Hubble, for the first time, directly measured the mass of a lone white dwarf

The unique observation yields insights into theories of the structure and composition of white dwarfs.

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According to research, the white dwarf has 56 percent of the mass of the Sun. This supports existing beliefs about how white dwarfs develop due to a typical star’s evolution and accords with past theoretical estimates about the white dwarf’s mass. The unique sighting provides new information about the composition and structure of white dwarfs.

Previous white dwarf mass measurements have been gleaned 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 surviving core of a burned-out Sun-like star.

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

Simple Newtonian physics allows astronomers to measure the mass of two co-orbiting stars by observing their motion. The companion star of the white dwarf, however, may be in orbit for a long period of hundreds or perhaps thousands of years, making these measurements imprecise. Telescopes may observe only a small portion of the dwarf’s orbital motion as orbital motion.

mass of a white dwarf sta
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 center of this Hubble Space Telescope image. Though its nuclear fusion furnace has shut down, trapped heat is sizzling on the surface at 180,000 degrees Fahrenheit, causing the stellar remnant to glow fiercely. 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 white dwarf without a companion. The foreground dwarf star’s gravitational warping of space caused a background star’s light to be somewhat deflected. Microlensing caused the background star to look 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 based at the University of California, Santa Cruz) used Hubble to precisely measure how light from a distant star bent 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 known white dwarf to the Sun.

McGill said, “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.”

ESA’s Gaia space observatory, which produces incredibly accurate measurements of over 2 billion star locations, helped the team focus on the white dwarf. The velocity of a star may be tracked using several Gaia observations. Based on this data, astronomers could foresee that LAWD 37 would fleetingly cross in front of a background star in November 2019.

After knowing this, they used Hubble 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.

COMPASS IMAGE FOR WHITE DWARF STAR LAWD 37
This graphic 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 tiny. For instance, the size of our measured offset is like measuring the length of a car on the Moon as seen from Earth.”

As the background star’s light was so faint, the main difficulty for astronomers was separating it from the white dwarf’s glare, 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 so super-compressed under gravity it behaves more like solid matter) under the extreme conditions inside 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 foreground white dwarf looks dimmer in infrared light, and the background star looks brighter.”

Journal 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
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