Sunday, March 19, 2023

924 - WHITE DWARF STAR - measuring its mass.

 

-   3924 - WHITE  DWARF  STAR  -  measuring its mass.    Astronomers using NASA’s Hubble Space Telescope have for the first time directly measured the mass of a single, isolated white dwarf star which is the surviving core of a burned-out, Sun-like star.

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------------  3924  -   WHITE  DWARF  STAR  -  measuring its mass.  

-    These astronomers  found that the white dwarf is 56 percent the mass of our Sun. This agrees with earlier theoretical predictions of the white dwarf’s mass and corroborates current theories of how white dwarfs evolve as the end product of a typical star’s evolution.

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-   The unique observation yields insights into theories of the structure and composition of white dwarfs.    Until now, previous white dwarf mass measurements have been gleaned from observing white dwarfs in binary star systems.

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-     By watching the motion of two co-orbiting stars, straightforward Newtonian physics can be used to measure their masses. However, these measurements can be uncertain if the white dwarf’s companion star is in a long-period orbit of hundreds or thousands of years. Orbital motion can be measured by telescopes only over a brief slice of the dwarf’s orbital motion.

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-    For this companion-less white dwarf, researchers had to employ a trick of nature, called gravitational microlensing. The light from a background star was slightly deflected by the gravitational warping of space by the foreground dwarf star. As the white dwarf passed in front of the background star, microlensing caused the star to appear temporarily offset from its actual position on the sky.

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-   Hubble telescope was used  to precisely measure how light from a distant star bent around the white dwarf, “LAWD 37”, causing the background star to temporarily change its apparent position in the sky.

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-   Astronomers first used microlensing in 2017 to measure the mass of another white dwarf, “Stein 2051 B”. But that dwarf is in a widely separated binary system.  These latest observation provides a new benchmark because LAWD 37 is all by itself.

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-    The collapsed remains of a star that burned out 1 billion years ago, LAWD 37 has been extensively studied because it is only 15 light-years away in the constellation Musca.   Astronomers got information about its spectrum of light, but the missing piece of the puzzle has been a measurement of its mass.

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-     ESA’s Gaia space observatory was used to make extraordinarily precise measurements of nearly 2 billion star positions. Multiple Gaia observations can be used to track a star’s motion. Based on this data, astronomers were able to predict that LAWD 37 would briefly pass in front of a background star in November 2019.

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-   Once this was known, Hubble was used to precisely measure over several years how the background star’s apparent position in the sky was temporarily deflected during the white dwarf’s passage.   The size of our measured offset is like measuring the length of a car on the Moon as seen from Earth.

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-   Since the light from the background star was so faint, the main challenge for astronomers was extracting its image from the glare of the white dwarf, which is 400 times brighter than the background star. Only Hubble can make these kinds of high-contrast observations in visible light.

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-    The precision of LAWD 37’s mass measurement allows us to test the mass-radius relationship for white dwarfs.  This means testing the theory of degenerate matter (a gas so super-compressed under gravity it behaves more like solid matter) under the extreme conditions inside this dead star.

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-    These alignments can now be detected with NASA’s James Webb Space Telescope.  Because Webb works at infrared wavelengths, the blue glow of a foreground white dwarf looks dimmer in infrared light, and the background star looks brighter.

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-    Based on Gaia’s predictive powers observing another white dwarf, LAWD 66, with NASA’s James Webb Space Telescope. The first observation was done in 2022. More observations will be taken as the deflection peaks in 2024 and then subsides.

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-    In his 1915 theory of general relativity, Einstein predicted that when a massive compact object passes in front of a background star, the light from the star would bend around the foreground object due to the warping of space by its gravitational field.

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-    Exactly a century before this latest Hubble observation, in 1919, two British-organized expeditions to the southern hemisphere first detected this lensing effect during a solar eclipse on May 19th.

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-    It was hailed as the first experimental proof of general relativity that gravity warps space. However, Einstein was pessimistic that the effect could ever be detected for stars outside our solar system because of the precision involved.  These measurements were 625 times smaller than the effect measured at the 1919 solar eclipse.

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                   March 19, 2023     WHITE  DWARF  STAR  -  measuring its mass.               3924                                                                                                                         

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--------------------- ---  Sunday, March 19, 2023  ---------------------------

 

 

 

 

         

 

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