Free Astronomy Magazine May-June 2023

27 MAY-JUNE 2023 ASTRO PUBLISHING McGill used Hubble to precisely measure how light from a distant star bent around the white dwarf, known as LAWD 37, causing the background star to temporarily change its apparent position in the sky. Kailash Sahu of the Space Tele- scope Science Institute in Baltimore, Maryland, USA, the principal Hub- ble investigator on this latest obser- vation, first used microlensing in 2017 to measure the mass of an- other white dwarf, Stein 2051 B. But that dwarf is in a widely sepa- rated binary system. “Our latest ob- servation provides a new benchmark because LAWD 37 is all by itself,” Sahu said. The collapsed remains of a star that burned out 1 billion years ago, LAWD 37 has been extensively studied because it is only 15 light-years away in the constellation Musca. “Because this white dwarf is relatively close to us, we’ve got lots of data on it — we’ve got information about its spectrum of light, but the missing piece of the puzzle has been a measurement of its mass,” said McGill. The team zeroed-in on the white dwarf thanks to ESA’s Gaia mission, which makes extraordinarily precise measurements of nearly two billion star positions. Multiple Gaia obser- vations can be used to track a star’s motion. Based on these data, as- tronomers were able to predict that LAWD 37 would briefly pass in front of a background star in November 2019. Once this was known, Hubble was used to precisely measure over several years how the background star’s apparent position in the sky was temporarily deflected during the white dwarf’s passage. “These events are rare, and the ef- fects are tiny,” said McGill. “For in- stance, the size of our measured offset is like measuring the length of a car on the Moon as seen from Earth.” Since the light from the background star was so faint, the main challenge for astronomers was extracting its image from the glare of the white dwarf, which is 400 times brighter than the background star. Only Hub- ble can make these kinds of high- T his animation shows the motion of a white dwarf star passing in front of a distant background star. During the passage, the faraway star appears to change its position slightly, because the light from it has been deflected by the white dwarf’s grav- ity. Employing this trick of nature, astronomers using the NASA/ESA Hubble Space Telescope have for the first time directly meas- ured the mass of a single, isolated star other than our Sun. This effect, called gravitational lensing, was predicted as a consequence of Einstein’s general theory of relativity from a century ago. Observations of a solar eclipse in 1919 provided the first di- rect evidence for general relativity. But Einstein didn’t think the same experiment could be done for stars beyond our Sun because of the precision required. [NASA, ESA, G. Bacon (STScI)] ing the motion of two co-orbiting stars, straightforward Newtonian physics can be used to measure their masses. However, these measure- ments can be uncertain if the dwarf’s companion star is in a long- period orbit of hundreds or thou- sands of years. Orbital motion can be measured by telescopes only over a brief slice of the dwarf’s or- bital motion. For this companion- less white dwarf, researchers had to employ a trick of nature, called gravitational microlensing. The light from a background star was slightly deflected by the gravitational warp- ing of space by the foreground dwarf star. 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. The results are reported in the jour- nal Monthly Notices of the Royal As- tronomical Society. The lead author is Peter McGill, formerly of the Uni- versity of Cambridge in the United Kingdom and now based at the Uni- versity of California, Santa Cruz.