Distant star shines light on Einstein theory

For the first time ever, scientists have measured the mass of a white dwarf by using the Hubble Telescope to observe how it deflects light from a background star.

The white dwarf, Stein 2051B, was observed passing in front of a star, and its gravity deflected the star’s light by two milliarcseconds (a unit of angle). Using this measurement, the astronomers calculated that the white dwarf’s mass is roughly 68% of our sun’s mass – matching theoretical predictions.

Jay Anderson, part of the Space Telescope Science Institute (STScI) and co-author of the study in Science, said:  "Stein 2051B appears 400 times brighter than the distant background star. So measuring the extremely small deflection is like trying to see a firefly move next to a light bulb. The movement of the insect is very small, and the glow of the light bulb makes it difficult to see the insect moving.”

The deviation observed by the scientist is equivalent to observing an ant crawl across the surface of a coin from 1,500 miles away. The astronomers were investigating gravitational microlensing – a phenomenon in which gravity affects light from a background star – first predicted by Einstein in 1915.

Kailash Sahu, also from STScI and co-author, said: “This microlensing method is a very independent and direct way to determine the mass of a star. It's like placing the star on a scale: the deflection is analogous to the movement of the needle on the scale."

Stein 2051B is 17 light years away from Earth and is believed to be almost 3 billion years old. Its background star is approximately 5,000 light years away. The researchers’ next steps will see Hubble used for a similar study on Proxima Centauri – the closest neighbour to our Solar System.