        APOD: 2017 December 17 - The Einstein Cross Gravitational Lens

                         Astronomy Picture of the Day

    Discover the cosmos! [1] Each day a different image or photograph of our
 fascinating universe is featured, along with a brief explanation written by a
                           professional astronomer.

                               2017 December 17
                                      [2]
                    The Einstein Cross Gravitational Lens
  Image Credit & Copyright: J. Rhoads [3] (Arizona State U. [4] ) et al. [4] ,
                   WIYN [5] , AURA [6] , NOAO [7] , NSF [8]

Explanation: Most galaxies have a single nucleus -- does this galaxy have
four? The strange answer leads astronomers [9] to conclude that the nucleus of
the surrounding galaxy is not even visible in this image [10] . The central
cloverleaf [11]  is rather light emitted from a background quasar [12] . The
gravitational [13]  field of the visible foreground galaxy breaks light [14]
from this distant quasar [15] into four distinct images. The quasar [16]  must
be properly aligned [17] behind the center of a massive galaxy for a mirage
like this [18]  to be evident. The general effect is known as gravitational
lensing [19] , and this specific case is known as the Einstein Cross [20] .
Stranger still, the images of the Einstein Cross [21]  vary in relative
brightness, enhanced occasionally by the additional gravitational microlensing
[22] effect of specific stars in the foreground galaxy.

                  Free Download: The 2018 APOD Calendar [23]
                 Tomorrow's picture: star system similar [24]

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Site notes:
  [1] archivepix.html
  [2] image/1712/qso2237_wiyn_1024.jpg
  [3] http://rhoads.asu.edu/
  [4] http://sese.asu.edu/
  [5] http://www.noao.edu/wiyn/wiyn.html
  [6] http://www.noao.edu/scope/copyright.html
  [7] http://www.noao.edu/
  [8] http://www.nsf.gov/
  [9] https://aas.org/learn/careers-astronomy
  [10] http://www.noao.edu/image_gallery/html/im0553.html
  [11] http://en.wikipedia.org/wiki/Four-leaf_clover
  [12] ap980409.html
  [13] https://spaceplace.nasa.gov/what-is-gravity/en/
  [14] http://www.youtube.com/watch?v=BkBNf_nFuhM
  [15] ap000419.html
  [16] https://imagine.gsfc.nasa.gov/ask_astro/quasar.html
  [17]
https://lh3.ggpht.com/_LlfXWxcpJyU/SkXvJ2cC99I/AAAAAAAAMxE/hC0Z002bgv4/
7%20Maine%20Coon%20Kittens%5B9%5D.jpg
  [18] ap990331.html
  [19] http://en.wikipedia.org/wiki/Gravitational_lens
  [20] http://www.astr.ua.edu/keel/agn/qso2237.html
  [21] ap961215.html
  [22] https://en.wikipedia.org/wiki/Gravitational_microlensing
  [23] https://apod.nasa.gov/apod/fap/APOD_Calendar_2018_300dpi.pdf
  [24] ap171218.html
  [25] ap171216.html
  [26] archivepix.html
  [27] lib/apsubmit2015.html
  [28] lib/aptree.html
  [29] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  [30] calendar/allyears.html
  [31] /apod.rss
  [32] lib/edlinks.html
  [33] lib/about_apod.html
  [34] http://asterisk.apod.com/discuss_apod.php?date=171217
  [35] ap171218.html
  [36] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [37] http://www.phy.mtu.edu/
  [38] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  [39] http://www.astro.umd.edu/
  [40] lib/about_apod.html#srapply
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