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Einstein's 'impossible hope' comes true: Weighing a star with gravity

June 07, 2017

A little more than 100 years after Einstein developed his theory of general relativity, researchers have used its laws to observe something the iconic scientist claimed, in a 1936 Science paper, "there [was] no hope of observing ... directly." These scientists' observation -- of the bending of distant starlight by gravity -- has allowed them to determine the mass of a white dwarf star, only possible in theory until now. The result demonstrates a way for determining the masses of objects that scientists can't easily measure by other means. One of the key predictions of general relativity set forth by Einstein was that the curvature of space near a massive body, such as a star, causes a ray of light passing near it to be deflected by twice the amount that would be expected based on classical laws of gravity. When a star in the foreground passes exactly between us and a background star, Einstein predicted, a phenomenon called gravitational microlensing results in a perfectly circular ring of light -- a so-called "Einstein ring." The first evidence of the bending of light came in the form of an eclipse in 1919, providing one of the first convincing proofs of Einstein's general theory of relativity. Yet, despite 100 years of technological advances, observing a slightly different scenario - two stars just out of alignment, resulting in an asymmetrical Einstein ring -- has not been achieved for stars outside our solar system. Such asymmetry is notable because it would cause the background star to appear off-center in a way that could be used to directly determine the mass of the foreground star, Einstein had said. Here, Kailash Chandra Sahu and colleagues took advantage of the superior angular resolution of the Hubble Space Telescope and proactively searched more than 5,000 stars for such an asymmetric alignment. They realized that the white dwarf Stein 2051 B was set to be in such a position in March of 2014. The researchers directed the Hubble Space Telescope to observe the phenomenon, measuring tiny shifts in the apparent position of a background star behind it. Based on the data, the authors estimate the white dwarf star's mass to be roughly 68% of that of our sun. The measurement of Stein 2051 B's mass holds important implications for understanding the evolution of white dwarf stars; the majority of the stars that have ever formed in the galaxy, including the sun, will become or already are white dwarfs. This advancement is highlighted in a Perspective and related video by T. D. Oswalt.

American Association for the Advancement of Science

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