An international team, involving an astronomer from the University of St Andrews, has measured the mass of a white dwarf star using the fact that massive bodies curve space-time around them according to Einstein's Theory of General Relativity.

The team used data from two space telescopes to measure how light from a distant star was bent around the white dwarf, known as LAWD 37, causing the distant star to temporarily change its apparent position in the sky. The mass measurement provides a test for the predicted mass-radius relationship for white dwarfs based on theoretical models for the extreme conditions inside such dead stars.

Dr Martin Dominik, Reader in Physics & Astronomy at the University of St Andrews, said: "You cannot hide your gravity. As you curve space-time, you create an optical weighting scale."

White dwarfs are one of the possible end states of stars that arise once they have burnt all their nuclear fuel. They are the leftovers of less massive stars like the Sun, whereas more massive stars will evolve into neutron stars or black holes. White dwarfs owe their stability to the fact that electrons are not able to occupy the same quantum state. Thereby, they constitute an interesting test laboratory for the properties of such dense matter.

The study was published in Monthly Notices of the Royal Astronomical Society. Lead author, Dr Peter McGill, carried out the research while completing his PhD at the Institute of Astronomy at the University of Cambridge and is now based at the University of California in Santa Cruz.

He said: "The precision of LAWD 37's mass measurement allows us to test the mass-radius relationship for white dwarfs. This means testing the properties of matter under the extreme conditions inside this dead star."

Dr McGill and his international team of colleagues were able to use a pair of telescopes – the Gaia Satellite and Hubble Space Telescope – to get the first accurate mass measurement for LAWD 37 by predicting, and then observing, an astrometric effect first predicted by Einstein.

The key challenge is that the arising change in position angle that needs to be measured is tiny. With his Theory of General Relativity, Einstein predicted that when a massive body passes in front of a distant star, the light from the star would bend around the foreground body due to its gravity. In 1919, two British astronomers – Arthur Eddington from Cambridge and Frank Dyson from the Royal Greenwich Observatory – first detected this effect during a Solar eclipse, in what was the first confirmation of General Relativity. However, Einstein did not believe that the effect would ever be observable for any other star.

Exactly this happened in 2017, when the bending of light by another nearby white dwarf, Stein 2051 b, was observed, almost a century after Eddington's and Dyson's findings, using the Hubble Space Telescope at the very limit of its capabilities. While Eddington measured an already incredibly small angle corresponding to the diameter of a human hair seen from 10m distance, the measured displacements were 1000 times smaller, corresponding to the angle subtended by a virus at the same distance. Unlike LAWD 37, however, Stein 2051 b is part of a binary system.

For LAWD 37, the astronomers were able to predict the movement using the Gaia space observatory, which is creating the most accurate and complete multi-dimensional map of the Milky Way, and they identified the point where it would align close enough to a background star for detecting a measurable shift in position due to the gravitational bending of light. Thereby, the astronomers were able to point the Hubble Space Telescope in the right place at the right time to observe this phenomenon, which happened in November 2019.

LAWD 37 has been extensively studied, as it is relatively close to us. This white dwarf is 15 light years away in the Musca constellation and is what remains of a star that died around 1.15 billion years ago. The missing piece of the puzzle has been a measurement of its mass, and we now know it to be 0.56 Solar masses. This agrees with earlier theoretical predictions of LAWD 37's mass and corroborates current theories of how white dwarfs evolve.

Mass is one of the most important factors in a star's evolution. For most stars, astronomers infer mass indirectly, relying on strong, often untested modelling assumptions. In rare cases where the mass can be directly inferred, the massive body needs to be gravitationally bound to a companion. For single isolated bodies, such as LAWD 37, other methods for determining mass are needed.

The researchers say their results open the door for predicting further events from Gaia data, and these can then be observed with a new generation of space-based observatories such as JWST, the successor to Hubble.

Dr McGill continued: "Gaia has really changed the game – it's exciting to be able to use Gaia data to predict when events will happen, and then observe them happening. We want to continue our efforts and obtain mass measurements for many more types of stars."

The research was supported in part by NASA, the Science and Technology Facilities Council (STFC), part of UK Research & Innovation (UKRI), and the European Space Agency.

The paper 'First semi-empirical test of the white dwarf mass-radius relationship using a single white dwarf via astrometric microlensing' is published in Monthly Notices of the Royal Astronomical Society (2022) and is available online.

The Hubble Space Telescope (HST) is a joint mission between the US National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). Gaia is a space observatory of the European Space Agency (ESA).

Hubble artwork: NASA, ESA, Ann Feild (STScI)

Issued by the University of St Andrews Communications Office.