 Here's a Hubble image of a white dwarf star named Laude 37, 15 light years away. Until now, white dwarf mass measurements have been determined by observing them in binary star systems. The motion of two co-orbiting stars enables us to use straightforward Newtonian physics to measure their masses. This inset box plots how the dwarf passed in front of a background star in 2019. The wavy blue line traces the dwarf's apparent motion across the sky, as seen from Earth. Though the dwarf is following a straight trajectory, the motion of Earth orbiting the sun imparts an apparent sinusoidal offset. As the white dwarf passed in front of the background star, the star appears temporarily offset from its actual position on the sky. Given the amount of deflection, Einstein's equations give us the mass of the star. For this white dwarf, it's 56% of our sun's mass.