Astronomers using NASA/ESA’s Hubble Space Telescope have, for the first time, directly measured the mass of a single isolated white dwarf star, the surviving core of a burnt-out Sun-like star. The researchers found that the white dwarf makes up 56 percent of the mass of our Sun. This is consistent with previous theoretical estimates of its mass and supports current theories of how white dwarfs evolved as the end product of the evolution of a typical star. The unique observation makes it possible to understand the structure and composition theories of white dwarfs.

So far, previous measurements of the mass of white dwarfs have been derived from observations of white dwarfs in binary star systems. By observing the motion of two stars in the same orbit, simple Newtonian physics can be used to measure their masses. But these measurements can be uncertain as to whether the dwarf companion star is in a long orbit of hundreds or thousands of years. Orbital motion can be measured with telescopes for only a short slice of the dwarf’s orbital motion.

Astronomers using NASA/ESA’s Hubble Space Telescope have been able to directly measure the mass of an isolated star other than our Sun for the first time, thanks to nature’s optical play. The target was a white dwarf, the surviving core of a burned-out sun-like star. The greater the temporal infinitesimal deviation in the image of the background star, the greater the mass of the foreground star. The researchers found that the dwarf is 56 percent of the mass of our Sun.

For this unique white dwarf, the researchers had to apply a nature trick called gravitational microlensing. Light from the background star is slightly deflected by the foreground dwarf star by the gravitational bending of space. As the white dwarf passed in front of the background star, microlenses made the star appear to be temporarily deviated from its original position in the sky.

The results are reported in the Monthly Notices of the Royal Astronomical Society. Lead author Peter McGill is formerly from the University of Cambridge in the UK and now the University of California at Santa Cruz.

McGill used Hubble to measure exactly how light from a distant star bends around a white dwarf known as LAWD 37, causing the background star to temporarily change its apparent position in the sky.

Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, USA, Hubble’s principal investigator for this latest observation, used microlensing for the first time in 2017 to measure the mass of another white dwarf, Stein 2051 B. But this dwarf is far away. binary system. “Our last observation provides a new milestone because it’s LAWD 37 itself,” Sahu said.

The collapsed remains of a star called LAWD 37 that burned 1 billion years ago have been carefully studied as it lies just 15 light-years away in the constellation Amulet. “Because this white dwarf is relatively close to us, we have a lot of data about it – we know about the light spectrum, but the missing piece of the puzzle is measuring its mass,” McGill said.

The team focused on the white dwarf thanks to ESA’s Gaia mission, which made extremely precise measurements of the position of nearly two billion stars. Several Gaia observations can be used to track the movement of the star. Based on this data, astronomers were able to predict that LAWD 37 will pass briefly in front of the background star in November 2019.

A dwarf named LAWD 37 is the burnt star at the center of this Hubble Space Telescope image. Even though the nuclear fusion furnace is turned off, the trapped heat sizzles at the surface at about 100,000 degrees Celsius, causing the stellar remnant to shine brightly. Once this was known, Hubble was used for several years to precisely measure how the apparent position of the background star in the sky was temporarily deviated during the pass of the white dwarf.