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   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Hubble directly measures mass of a lone white dwarf    
      
     Date:   
         February 2, 2023   
     Source:   
         NASA/Goddard Space Flight Center   
     Summary:   
         Astronomers have directly measured the mass of a single, isolated   
         white dwarf -- the surviving core of a burned-out, Sun-like   
         star. Researchers 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. The unique observation yields insights into theories   
         of the structure and composition of white dwarfs.   
      
      
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   FULL STORY   
   ==========================================================================   
   Astronomers using NASA's Hubble Space Telescope have for the first   
   time directly measured the mass of a single, isolated white dwarf --   
   the surviving core of a burned-out, Sun-like star.   
      
      
   ==========================================================================   
   Researchers 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. 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. 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.   
      
   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.   
      
   The results are reported in the Monthly Notices of the Royal Astronomical   
   Society. The lead author is Peter McGill, formerly of the University of   
   Cambridge (now based at the University of California, Santa Cruz).   
      
   McGill used Hubble to precisely measure how light from a distant star   
   bent around the white dwarf, known as LAWD 37, causing the background   
   star to temporarily change its apparent position in the sky.   
      
   Kailash Sahu of the Space Telescope Science Institute in Baltimore,   
   Maryland, the principal Hubble investigator on this latest observation,   
   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. "Our latest   
   observation provides a new benchmark because LAWD 37 is all by itself,"   
   Sahu said.   
      
   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. "Because this white dwarf is relatively   
   close to us, we've got lots of data on it -- we've got information about   
   its spectrum of light, but the missing piece of the puzzle has been a   
   measurement of its mass," said McGill.   
      
   The team zeroed in on the white dwarf thanks to ESA's Gaia space   
   observatory, which makes 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.   
      
   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.   
      
   "These events are rare, and the effects are tiny," said McGill. "For   
   instance, the size of our measured offset is like measuring the length   
   of a car on the Moon as seen from Earth."  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.   
      
   "The precision of LAWD 37's mass measurement allows us to test the   
   mass-radius relationship for white dwarfs," said McGill. "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," he added.   
      
   The researchers say their results open the door for future event   
   predictions with Gaia data. In addition to Hubble, 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.   
      
   Based on Gaia's predictive powers, Sahu is 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.   
      
   "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," said McGill. "We want to continue measuring the gravitational   
   microlensing effect and obtain mass measurements for many more types   
   of stars."  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.   
      
   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. 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. "Our measurement is 625 times smaller than the effect measured   
   at the 1919 solar eclipse," said McGill.   
      
   The Hubble Space Telescope is a project of international cooperation   
   between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt,   
   Maryland, manages the telescope. The Space Telescope Science Institute   
   (STScI) in Baltimore conducts Hubble science operations. STScI is operated   
   for NASA by the Association of Universities for Research in Astronomy,   
   in Washington, D.C.   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Stars # NASA # Astronomy # Space_Telescopes #   
                   Space_Exploration # Extrasolar_Planets # Black_Holes #   
                   Satellites   
       * RELATED_TERMS   
             o Supernova o Astronomy o Chandra_X-ray_Observatory o Planet   
             o Globular_cluster o Brown_dwarf o Gravitational_wave o Pluto   
      
   ==========================================================================   
   Story Source: Materials provided by   
   NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style   
   and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Peter McGill, Jay Anderson, Stefano Casertano, Kailash C Sahu,   
      Pierre   
         Bergeron, Simon Blouin, Patrick Dufour, Leigh C Smith, N Wyn   
         Evans, Vasily Belokurov, Richard L Smart, Andrea Bellini, Annalisa   
         Calamida, Martin Dominik, Noe' Kains, Jonas Klu"ter, Martin Bo   
         Nielsen, Joachim Wambsganss. First semi-empirical test of the   
         white dwarf mass-radius relationship using a single white dwarf   
         via astrometric microlensing.   
      
         Monthly Notices of the Royal Astronomical Society, 2023; 520 (1):   
         259 DOI: 10.1093/mnras/stac3532   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/02/230202112706.htm   
      
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