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Einstein’s method used to calculate mass of a star

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Albert Einstein’s prediction of gravity acting as a kind of magnifying lens, brightening and bending the distant starlight has been utilized to do something that hasn’t been done before – ever – measure the mass of stars.

An international team of scientists describe in their study published in Science a particular type of Einstein’s “gravitational microlensing” by a star other than the Sun. This has enabled scientists to determine the masses of objects that can otherwise be easily measure by other means.

The team determined the mass of a collapsed stellar remnant called a white dwarf star. Such objects have completed their hydrogen-burning life cycle, and thus are the fossils of all prior generations of stars in our galaxy, the Milky Way.

The gravitational microlensing of stars, predicted by Einstein, has previously been observed. Famously, in 1919, measurements of starlight curving around a total eclipse of the Sun provided one of the first convincing proofs of Einstein’s general theory of relativity — a guiding law of physics that describes gravity as a geometric function of both space and time, or spacetime.

Researchers observed a much more likely scenario: Two objects were slightly out of alignment, and therefore an asymmetrical version of an Einstein ring formed. The ring and its brightening were too small to be measured, but its asymmetry caused the distant star to appear off-center from its true position. This particular part of Einstein’s prediction is called ‘astrometric lensing’ and astronomers involved with the latest study were able to observe it in a star other than the Sun.

Scientists took advantage of the superior angular resolution of the Hubble Space Telescope (HST) and measured shifts in the apparent position of a distant star as its light was deflected around a nearby white dwarf star called Stein 2051 B on eight dates between October 2013 and October 2015. They determined that Stein 2051 B — the sixth-closest white dwarf star to the Sun — has a mass that is about two-thirds that of the Sun.

One of the authors of the study explains: “The basic idea is that the apparent deflection of the background star’s position is directly related to the mass and gravity of the white dwarf — and how close the two came to exactly lining up.”

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