Hubble Directly Measures Mass of Lone White Dwarf for the First Time

Astronomers using the NASA/ESA Hubble Space Telescope have for the first time directly measured the mass of a single, isolated white dwarf star – the surviving core of a burned-out sunlike star.

For the first time in history, scientists have been able to directly measure the mass of a solitary white dwarf star using the Hubble Space Telescope. This achievement has opened new doors in our understanding of the structure and composition of white dwarfs and how they evolve over time.

The end product of a typical star's evolution is a white dwarf star, which is a collapsed star that burned out over a billion years ago. Until now, mass measurements of white dwarfs have been conducted by observing them in binary star systems. However, these measurements have been uncertain if the dwarf's companion star is in a long-period orbit.

In order to measure the mass of a companionless white dwarf, researchers had to employ a technique known as gravitational microlensing. The light from a background star was slightly deflected by the gravitational warping of space by the foreground dwarf star. This is the first time scientists have been able to directly measure the mass of a solitary white dwarf.

The Study

Peter McGill, a researcher from the University of California, Santa Cruz, used the Hubble Space Telescope to precisely measure how light from a distant star bent around the white dwarf, known as LAWD 37. This caused the background star to temporarily change its apparent position in the sky. The results were reported in the journal Monthly Notices of the Royal Astronomical Society.

Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, USA, the principal Hubble investigator on this latest observation, first used microlensing in 2017 to measure the mass of another white dwarf, Stein 2051 B. However, that dwarf was in a widely separated binary system. "Our latest observation provides a new benchmark because LAWD 37 is all by itself," Sahu said.

The researchers zeroed in on the white dwarf thanks to the European Space Agency's Gaia mission, which makes precise measurements of nearly two billion star positions. Based on these data, astronomers were able to predict that LAWD 37 would briefly pass in front of a background star in November 2019.

Hubble was then 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. The researchers found that the white dwarf was 56 percent of the mass of our Sun, which agrees with earlier theoretical predictions of its mass and corroborates current theories of how white dwarfs evolve.

Challenges

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 Hubble could make these kinds of high-contrast observations in visible light.

"Even when you’ve identified such a one-in-a-million event, it’s still extremely difficult to make these measurements,” said Leigh Smith of the University of Cambridge. "The glare from the white dwarf can cause streaks in unpredictable directions, meaning we had to analyze each of Hubble’s observations extremely carefully, and their limitations, to model the event and estimate the mass of LAWD 37."

Significance of Results

The precision of LAWD 37's mass measurement allows researchers to test the mass-radius relationship for white dwarfs. This means testing the theory of degenerate matter (a gas so super-compressed under gravity that it behaves more like solid matter) under extreme conditions inside the dead star.

The results also open the door for future event predictions with Gaia data. In addition to Hubble, these alignments can now be detected with the NASA/ESA/CSA James Webb Space Telescope, which is set to launch in 2021. The James Webb Space Telescope will have even greater capabilities for observing distant stars and galaxies, and its observations will complement those made by Hubble and Gaia to create a more complete picture of our universe. By combining data from multiple telescopes, astronomers will be able to make even more accurate predictions about future events in the cosmos, including the formation and evolution of stars, the movement of galaxies, and the detection of exoplanets. The results of this research will not only deepen our understanding of the universe but also have practical applications, such as helping to predict potentially hazardous asteroids and other objects that could pose a threat to our planet.