How to weigh stars with gravitational lensing

Astronomy & Astrophysics publishes the predictions of the passages of foreground stars in front of background stars. A team of astronomers, using ultra-precise measurements from the Gaia satellite, have accurately forecast two passages in the next months. Each event will produce shifts in the background star’s position due to the deflection of light by gravity, and will allow the measurement of the mass of the foreground star, which is extremely difficult to determine by other means.
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