Hubble Telescope demonstrates Einstein's general relativity

white dwarf

Since its launch in 1990, the Hubble space telescope has offered us the possibility of discovering the universe in high quality. Thanks to its numerous instruments and improvements, it has made it possible to test various hypotheses. One of Hubble's tasks is to test the theory of relativity, which predicts the existence of gravitational lensing. On this occasion, this phenomenon has been used to measure with great precision the mass of a solitary white dwarf star.

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With a temperature of over 100 000 degrees Celsius, the white dwarf LAWD 37 appears at the center of this Hubble image. The peaks are produced by diffraction between the light from the star and the mirror mounts on the telescope. Credits: NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI)

Bending the light

Thanks to Albert Einstein's work on the theories of general relativity, we have a great way to understand the universe. This tells us how the presence of mass deforms space-time. This in turn affects other objects in the vicinity. In turn, it predicts disturbances in the path of light as it passes near very massive objects. Altering its trajectory and generating effects similar to those caused by lenses, which are observed by Hubble.

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Illustration of how the light from the background star is "bent" by the gravity of LAWD 37 in the gravitational lensing process. This generates a change in the sky position where it appears as seen by the telescope.

White dwarfs are the hot remnants of Sun-like stars.. In its last stages of life, after having burned hydrogen, helium and even iron and losing the ability to sustain a force opposite to gravity to keep it stable, the star will eject its outer layers. Leaving behind a core that spins at high speed and despite not having a constant nuclear fusion, it maintains part of the original temperature.

Measuring mass at a distance

Previously, the vast majority of measurements of the mass of white dwarfs were made by observations of binary systems.. Where by measuring the perturbations of its companion the mass of the star could be inferred.. Although this was limited to those with an orbital period of several months or even a few years.

On this occasion the star LAWD 37, a solitary white dwarf 15 light-years away and a billion years oldwould present a stellar occultation from Hubble's perspective. That is, it would apparently pass in front of a background star. By studying the behavior of its light as it is affected by the lensing effect, it is desired to be able to infer the mass of the white dwarf.

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Plots of how the light from the background star has been affected by LAWD 37, changing its apparent position in the sky. Credits: NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI)

Hubble pointed several times over the years to how the light from the background star was affected according to the theory of general relativity. By making a great effort to eliminate the glare from the white dwarf, which is at least 400 times brighter, the trajectory of the background light was reconstructed. At the same time, the mass of LAWD 37 was obtained with great precision, which corresponds to 56 % of that of our Sun. The same result supports our stellar models and the theory of general relativity.

By studying the mass-radius relationship of these white dwarfs, we can learn more about their structure and composition. And offer clues to theories of degenerate matter, gases supercompressed by gravitational effects that come to resemble solids. As well as continuing Hubble's work in demonstrating general relativity. Thanks to new observatories, these observations can be made in even greater detail and continue to unravel the mysteries of the universe.

Francisco Andrés Forero Daza