Free Astronomy Magazine May-June 2023

28 MAY-JUNE 2023 ASTRO PUBLISHING T his graphic shows how microlensing was used to measure the mass of a white dwarf star. 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 mo- tion across the sky as seen from Earth. Though the dwarf is following a straight trajectory, the motion of Earth as it or- bits the Sun imparts an apparent sinusoidal off- set due to parallax. (The star is only 15 light- years away, and there- fore 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. [NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI)] contrast observations in visible light. “Even when you’ve identified such a one-in-a-million event, it’s still ex- tremely difficult to make these meas- urements,” said Leigh Smith of the University of Cambridge. “The glare from the white dwarf can cause streaks in unpredictable directions, meaning we had to analyse each of Hubble’s observations extremely carefully, and their limitations, to model the event and estimate the mass of LAWD 37.” “The precision of LAWD 37’s mass measurement allows us to test the mass-radius relationship for white dwarfs,” said McGill. “This means testing the theory of degenerate matter (a gas so super-compressed under gravity that it behaves more like solid matter) under the extreme conditions inside this dead star,” he added. The researchers say their re- sults open the door for future event In his 1915 general theory of relativ- ity, Einstein predicted that when a massive compact object passes in front of a background star, the light from the star would bend around the foreground object because of the warping of space by its gravita- tional field. Exactly a century before this latest Hubble observation, in 1919, two British-organised expeditions to the southern hemisphere first detected this lensing effect during a solar eclipse on 19 May. It was hailed as the first experimental proof of gen- eral relativity — that gravity warps space. However, Einstein was pes- simistic that the effect could ever be detected for stars outside our Solar System because of the precision re- quired. “Our measurement is 625 times smaller than the effect meas- ured at the 1919 solar eclipse,” said McGill. predictions with Gaia data. In addi- tion to Hubble, these alignments can now be detected with the NASA/ESA/CSA James Webb Space Telescope. Because Webb works at infrared wavelengths, the blue glow of a foreground white dwarf looks dimmer in infrared light, and the background star looks brighter. Based on Gaia’s predictive powers, Sahu is observing another white dwarf, LAWD 66, with Webb. The first observation was made in 2022. More observations will be taken as the deflection peaks in 2024 and then subsides. “Gaia has really changed the game — it’s exciting to be able to use Gaia data to predict when events will happen, and then observe them happening,” said McGill. “We want to continue meas- uring the gravitational microlensing effect and obtain mass measure- ments for many more types of stars.” !