For the first time, astronomers have directly measured the mass of a single, isolated white dwarf using NASA's Hubble Space Telescope. A white dwarf is the core of a Sun-like star that has burned out and died.

According to research, the white dwarf has 56 per cent 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 unusual sighting provides new information about the composition and structure of white dwarfs.

The white dwarf mass has been measured from binary star systems until now. Newtonian physics can measure the masses of two co-orbiting stars by measuring their velocity. 

If the white dwarf's companion star orbits for hundreds or thousands of years, these measurements may be inaccurate. Telescopes can measure dwarf orbital motion only briefly.

This companion-less white dwarf required gravitational microlensing. The dwarf star's gravitational warping diverted background starlight. Microlensing temporarily offset the background star when the white dwarf passed in front of it.

Royal Astronomical Society Monthly Notices publishes the findings. Peter McGill, previously of Cambridge, is the principal author (now based at the University of California, Santa Cruz).

McGill utilised Hubble to carefully analyse how light from a distant star curved around the white dwarf LAWD 37, temporarily causing the background star to change its sky location.

In 2017, Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, measured the mass of Stein 2051 B using microlensing. That dwarf is in a distant binary system. 'LAWD 37 is all by itself,' Sahu stated.

Because it is only 15 light-years away in Musca, LAWD 37, the collapsing remains of a 1 billion-year-old star, has been widely examined. 

'Because this white dwarf is pretty close to us, we've got loads of data on it, including its spectrum of light, but the missing piece of the puzzle has been a determination of its mass,' McGill said.

Gaia's precise measurements of roughly 2 billion star locations helped astronomers find the white dwarf. Multiple Gaia observations can track a star. Based on this data, astronomers predicted that LAWD 37 would temporarily cross a background star in November 2019.

Once this was discovered, Hubble was used to carefully quantify over several years how the background star's apparent sky location was temporarily deflected during the white dwarf's passage.

McGill said these events are rare and have minor effects. For example, measuring our offset is like measuring a car on the Moon from Earth.

Astronomers had to separate the weak background star's picture from the 400-times-brighter white dwarf. Only Hubble can do visible-light high-contrast observations.

McGill said LAWD 37's precise mass measurement lets us examine the mass-radius relationship for white dwarfs. 'This involves testing the hypothesis of degenerate matter (a gas so super-compressed under gravity that behaves more like solid stuff) under extreme conditions within this dead star,' he said.

Researchers claim Gaia data can anticipate future events. Hubble and NASA's James Webb Space Telescope can now detect these alignments. Webb's infrared light dims a foreground white dwarf's blue glow and brightens the background star.

NASA's James Webb Space Telescope is observing LAWD 66, another white dwarf, based on Gaia's predictions. Observations began in 2022. The deflection will peak in 2024 and subside, requiring more observations.

'Gaia has altered the game — it's amazing to be able to utilise Gaia data to predict when events will happen and then monitor them,' McGill said. 'We aim to measure the gravitational microlensing effect and get mass data for many more star types.'

Einstein predicted in his 1915 theory of general relativity that the light from a background star would bend around a big compact object passing in front of it due to its gravitational field distorting space.

On May 19, 1919, two British-led missions to the southern hemisphere discovered these lensing phenomena during a solar eclipse. It proved that gravity warps space. Due to precision, Einstein doubted the effect could be noticed for stars outside our solar system. McGill stated, 'Our measurement is 625 times smaller than the 1919 solar eclipse.'

The Hubble Space Telescope is a NASA-ESA collaboration. NASA's Goddard Space Flight Center in Greenbelt manages the telescope. STScI in Baltimore manages Hubble science. The Washington-based Association of Universities for Research in Astronomy runs STScI for NASA.