NASA / ESA’s Hubble Space Telescope has for the first time directly measured the mass of a single, isolated white dwarf, called LAWD 37. This white dwarf is around 56% the mass of our Sun and provides insights into theories of their structure as well as composition. For this discovery, researchers had to use gravitational microlensing.
What is gravitational microlensing? Light from a background star was basically deflected by the gravitational warping of space by the foreground dwarf star, and as the white dwarf passed in front of the background star, microlensing caused the star to appear temporarily offset from its actual position on the sky. LAWD 37’s collapsed remains burned out 1 billion years ago, but it has been extensively studied due to it being just 15 light-years away in the Musca constellation.
"These events are rare, and the effects are tiny. For instance, the size of our measured offset is like measuring the length of a car on the Moon as seen from Earth. The precision of LAWD 37’s mass measurement allows us to test the mass-radius relationship for white dwarfs. This means testing the theory of degenerate matter (a gas so super-compressed under gravity it behaves more like solid matter) under the extreme conditions inside this dead star,” said Peter McGill, lead author from the University of California, Santa Cruz."