona Beach, Fla., who was not involved in the new work. “Einstein would be very proud.”
While the stars literally aligned to make the measurement possible, this was no lucky accident. Sahu and colleagues scoured a catalog of 5,000 stellar motions to find a pair of stars likely to pass close enough on the sky that Hubble could sense the shift.

There were a few possible candidates, and one of them, called Stein 2051 B, was already a mysterious character.

Located about 18 light-years from Earth, Stein 2051 B is a white dwarf, a common end-of-life state for a sunlike star. When low-mass stars run out of fuel, they puff up into a red giant while fusing helium into carbon and oxygen. Eventually, they slough off outer layers of gas, leaving this carbon-oxygen core — the white dwarf — behind. About 97 percent of the stars in the Milky Way, including the sun, are or someday will be white dwarfs.

White dwarfs are extremely dense. They are prevented from collapsing into a black hole only by the pressure their electrons produce in trying not to be in the same quantum state as each other. This bizarre situation sets strict limits on their sizes and masses: For a given radius, a white dwarf can be only so massive, and only so large for a given mass.

This mass-radius relation was laid out in Nobel prize‒winning work by Subrahmanyan Chandrasekhar in the 1930s, but it has been difficult to prove. The only white dwarfs weighed so far share their orbits with other stars whose mutual motions help astronomers calculate their masses. But some astronomers worry that those companions could have added mass to the white dwarfs, throwing off this precise relationship.

Stein 2051 B also has a companion, but it is so far away that the two stars almost certainly evolved independently. That distance also means it would take hundreds of years to precisely measure the white dwarf’s mass. The best efforts to find a rough mass so far created a conundrum: Stein 2051 B appeared to be much lighter than expected. It would need an exotic iron core to explain it.

Measuring the shift of a background star provides a way to measure the white dwarf’s mass directly. The more massive the foreground star — in this case, the white dwarf — the greater the deflection of light from the background star.

“This is the most direct method of measuring the mass,” Sahu says. “It’s almost like putting somebody on a scale and reading off their weight.”

The white dwarf was scheduled to pass near a background star on March 5, 2014. Sahu’s team made eight observations of the two stars’ positions between October 2013 and October 2015.

The team found that the background star appeared to move in a small ellipse as the white dwarf approached and then moved away from it, exactly as predicted by Einstein’s equations. That suggests its mass is 0.675 times the mass of the sun — well within the normal range for its size.

This first measurement won’t be the last, Oswalt says. Several new star surveys are coming online in the next few years that will track the motions of billions of stars at once. That means that even though light-bending alignments are rare, astronomers should catch several more soon.