Scientists defy Einstein's prediction and use relativity to measure a star's mass

This illustration shows how the gravity of a white dwarf star warps space and bends the light of a distant star behind it. (NASA)

Astronomers scanning the skies with NASA’s Hubble Space Telescope have pulled off a feat that even Albert Einstein had declared all but impossible: they’ve witnessed the subtle bending of one star’s light by another star’s gravity and used that distortion to measure a star’s mass.

The findings, unveiled Wednesday at a meeting of the American Astronomical Society and published in the journal Science, confirm a key tenet of Einstein’s landmark general theory of relativity and introduce a new tool with which to explore a fundamental property of stars.

The general theory of relativity, presented in 1915, describes how gravity can distort the path of light, altering its trajectory. In 1919, the theory was proved correct when, during a solar eclipse, an expedition led by Sir Arthur Eddington discovered that stars near the edge of the blocked sun’s disc were not where they were supposed to be. Their apparent position had moved because the sun’s gravity had distorted the path of their starlight, just as Einstein had predicted.

Since then, astronomers have used this insight as a powerful tool with which to observe distant phenomena. That’s because, when lined up just right, a massive object in the foreground can bend the light of a background light source and magnify it the way a lens does. This phenomenon, known as gravitational lensing, has allowed astronomers to observe distant galaxies that otherwise would be too faint to study.

But lensing events enabled by galaxies and other large structures have been fuzzy at best, said Terry D. Oswalt, an astronomer at Embry-Riddle Aeronautical University’s Daytona Beach campus, who was not involved in the study.

“They are lousy lenses because they’re not point sources,” Oswalt said. “They’re big and splotchy. They’ve got spiral arms and nuclei and sometimes companion galaxies, and sometimes there’s clusters of galaxies.”

But stars are point sources, not large and lumpy like galaxies. If you could catch a lensing event between two stars, it could offer a much more focused effect. You might even be able to capture an Einstein ring — a phenomenon in which a lensing object eclipses a background light source so perfectly that the background object is rendered as a luminous circle. (This has been documented for galaxies, but not for individual stars.)

For this paper, lead author Kailash Sahu of the Space Telescope Science Institute in Baltimore and his colleagues set out to find a lensing event between two stars. This was a much more difficult feat, partly because the effect for single stars is so tiny compared with the size of galaxies. To make matters worse, astronomers are far less likely to catch two stars overlapping than to find two galaxies doing so.

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