Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                               2019 October 26

                               Gravity's Grin
       Image Credit: X-ray - NASA / CXC / J. Irwin et al. ; Optical -
                                 NASA/STScI

   Explanation: Albert Einstein's general theory of relativity, published
   over 100 years ago, predicted the phenomenon of gravitational lensing.
   And that's what gives these distant galaxies such a whimsical
   appearance, seen through the looking glass of X-ray and optical image
   data from the Chandra and Hubble space telescopes. Nicknamed the
   Cheshire Cat galaxy group, the group's two large elliptical galaxies
   are suggestively framed by arcs. The arcs are optical images of distant
   background galaxies lensed by the foreground group's total distribution
   of gravitational mass. Of course, that gravitational mass is dominated
   by dark matter. The two large elliptical "eye" galaxies represent the
   brightest members of their own galaxy groups which are merging. Their
   relative collisional speed of nearly 1,350 kilometers/second heats gas
   to millions of degrees producing the X-ray glow shown in purple hues.
   Curiouser about galaxy group mergers? The Cheshire Cat group grins in
   the constellation Ursa Major, some 4.6 billion light-years away.

                       Tomorrow's picture: ghostly sky
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                      A service of: ASD at NASA / GSFC
                             & Michigan Tech. U.

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