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Measuring the mass of stars isn’t an easy feat –  you can’t exactly pop them on a pair of scales. But thanks to one of Einstein’s key predictions of general relativity, astrophysicists have directly measured the mass of a white dwarf star for the first time.

Einstein’s prediction, called gravitational lensing, says that the curvature of space near a massive body, such as a star or black hole, causes light passing that body to bend instead of travel in a straight line as it normally would. But in a 1936 paper he claimed that it would be impossible to directly observe the phenomenon since telescopes wouldn’t be able to see this level of detail.

It seems he underestimated the power of later generations of telescopes, however, as this new observation was made from the .

at the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues measured the deflection of light by white dwarf Stein 2051 B to measure its mass. This star is 18 light years from Earth.

“The mass of the star essentially decides everything about the star,” says Sahu. Knowing a star’s mass can help astrophysicists work out how old a star is and what it will become after it dies.

Bending background light

Stein 2015 B, it turns out, is about 68 per cent the mass of the sun. That’s not too far from previous estimates of the star’s mass, but this most recent calculation is important because, unlike previous estimates, it doesn’t rely on assumptions about the star’s composition or its orbit around other stars. For example, early mass estimates for this white dwarf assumed it had an iron core, but this direct measurement suggests that is incorrect.

at the University of Leicester in the UK, who wasn’t involved in this study, is impressed with the results. “Measuring the mass of white dwarfs without using models is really challenging,” he says.

Sahu and his colleagues observed the white dwarf as it moved in front of another star over a two-year period. By measuring how the light from that background star changed course as the white dwarf moved in front of it, Sahu was able to calculate its mass, as the greater a star’s mass, the greater the deflection.

The team is now trying to measure the mass of Proxima Centauri, a red dwarf that’s just 4.25 light years from Earth.

Singular stars

Gravitational lensing is a handy way to independently measure the mass of a star, but finding stars that are passing in front of another star with the right alignment to take these measurements can be tricky, says Barstow. Sahu and his colleagues started with a survey of more than 5000 stars before they settled on Stein 2051 B.

Nonetheless, Barstow says that gravitational lensing is the only way to accurately measure the mass of stars that don’t come in pairs. In binary systems, where two stars orbit around a common centre, astrophysicists can estimate stellar masses by studying their orbits.

The mass measurements for Stein 2051 B may be off by up to 7.5 per cent due to a lack of perfect resolution, says Sahu, but it is still our best calculation of the white dwarf’s mass. Future space telescopes like NASA’s could help make more accurate measurements, he says.