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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 fro
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