Astronomers weigh a white dwarf using gravitational lensing – Astronomy Magazine

Einsteins theory of general relatively changed the way scientists look at the universe. The presence of mass bends spacetime like a bowling ball depressing a mattress, causing light to curve as it travels through these depressions on its way to Earth. In 1919, Sir Arthur Eddington confirmed this effect by measuring the deflection of background stars caused by our Sun during a total solar eclipse. Nearly a century later, astronomers have used the Hubble Space Telescope (HST) to measure this effect caused by a star outside our solar system for the first time.

This groundbreaking result was announced today at the 230th Meeting of the American Astronomical Society by Kailash Sahu of the Space Telescope Science Institute. Sahus team used HST to capture the deflection of light from a background star as a white dwarf, the remnant core of a star once like our Sun, passed in front of it as seen from Earth. Although this deflection was tiny about 1,000 times smaller than the deflection measured by Eddington in 1919 the precision achievable with Hubble allowed astronomers to see it clearly. From the deflection, they were able to measure the mass of the white dwarf, called Stein 2051B, in a new way that independently confirms the theoretical mass-radius relationship for white dwarfs. This is good news, because the mass-radius relationship is the foundation for astronomers use of these objects as standard distance indicators in cosmology. The work will appear this month in the journal Science.

To find a suitable pair of stars to accomplish this task, Sahus team first combed through a catalog of 10,000 stars with large proper motions, or movements on the sky as seen from Earth. Based on the motions of these stars, the team projected the stars positions forward in time to find a pair that would pass close enough to each other (when projected on the sky, not in physical space) to produce a bend in starlight measurable with HST.

Their choice: Stein 2051B, a white dwarf 17 light-years from Earth. According to the teams calculations, Stein 2051B would pass in front of a distant background star, about 5,000 light-years away, causing the background starlight to bend by 2 milliarcseconds. In more understandable terms, seeing that bend would be like trying to watch an insect crawl across the face of a quarter from a distance of about 1,500 miles (2,400km).

The team enlisted Hubble to observe the stars over eight epochs, or points in time, with observations taken in the time leading up to, during, and after the event, which occurred in March 2014. And, indeed, they did observe a deflection of the background light as the white dwarf passed in front of the distant source.

This work represents two firsts in astronomy. One, its the first time a deflection due to general relativity has been measured using a star other than our Sun. And two, as Sahu explained during the press conference, measuring the mass of Stein 2051B is the first clean test for [the] mass-radius relationship.

The mass-radius relationship for white dwarfs leads to a limit called the Chandrasekhar limit. If a white dwarf accumulates mass past this limit (by stealing it off a binary companion), it will explode as a supernova, which can be seen from vast distances and can be used by astronomers to measure very large distances accurately. But if this relationship is different than we currently understand it, it would affect distance measurements based on white dwarf supernovae.