Astronomers have used, for the first time, a novel method to determine the mass of a nearby dead star. The star is a “white dwarf,” the shrunken corpse of a star like our sun after it has burned up its nuclear fuel. The new method is based on the bending of a beam of light near a massive object — the same phenomenon that was seen during the total eclipse of the Sun that was used to test Einstein’s general theory of relativity a century ago. Now, astronomers have achieved a solid estimate of the mass of a white dwarf by measuring the deflection of light rays as they pass near the star.
Using the sharp vision of NASA’s Hubble Space Telescope, the research team was able to see how much the white dwarf is bending the light from a background star — a measurement astronomers need in order to gauge the white dwarf’s mass. The team’s result will appear in the journal Science on June 9, 2017.
“This measurement is a triumph for the Hubble Space Telescope, a wonderful confirmation of theoretical predictions, and a beautiful reprise of the Einstein solar eclipse observations of a century ago,” said team member Howard Bond, Professor of Practice in the Department of Astronomy and Astrophysics at Penn State, and Astronomer Emeritus at NASA’s Space Telescope Science Institute, the science operations center for the Hubble Space Telescope.
This observation is the first time Hubble has witnessed this type of effect created by a star. The data provide a solid estimate of the white dwarf’s mass and yield insights into theories of its structure and composition. Hubble observed the white dwarf, Stein 2051B, as it passed in front of a background star. During the close alignment, the white dwarf’s gravity bent the light from the more distant star, making it appear offset by about 2 milliarcseconds from its actual position.
Bond compared the mass that the Hubble team determined for the white dwarf — 68 percent of the mass of our sun — with the theoretical predictions of its mass, based on the known radius of the star and the properties of the extremely dense matter that makes up a white dwarf. “The agreement of the theoretical prediction with the measurement we were able to make with Hubble was astonishingly good,” Bond said.
The researchers plan to use Hubble to conduct a similar microlensing study with Proxima Centauri, our solar system’s closest stellar neighbor.
The Hubble analysis also helped the astronomers to verify independently the theory of how a white dwarf’s radius is determined by its mass, an idea first proposed in 1935 by astronomer Subrahmanyan Chandrasekhar. “Our measurement is a nice confirmation of white-dwarf theory, and it even tells us the internal composition of a white dwarf — that it is made of carbon and oxygen,” Bond said.