According to research, the white dwarf has 56 percent of the mass of the sun. This supports existing views on how white dwarfs develop as a result of the evolution of a typical star and is consistent with previous theoretical estimates on the mass of the white dwarf. The unique observation provides new information about the composition and structure of white dwarfs.

Previous white dwarf mass measurements have been derived from observing white dwarfs in binary star systems.

Astronomers using NASA’s Hubble Space Telescope have for the first time directly measured the mass of a single, isolated white dwarf — the remnant core of a burned-out Sun-like star.

They found that the white dwarf is 56 percent of the mass of our sun. This is consistent with previous theoretical predictions of the white dwarf’s mass and confirms current theories of how white dwarfs evolve as the end product of the evolution of a typical star.

Simple Newtonian physics allows astronomers to measure the masses of two stars orbiting side by side by observing their motion. However, the white dwarf’s companion star could be in orbit for a long period of hundreds or perhaps thousands of years, making these measurements inaccurate. Telescopes can observe only a small portion of the dwarf’s orbital motion as orbital motion.

mass of a white dwarf staff
This image shows how microlensing was used to measure the mass of a white dwarf star. The dwarf, called LAWD 37, is a burnt-out star at the center of this Hubble Space Telescope image. Although the nuclear fusion furnace is turned off, the trapped heat at the surface hisses at 180,000 degrees Fahrenheit, causing the stellar remnant to glow brightly. Credits SCIENCE: NASA, ESA, Peter McGill (UC Santa Cruz, IoA), Kailash Sahu (STScI) IMAGE PROCESSING: Joseph DePasquale (STScI)

The gravitational microlensing technique had to be used for this companionless white dwarf. The gravitational warping of space from the dwarf star in the foreground caused the light from a background star to be slightly deflected. Microlensing made the background star appear momentarily offset from its true position in the sky as the white dwarf moved in front of it.

The lead author is Peter McGill, formerly of the University of Cambridge (now located at the University of California, Santa Cruz) used Hubble to measure exactly how light from a distant star arcs around the white dwarf known as LAWD 37. Also known as LP 145-141, the LAWD 37 is an isolated white dwarf located 15 light-years from the Solar System. It is the fourth closest white dwarf to the sun.

McGill said, “Because this white dwarf is relatively close to us, we have a lot of data on it — we have information on its light spectrum, but the missing piece of the puzzle was a measurement of its mass.”

ESA’s Gaia space observatory, which produces incredibly accurate measurements of more than 2 billion star locations, helped the team focus on the white dwarf. The speed of a star can be tracked using various Gaia observations. Based on this data, astronomers could foresee LAWD 37 passing fleetingly in front of a background star in November 2019.

Knowing this, they used Hubble to accurately measure over several years how the apparent position of the background star in the sky was temporarily deflected during the passage of the white dwarf.

This image shows how microlensing was used to measure the mass of a white dwarf star. Credits SCIENCE: NASA, ESA, Peter McGill (UC Santa Cruz, IoA), Kailash Sahu (STScI) IMAGE PROCESSING: Joseph DePasquale (STScI)

McGill said, “These events are rare and the effects are small. For example, the size of our measured offset is comparable to measuring the length of a car on the moon as seen from Earth.”

Because the background star’s light was so dim, the main difficulty for astronomers was separating it from the glare of the white dwarf, which is 400 times brighter than the background star. Hubble can only make these high-contrast observations in visible light.

McGill said, “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 that is so super-compressed under gravity that it behaves more like solid matter) under the extreme conditions in this dead star.”

Scientists noted, “The results open the door for future event predictions with Gaia data. In addition to Hubble, these alignments can now be detected with NASA’s James Webb Space Telescope. Because Webb works at infrared wavelengths, the blue glow of a white dwarf in the foreground appears fainter in infrared light and the star in the background appears brighter.”

Magazine reference:

  1. Peter McGill, Jay Anderson, et al. First semi-empirical test of the white dwarf mass-radius relationship using a single white dwarf via astrometric microlensing. Monthly Notices of the Royal Astronomical Society, Volume 520. DOI: 10.1093/mnras/stac3532