Astronomers have directly measured the mass of a dead star using an effect known as gravitational microlensing, first predicted by Albert Einstein in his Theory of Relativity.
The international team, led by the University of Cambridge, used data from two telescopes to measure how light from a distant star was bent around a white dwarf known as LAWD 37, causing the distant star to temporarily shift its apparent position in the sky. . It is the first time that this effect has been detected in an isolated star other than our Sun, and the first time that the mass of such a star has been directly measured. The results are published in the Monthly Notices of the Royal Astronomical Society.
LAWD 37 is a white dwarf, the result of the death of a star like ours. When a star dies, it stops burning its fuel and ejects its outer material, leaving only a hot, dense core. Under these conditions, matter as we know it behaves very differently and becomes something called electron-degenerate matter. “White dwarfs give us clues about how stars evolve: one day our own star will end up as a white dwarf,” said Dr. Peter McGill, lead author of the study and a PhD candidate at the Cambridge Institute of Astronomy. McGill now works at the University of California at Santa Cruz. LAWD 37 has been the subject of numerous studies, as it is relatively close to us. This white dwarf is 15 light-years away, in the constellation Musca, and is what remains of a star that died about 1.15 billion years ago.
“Because this white dwarf is relatively close to us, we have a lot of data about it: information about its light spectrum, but the missing piece of the puzzle was to measure its mass,” explains McGill. Mass is one of the most important factors in the evolution of a star. For most stellar objects, astronomers infer the mass indirectly, based on strong and often unproven modeling assumptions. In the rare cases where the mass can be inferred directly, the object must have a companion, such as a binary star system. But for individual objects, like LAWD 37, other methods are needed to determine the mass.
McGill and his international team of colleagues were able to use a pair of telescopes – the European Space Agency’s Gaia Telescope and the Hubble Space Telescope – to obtain the first precise measurement of LAWD 37’s mass by predicting, and subsequently observing, of an astrometric effect first predicted by Einstein. In his General Theory of Relativity, Einstein predicted that when a massive compact object passes in front of a distant star, the star’s light would bend around the foreground object due to its gravitational field. This effect is known as gravitational microlensing.
In 1919, two British astronomers – Arthur Eddington of Cambridge and Frank Dyson of the Royal Greenwich Observatory – first detected this effect during a solar eclipse, in what was the first popular confirmation of General Relativity. However, Einstein was pessimistic about the possibility that the effect would ever be detected in stars outside our solar system. In 2017, astronomers detected this gravitational microlensing effect for another nearby white dwarf in a binary system, Stein 2051 b, marking the first detection of this effect for a star other than our Sun. Now, the Cambridge-led team has detected the effect in LAWD 37, which is the first direct measurement of the mass of a single white dwarf.
Using ESA’s Gaia satellite, which is creating the most accurate and comprehensive multi-dimensional map of the Milky Way, astronomers were able to predict LAWD 37’s motion and identify the point at which it would align close enough to a background star as to detect the lens signal. Using the Gaia data, astronomers were able to point the Hubble Space Telescope in the right place at the right time to observe this phenomenon, which occurred in November 2019, 100 years after the famous Eddington/Dyson experiment. Since the light from the background star was so weak, the main challenge for astronomers was to extract the lensing signal from the noise. “These phenomena are rare and their effects are tiny,” explains McGill. “For example, our measured effect size is like measuring the length of a car on the Moon as seen from Earth, and is 625 times smaller than the effect measured in the 1919 solar eclipse.”