          APOD: 2018 July 8 - The Extraordinary Spiral in LL Pegasi

                         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.

                                  2018 July 8
                                      [2]
                    The Extraordinary Spiral in LL Pegasi
    Image Credit: NASA [3] , ESA [4] , Hubble [5] , HLA [6] ; Processing &
            Copyright: Domingo Pestana [7]  & Raul Villaverde [8]

Explanation: What created the strange spiral structure on the upper left? No
one is sure, although it is likely related to a star in a binary star [9]
system entering the planetary nebula [10]  phase, when its outer atmosphere is
ejected. The huge spiral [11]  spans about a third of a light year [12]
across and, winding four or five complete turns [13] , has a regularity that
is without precedent. Given the expansion rate of the spiral [14]  gas, a new
layer must appear about every 800 years, a close match to the time it takes
for the two stars to orbit each other. The star system [15]  that created it
is most commonly known as LL Pegasi [16] , but also AFGL 3068. The unusual
structure itself has been cataloged as IRAS 23166+1655 [17] . The featured
image was taken in near- infrared [18]  light by the Hubble Space Telescope
[19] . Why the spiral glows [20]  is itself a mystery, with a leading
hypothesis being illumination by light reflected from nearby stars.

                    Tomorrow's picture: road to mars [21]

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    Authors & editors: Robert Nemiroff [33] (MTU [34] ) & Jerry Bonnell [35]
                                  (UMCP [36] )
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                           & Michigan Tech. U. [42]
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Site notes:
  [1] archivepix.html
  [2] image/1807/LLPeg_HubblePestana_2000.jpg
  [3] https://www.nasa.gov/
  [4] http://www.esa.int/
  [5] https://www.nasa.gov/mission_pages/hubble/main/index.html
  [6] https://hla.stsci.edu/
  [7] https://www.flickr.com/photos/122467237@N08/
  [8] https://www.flickr.com/photos/113243238@N08/
  [9] ap970219.html
  [10] http://en.wikipedia.org/wiki/Planetary_nebula
  [11] ap151130.html
  [12] http://chandra.harvard.edu/photo/cosmic_distance.html
  [13] https://www.eso.org/public/images/potw1710a/
  [14] http://en.wikipedia.org/wiki/Spiral
  [15] https://www.eso.org/public/videos/potw1710a/
  [16] http://adsabs.harvard.edu/abs/2012A%26A...544L..18L
  [17] http://en.wikipedia.org/wiki/IRAS_23166%2B1655
  [18] https://science.nasa.gov/ems/07_infraredwaves
  [19] ap090525.html
  [20] http://flyingpudding.com/projects/florets/applet/
  [21] ap180709.html
  [22] ap180707.html
  [23] archivepix.html
  [24] lib/apsubmit2015.html
  [25] lib/aptree.html
  [26] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  [27] calendar/allyears.html
  [28] /apod.rss
  [29] lib/edlinks.html
  [30] lib/about_apod.html
  [31] http://asterisk.apod.com/discuss_apod.php?date=180708
  [32] ap180709.html
  [33] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [34] http://www.phy.mtu.edu/
  [35] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  [36] http://www.astro.umd.edu/
  [37] lib/about_apod.html#srapply
  [38] https://www.nasa.gov/about/highlights/HP_Privacy.html
  [39] https://astrophysics.gsfc.nasa.gov/
  [40] https://www.nasa.gov/
  [41] https://www.nasa.gov/centers/goddard/
  [42] http://www.mtu.edu/
