Astronomers have measured the mass of a lone white dwarf for the first time. This type of smoldering stellar remnant forms at the end of the life of low-mass stars and will be what the Sun leaves behind when it dies in about 5 billion years.
The Hubble Space Telescope measured the mass of a white dwarf, designated LAWD 37, that burned up over 1 billion years ago. In the work, scientists used a phenomenon first predicted in 1915 by Albert Einstein called “gravitational lensing”, which involves the bending of light by objects of large mass. The team determined that LAWD 37 has a mass of about 56% that of the Sun. The discovery confirms current theories about how these stellar remnants form and evolve. This particular white dwarf is well studied because it is relatively close to Earth, only 15 million light-years away in the constellation Muhi.
“Because this white dwarf is relatively close to us, we have a lot of data about it – we have information about its spectrum of light, but the missing piece of the puzzle was measuring its mass,” Peter McGill. This is stated in a statement by an astronomer from the University of California at Santa Cruz, who led the study.
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An image of the white dwarf LAWD 37 taken by the Hubble Space Telescope. (Image credit: NASA, ESA, P. McGill (UC Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI))
This is the first time astronomers have calculated the mass of a lone white dwarf, but they have previously made similar measurements for white dwarfs in double partnerships with other stars.
In pairs, astronomers can get a measurement of mass by applying Newton’s theory of gravity to the motion of two stars orbiting each other. However, this can be an uncertain process, especially when the companion star has a long orbit of hundreds or thousands of years.
To measure the mass of this single star, the researchers turned to Einstein’s formulation of gravity, his general theory of relativity.
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General relativity suggests that objects of high mass “distort” the very fabric of space-time. The larger the mass, the larger the “dent” in space it causes.
As light from the background object passes this warp, it is deflected, an effect that can amplify the light or even cause the background object to appear in multiple places at the same time. More often, however, the deformation causes a shift in the apparent position of the background object.
The mass of the lensing object causing the effect can be determined by measuring how much the light is deflected and therefore the shift in position it causes when astronomers look at the background object. This is true even if the offset is small, as is the case with microlensing, for example, involving this particular white dwarf.
In the new observations, LAWD 37 acted as a gravitational lens in the foreground, slightly deflecting light passing by it from the background star and shifting its position in the sky. This shift in position allowed McGill and his team to measure the mass of LAWD 37. The researchers used a similar process to determine the mass of another white dwarf in 2017, but this stellar remnant was in a binary system, not in a single dead star like LAWD 37.
The diagram shows how a massive object, such as a white dwarf, can warp spacetime, causing a background star to appear in a different place from where it actually is. (Image credit: NASA, ESA, A. Feild)
McGill and his colleagues were able to hone LAWD 37 thanks to the European Space Agency’s Gaia mission, which accurately measures the position of some 2 billion stars. Using multiple Gaia images allows astronomers to track the star’s movement so the team can predict that LAWD 37 will pass in front of the background star in November 2019.
Armed with this prediction, the scientists used Hubble for several years to measure the change in the apparent position of the background star as the white dwarf passed in front of it.
“These events are rare and their consequences are minor,” McGill said. “For example, the size of our measured displacement is like measuring the length of a car on the Moon as viewed from Earth.”
The team also had to extract the faint light of background stars from the bright light of LAWD 37, which was about 400 times brighter. Fortunately, Hubble has enough power to make such high-contrast observations in visible light.
“Even if you’ve identified such a one-in-a-million event, it’s still extremely difficult to make these measurements,” said Lee Smith, an astronomer at the University of Cambridge in the UK and co-author of the study. in a statement. “Aurora from a white dwarf can cause bands in unpredictable directions, which means we had to analyze each of the Hubble observations and their limitations very carefully in order to model the event and estimate the mass of LAWD 37.”
The team’s research is described in a paper published Dec. 6 in the Monthly Notices of the Royal Astronomical Society.
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