Scientists have done what Einstein said was impossible — used relativity to measure a star’s mass

Astronomers scanning the skies with NASA’s Hubble Space Telescope have discovered the bending of one star’s light by another star’s gravity — and have even used that distortion to measure a star’s mass.

The findings, published in the journal Science, confirm a key tenet of Albert Einstein’s theories even as they offer a new tool with which to explore a fundamental property of stars.

Einstein’s general theory of relativity, presented in 1915, describes how gravity can distort the path of light, altering its trajectory. In 1919, the theorist was proved right when, during a solar eclipse, an expedition 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 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, without this effect, would be too faint to study.

But lensing events by large structures such as galaxies 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.”

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