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Some Scientists Have Weighed A Star Using Gravity, Light, And Einstein's Equations

It's a powerful confirmation not only of Einstein's theories, but also of our understanding of how stars form, evolve, and die.

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Almost 100 years after British scientist Sir Arthur Eddington confirmed Albert Einstein's theory of relativity during the 1919 solar eclipse, those same equations – and some far, far more accurate equipment – have been used not just to see that light bends around a star, but also to use the precise degree to which it bends to measure the mass of the star that bends it.

Einstein's theory predicts that the sun’s gravity should bend starlight that passes near it, more than previously expected. The 1919 eclipse meant that Eddington could see the stars emitting that light and see how much they appeared to move. His measurements backed Einstein and ushered in a new era for physics.

In a new study, published today in the journal Science, astronomers used the Hubble space telescope to look at Stein 2051B, a white dwarf star about 18 light years away in the constellation Camelopardalis, after rejecting 5,000 other candidate stars. Over two years, the telescope watched it pass in front of a background star eight times, and saw how it moved.

"I thought it was a really nice result," Eamonn Kerins, an astronomer at the University of Manchester who specialises in gravitational lensing and was not involved in the study, told BuzzFeed News. "It's another nice confirmation of general relativity."

Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore and one of the authors of the study, told BuzzFeed News that "it opens a new way to measure the mass of stars, so potentially it is a big deal".

What's really interesting, he said, is not so much the confirmation of Einstein's theories – "relativity has passed all the tests, hundreds of times; it's just physics now", he said – but what it says about white dwarfs, and therefore about how stars evolve.

White dwarfs are the last stage of medium-sized stars like our sun. The classic theory of stellar evolution says that those stars burn hydrogen into helium through nuclear fusion, and then form cores of carbon and oxygen. Eventually, when all the fuel has been used, it expands into a red giant, then collapses to a white dwarf. Scientists think they've known since 1930 what white dwarfs are made of, and therefore how heavy a white dwarf of a given size should be.

The trouble is, it's been really hard to measure the mass of white dwarfs and confirm that model. "There have only been a few it's been done for," said Sahu, "and they're all in binary systems." Binary systems – pairs of stars orbiting each other – are unusual, so they may not be representative. (Stein 2051 is also a binary system, but the two stars are so far apart – about 55 times as far as the Earth from the sun – that they probably didn't affect each others' formation.)

Attempts have been made to measure the mass of 2051B, by looking at its orbit, but they came up with a really weird result – about half the mass of the sun. That doesn't fit at all with our understanding, Matt Burleigh, an astronomer at the University of Leicester, told BuzzFeed News. "It would mean it would have to have a core of iron to make it work, which was counter to all theories," he said. "It would have blown apart stellar evolution theory. It would have been horrible."

Instead, the gravitational lensing technique returned a mass of about 0.67 solar masses. "It's nicely on the model prediction," said Burleigh. "That’s great because you need to be satisfied that your models are working. Then you can apply it to stars where you can't do the measurement, because you've tested the model and it appears to be right." Kerins agrees: "The measurement supports our understanding of white dwarf physics, the relation between the mass and the radius. It's a nice confirmation."

The technical achievement of this study is as important as the scientific breakthrough, Alan Heavens, a professor of astrophysics specialising in gravitational lensing at Imperial College London, told BuzzFeed News. "It's very significant," he said. "It's the first time it's been done [for a lensing star] outside the solar system. It's very, very hard to measure it so accurately."

The New York Times headline after Eddington's experiment in 1919.
New York Times

The New York Times headline after Eddington's experiment in 1919.

Eddington had to detect a movement of a few thousands of a degree – too small to detect by eye, but manageable with the photographic techniques of the time. But because Stein 2051B is so much further away than our sun, the apparent movement of the star was several orders of magnitude smaller. According to Sahu, it was the equivalent of looking at a pound coin 1,000 miles away, and detecting something moving from one side of the coin to the other. It has to be done with a space telescope, because the twinkling of stars caused by the atmosphere would create far larger movements than the real movement they were trying to detect.

"I love the technique," said Burleigh. "You can only do it with Hubble. Even though it's 27 years old, it's still doing new science."

What will be interesting, according to Heavens, is what happens next. "I think many more of these things will come," he said. The European Space Agency's space telescope Gaia is measuring the position of huge numbers of stars to enormous accuracy, he said, and they're predicting that they will see perhaps thousands more, allowing for the confirmation of the masses of different kinds of stellar objects. Sahu said that his current project involves measuring the mass of Proxima Centauri, our nearest star. "This is a new technique, which people can pick up and use," he said.

Tom Chivers is a science writer for BuzzFeed and is based in London.

Contact Tom Chivers at

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