How We Managed What Einstein Thought Was Impossible – And Used His Theory To Weigh A Star

Image Credit: X-ray: NASA/CXC/UA/J.Irwin et al; Optical: NASA/STScI

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Gravity of a white dwarf star warps space and bends the light of a distant star behind it.
NASA, ESA, and A. Feild (STScI

Guest Author Martin Dominik, University of St Andrews

If you are looking at a glass lens you will not see the lens itself, but rather notice objects behind it being displaced and distorted. Gravitation has a similar effect. Any massive body warps the space and time around it, according to Albert Einstein’s theory of general relativity. As a result, light rays should take an apparent turn around the object rather than travelling in a straight line.

In 1919, during a solar eclipse, Arthur Eddington was able to show that this is true. He measured the apparent displacement of stars caused by the bending of their light around the sun, which acted as a “gravitational lens”. It was this result that made Einstein rise to instant fame. Now, a century later, we have for the first time measured this effect on a star other than our sun – something Einstein himself thought was impossible. Our results have been published in Science.

Using the Hubble Space Telescope repeatedly over the course of two years (from October 2013 to October 2015), we monitored the changing position of a background star at the moment it moved to be closely aligned with the nearby white dwarf star Stein 2051 B. White dwarfs are of peculiar interest because, along with neutron stars and black holes, they are faint or invisible remnants of stars at the end of their lives.

Curved space-time.

Perfect scale

The “gravitational lens” works like a weighing scale, with the light deflection of the background star being analogous to the movement of the needle on the scale. That’s because gravitational strength depends on mass – the bigger the mass, the bigger the effect of gravitational lensing. Consequently, after spending a further year and a half on careful analysis of the acquired data, we were able to directly obtain the mass of Stein 2051 B from the measured deflection of the background star. Stein 2051 B turned out to be 68% the mass of the sun.

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