by Astronomers have used NASA’s Hubble Space Telescope for the first time to directly measure the mass of a single, isolated white dwarf — the surviving core of a burned-out Sun-like star.
Researchers found that the white dwarf accounts for 56 percent of the mass of our Sun. This is consistent with previous theoretical predictions about the mass of the white dwarf and confirms current theories about how white dwarfs evolve as the end product of the evolution of a typical star. The unique observation provides insight into theories on the structure and composition of white dwarfs.
So far, previous mass measurements of white dwarfs have been obtained from observations of white dwarfs in binary systems. By observing the motion of two orbiting stars, simple Newtonian physics can be used to measure their masses. However, these measurements can be uncertain when the white dwarf’s companion star is in a long-term orbit of hundreds or thousands of years. Orbital motion can only be measured with telescopes over a short portion of the dwarf’s orbital motion.
For this companionless white dwarf, the researchers had to use a trick of nature, the so-called gravitational microlensing effect. Light from a background star was slightly deflected by the foreground dwarf star’s gravitational distortion of space. As the white dwarf passed in front of the background star, microlensing caused the star to appear temporarily offset from its actual position in the sky.
The results are reported in the Monthly Bulletins of the Royal Astronomical Society. The lead author is Peter McGill, formerly of the University of Cambridge (now based at the University of California, Santa Cruz).
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 momentarily change its apparent position in the sky.
Hubble’s principal investigator on this latest observation, Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, used microlensing for the first time in 2017 to measure the mass of another white dwarf, Stein 2051 B. However, this dwarf is in a widely separated binary system. “Our latest observation provides a new benchmark because LAWD 37 stands on its own,” Sahu said.
LAWD 37, the collapsed remains of a star that burned out 1 billion years ago, has been studied extensively as it lies just 15 light-years away in the constellation Musca. “Because this white dwarf is relatively close to us, we have a lot of data about it — we have information about its light spectrum, but the missing piece of the puzzle was a measurement of its mass,” he told McGill.
Thanks to ESA’s Gaia space observatory, which takes extraordinarily precise measurements of almost 2 billion star positions, the team was able to circle the white dwarf. Several Gaia observations can be used to track a star’s motion. Based on this data, astronomers were able to predict that LAWD 37 would pass just in front of a background star in November 2019.
Once this was known, Hubble was used to accurately measure over several years how the background star’s apparent position in the sky was momentarily deflected during the white dwarf’s flyby.
“These events are rare and the impact is tiny,” McGill said. “For example, the magnitude of our measured displacement is like measuring the length of a car on the moon as seen from Earth.”
Because the background star’s light was so faint, astronomers’ biggest challenge was extracting its image from the white dwarf’s glare, which is 400 times brighter than the background star. Only Hubble can make such high-contrast visible-light observations.
“The precision of the LAWD 37 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 that becomes so supercompressed under gravity that it behaves more like solid matter) under the extreme conditions inside this dead star,” he added.
The researchers say their findings open the door to future event predictions using Gaia data. In addition to Hubble, these alignments can now be detected with NASA’s James Webb Space Telescope. Because Webb works with 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 power, Sahu observes another white dwarf, LAWD 66, with NASA’s James Webb Space Telescope. The first observation was made in 2022. Further observations will be made as the diversion 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 watch them happen,” said McGill. “We want to continue measuring the gravitational microlensing effect and obtain mass measurements for many more types of stars.”
In his 1915 general theory of relativity, Einstein predicted that when a massive, compact object passed in front of a background star, the star’s light would be bent around the foreground object due to the curvature of space caused by its gravitational field.
Exactly a century before this last Hubble observation, in 1919, two expeditions to the southern hemisphere organized by Britain first detected this lens flare during a solar eclipse on May 19. It has been hailed as the first experimental proof of general relativity – that gravity distorts space. However, Einstein was pessimistic that the effect could ever be demonstrated for stars outside our solar system due to the precision involved. “Our measurement is 625 times smaller than the effect measured from the 1919 eclipse,” McGill said.
The Hubble Space Telescope is an international collaborative project between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, DC
