Einstein helps astronomers to weigh a star

08 June 2017
Posted by Chris Smith.

Einstein's theory of General Relativity, conceived a century ago, has enabled astronomers to weigh a nearby star, vindicate a Nobel laureate and settle a long-standing mystery.

The discovery hinges on the technique known as gravitational lensing, which is a central tenet of General Relativity and led to the observation that made Einstein an overnight celebrity.

General relativity holds that massive objects, like stars, deform or "warp" the fabric of space, dubbed "spacetime" around themselves. This causes the path of light rays passing through that patch of space to appear to bend.

In 1919, Astronomer Royal Frank Dyson conceived of an experiment to test Einstein's hypothesis, and Arthur Eddington was dispatched to the Gulf of Guinea to photograph a solar eclipse.

The idea was that, obscured by the Moon, the Sun's light would be blocked out temporarily, enabling observers to see a distant cluster of much dimmer, distant stars called the Hyades, which at this time would be sitting just behind the Sun.

If Einstein was right, the spacetime deformation caused by the Sun would make the positions of the Hyades appear to change slightly. Eddington's observations showed exactly that, proving Einstein was right. 

Now, almost 100 years after that breakthrough, US scientists have performed a similar feat from outer space to "weigh" a star called Stein 2051B.

Using the Hubble Space Telescope, astronomer Kailash Sahu and his colleagues from the Space Telescope Institute watched as, in 2014, Stein 2051B drifted in front of a more distant star, causing the remote star's apparent position to shift by a tiny amount, which they were able to measure.

The scale of the movement corresponds to the amount of spacetime deformation, which is in turn proportional to the mass of Stein 2051B.

Their measurements, published this week in the journal Science, show that Stein 2051B weighs about two-thirds as much as our own Sun.

This observation is enormously important, because Stein 2051B is an object known as a white dwarf. These are stellar cinders that form when stars like our own Sun, which account for two thirds of the stars in the Universe, burn themselves out. As such they are like cosmic fossils and are key to sorting out the evolution and histories of galaxies like our own.

Previous measurements of Stein 2051B had suggested that it had a mass that did not fit with the relationship discovered by 1930s Nobel prize-winning astrophysicist Subrahmanyan Chandrasekhar.

Alas for Stein 2051B, it's not that exotic after all. The new measurements show that it fits the trend perfectly!

Incredibly, Einstein himself published a paper in 1936 suggesting that an experiment like that carried out by Sahu and his colleagues, to weigh a distant star, should be theoretically possible, although he did caution in his paper in Science describing the idea that "There is no hope of observing this phenomenon directly."

Well, he was only human after all!

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