Hubble Uses Microlensing To Measure the Mass of a White Dwarf (Annotated)

This graphic shows how microlensing was used to measure the mass of a white dwarf star.

The dwarf, called LAWD 37, is a burned-out star in the centre of this Hubble Space Telescope image. Though its nuclear fusion furnace has shut down, trapped heat is sizzling on the surface at roughly 100 000 degrees Celsius, causing the stellar remnant to glow fiercely.

The inset boxes at right plot 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 as it orbits the Sun imparts an apparent sinusoidal offset due to parallax. (The star is only 15 light-years away, and therefore is moving at a faster rate against the stellar background.)

As it passed by the fainter background star, the dwarf’s gravitational field warped space (as Einstein’s general theory of relativity predicted a century ago). And this deflection was precisely measured by Hubble’s extraordinary resolution. The dwarf’s offset position is coloured orange.

The amount of deflection yields a mass for the white dwarf of 56 percent our Sun’s mass, and this provides insights into theories of the structure and composition of white dwarfs. This is the first time that astronomers have directly measured the mass of a single, isolated star other than our Sun.

The white dwarf has a ‘spike’ because it is so bright that the light ‘bled’ into the Hubble camera’s CCD detector. This interfered with one of the observing dates for measuring that background star’s position on the sky.

The compass graphic points to the object’s orientation on the celestial sphere. North points to the north celestial pole which is not a fixed point in the sky, but it currently lies near the star Polaris, in the circumpolar constellation Ursa Minor. Celestial coordinates are analogous to a terrestrial map, though east and west are transposed because we are looking up rather than down.