Astronomers measure the mass of a star—thanks to an old tip from Einstein

Weighing a star is hard. In fact, binary stars are the only ones scientists can directly gauge, because their orbits around each other reveal their masses. Now, a team of astronomers has succeeded in measuring the mass of an isolated star using a technique first suggested by Albert Einstein in 1936. The method exploits the fact that a large mass, like a star, can bend the path of light. Although the effect is tiny, measuring the deflection can reveal the mass of the light-bending star.  

“This is a really elegant piece of work they’ve done,” says astronomer Martin Barstow of the University of Leicester in the United Kingdom, “and a nice echo of a century of general relativity.”

Astronomers have seen many examples of gravity-bending light, including galaxies distorting images of even more distant ones, sometimes stretching them out into circular “Einstein rings.” In our own galaxy, when one star passes in front of another, astronomers see a brief brightening of the more distant one as the nearer star acts as a lens, bending more of its passing rays toward Earth. This effect, known as gravitational microlensing, has been used to detect exoplanets and search for dark matter, black holes, and brown dwarfs.

But Einstein also predicted that if the source of the light and light-bending star are not in exact alignment, the bending will cause the source star to appear to move when viewed from Earth. The size of that shift tells scientists the light-bending star’s mass. The effect is so tiny and the likelihood of such a near-alignment so rare, Einstein thought it could never be done.

But a team of astronomers from the United States, the United Kingdom, and Canada had a hunch that the keen-eyed view of the Hubble Space Telescope might be able to detect such a shift. They started by looking for stars that might be coming into alignment, and found that Stein 2051 B—a white dwarf just 18 light-years from Earth—was due to pass almost directly in front of another star in March 2014. When it did, the team captured the slightest shifts in position of the background star. That shift let the team calculate that Stein 2051 B’s mass is about two-thirds the mass of the sun, 0.675 solar masses, they report today in Science.

But this was more than just a skillful demonstration of a new technique. Stein 2051 B is something of an enigma for specialists in white dwarfs, the husks left behind when stars have burned up all their fuel. (This fate awaits 97% of stars, including the sun.) From observations of its size, temperature, and the light it emits, researchers had estimated that Stein 2051 B was a particular variety of white dwarf that should weigh about 0.67 solar masses. But, using the method to measure mass through binary stars, other researchers had paired it with another nearby star, Stein 2051 A, and had calculated a weight of just 0.5 solar masses. The latest calculation puts Stein 2051 B’s mass exactly where it should be, and it also casts doubt on the idea that A and B are actually a binary pair. “It’s a nice addition to our understanding of white dwarf composition,” Barstow says.

Astronomer Markus Hundertmark of the University of Heidelberg in Germany says that simply the detection of such a shift would have deserved publication in its own right. “Measuring the mass of a nearby white dwarf seems to make the result even better, and at first glance it is a surprising discovery.”

The technique’s application seems limited at the moment, because the near-alignment of stars is so rare, but that will change next year when the second star catalog from the European Space Agency’s Gaia satellite is released, giving exact positions and motions of many thousands of stars. “It’s likely we’ll find many more examples to pursue,” Barstow says. The new result “seems to be a most curious effect,” says team member Martin Dominik of the University of St. Andrews in the United Kingdom, but it will “turn into a quite useful astrophysical technique sooner rather than later.”