Artist’s depiction of a white dwarf (right) in orbit around a white giant star. (Image: NASA, ESA and G. Bacon (STScI))

Albert Einstein’s theory of gravity, called general relativity, is probably the best physics theory ever formulated. It just keeps working, often for things Einstein himself didn’t believe, like the accelerating expansion of the universe. Scientists only just proved some of its crazy predictions, like gravitational waves.

But in the 1930s, Einstein’s friend R.W. Mandl visited and asked him to publish on a peculiar prediction of general relativity: that stars should deflect the light that passes by them like a lens would, something we today call gravitational lensing. That paper, published in Science in 1936, said “Of course, there is no hope of observing this phenomenon directly.”


But a team of physicists have now measured the mass of a white dwarf star using the way it lensed starlight behind it. Einstein’s predictions have once again held true, and his own doubts about his theories have once again been unfounded. And more importantly, scientists have directly measured a single white dwarf star’s mass without the help of another star or another model of some kind. They essentially put the star on an interstellar balance scale.

Image: NASA, ESA and A. Feild (StSci)

“The fact that we could actually measure the mass of a white dwarf is exciting,” the paper’s lead author, astronomer Kailash Sahu from the Space Telescope Science Institute, told Gizmodo. And “almost all stars go through this white dwarf stage.”



It’s important to note that we’ve long observed gravitational lensing. In 1919, an experiment led by Sir Arthur Eddington showed the Sun’s gravitational field pushing the light of some stars over and around it during a solar eclipse. More recently, deep-field images have uncovered closer galaxies lensing more distant galaxies, and we’ve even seen stars brighten other stars as they pass in front of them. But measuring the mass of a star by observing the degree to which it bends the light of another star has proven to be an incredibly difficult challenge. Astronomers have to wait for the right conditions, and even then the shift would be incredibly tiny.

“It’s like if you put a firefly crossing the surface of a US quarter next to a light bulb in New York,” said Sahu, “and try to measure the motion of the firefly across that quarter... from Kansas City.”

Sahu’s firefly is the white dwarf star Stein 2051B—a stellar object around 20 light-years from Earth—which his team observed on several occasions with the Hubble Space Telescope. The star has a partner, Stein 2051A, that it circles from a distance further than that between Earth and Pluto. The team watched as Stein 2015B passed in front of light sources behind it and other random stars, and then measured the apparent shift in the light. They used the data and the relatively simple lensing equation to determine the mass, which aligned nicely with other white dwarf predictions and measurements with other methods. The team published their research in the journal Science today, and presented their results today at the 230th meeting of the American Astronomical Society in Austin, Texas.

This isn’t the first time scientists have measured the mass of a white dwarf either, but other methods usually require some theoretical model and other measurements or a much closer binary system than this one, astronomer Pier-Emmanuel Tremblay from the University of Warwick in the United Kingdom told Gizmodo. “One could argue this is the most model-independent mass measurement for a single white dwarf,” meaning this technique doesn’t use assumptions from other fields or observations.

None of the scientists I spoke to doubted the importance of the discovery. But everyone asked me to be cautious about overstating the applicability of the method for weighing stars in the future. There’s a lot of empty space in this here galaxy, and it’s just not that common for a close star to pass a distant star. Despite that, science requires repeated measurements. “I would definitely like to see this method repeated on other objects if possible,” professor B. Scott Gaudi from the Ohio State University told Gizmodo. “I think there are a few other good candidates for which this can be done.”


Sahu already has plans for the future. His team has a catalogue of stars to keep track of, and might be able to measure the mass of our neighboring star, Proxima Centauri, using this method. But they can’t just go measuring everything. “You have to apply for a telescope time, see if the panel thinks it’s worth it,” he said. “It’s a long process.”