US astronomers have found a new application for Albert Einstein's century-old theory of relativity - using it to directly measure the size of a star beyond the sun.
Scientists have used the Hubble Space Telescope to plot minute changes in the path of light coming from a distant background star as it passed by a relatively close target star, known as Stein 2051B.
In research published on Wednesday, experts applied Einstein's findings to measure how Stein 2051B's gravity warped the background star's light, a phenomenon the physicist predicted more than 100 years ago and a direct means to assess its mass.
The technique could be applied to other stars.
"It was like measuring the motion of a little firefly in front of a light bulb from 1,500 miles away," astronomer Kailash Sahu of the Space Telescope Science Institute in Baltimore said.
The research has been presented at a meeting of the American Astronomical Society in Texas and published in the journal Science.
The measurements showed that the mass of Stein 2051B was about two-thirds of the sun in Earth's solar system.
The Stein 2051B is known as a white dwarf, which is what all stars smaller than eight times the size of the sun become when they run out of nuclear fuel and collapse, leaving a hot core.
"The single most important thing for the star is its mass," Sahu said.
"If we know the mass, we known what its radius will be, how bright it will be, how long it will live, what will happen after it dies.
"Everything depends on the mass of the star."
Scientists previously determined the mass of three other white dwarf stars using an indirect technique that required the star to have an orbiting partner for measuring gravitational pull.
The bending of light was a key test for Einstein's general theory of relativity, which was published more than 100 years ago, and proven in 1919 when scientists measured the curving of starlight around the sun during a total solar eclipse.
Einstein's work laid the foundation for modern physics and cosmology, providing a perspective of space and time that is shaped by gravity.
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