        APOD: 2018 February 6 - Galaxy NGC 474: Shells and Star Streams

                         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 February 6
                                      [2]
                   Galaxy NGC 474: Shells and Star Streams
 Image Credit [3] : CFHT [4] , Coelum [5] , MegaCam [6] , J.-C. Cuillandre [7]
                (CFHT [8] ) & G. A. Anselmi [9] (Coelum [10] )

Explanation: What's happening to galaxy NGC 474? The multiple layers of
emission appear strangely complex and unexpected given the relatively
featureless appearance of the elliptical galaxy [11]  in less deep images. The
cause of the shells is currently unknown, but possibly tidal tails [12]
related to debris left over from absorbing numerous small galaxies in the past
billion years. Alternatively the shells [13] may be like ripples in a pond,
where the ongoing collision with the spiral galaxy just above NGC 474 [14] is
causing density waves [15]  to ripple through the galactic giant. Regardless
of the actual cause, the featured image [16] dramatically highlights the
increasing consensus that at least some elliptical galaxies [17] have formed
in the recent past, and that the outer halos of most large galaxies [18]  are
not really smooth but have complexities induced by frequent interactions [19]
with -- and accretions of -- smaller nearby galaxies [20] . The halo of our
own Milky Way Galaxy [21] is one example [22]  of such unexpected complexity
[23] . NGC 474 spans about 250,000 light years [24]  and lies about 100
million light years distant toward the constellation of the Fish (Pisces [25]
).

                      Tomorrow's picture: open space [26]

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Site notes:
  [1] archivepix.html
  [2] image/1802/NGC474_CfhtCoelum_1500.jpg
  [3] https://creativecommons.org/licenses/by-sa/4.0/deed.en
  [4] http://www.cfht.hawaii.edu/
  [5] http://www.coelum.com/
  [6] http://www.cfht.hawaii.edu/Instruments/Imaging/Megacam/
  [7] http://www.cfht.hawaii.edu/~jcc/
  [8] http://www.cfht.hawaii.edu/
  [9] http://www.coelum.com/coelum/autori/giovanni-anselmi
  [10] http://www.coelum.com/
  [11] http://en.wikipedia.org/wiki/Elliptical_galaxy
  [12] ap100926.html
  [13] ap021111.html
  [14] http://adsabs.harvard.edu/abs/1999IAUS..186..191T
  [15] http://www.youtube.com/watch?v=c5Us-jonCLA
  [16]
http://www.cfht.hawaii.edu/HawaiianStarlight/AIOM/English/2013/Images/
Jul-Image2012-CFHT-Coelum.jpg
  [17] http://asterisk.apod.com/viewtopic.php?f=24&t=18005
  [18] ap061011.html
  [19] https://www.youtube.com/watch?v=Su4gI2CApcQ
  [20] http://en.wikipedia.org/wiki/Local_Group
  [21] http://en.wikipedia.org/wiki/Milky_Way_galaxy
  [22] ap050529.html
  [23] https://utbblogs.com/wp-content/uploads/2014/10/toilet-paper-mess-01.jpg
  [24] http://chandra.harvard.edu/photo/cosmic_distance.html
  [25] http://en.wikipedia.org/wiki/Pisces_%28constellation%29
  [26] ap180207.html
  [27] ap180205.html
  [28] archivepix.html
  [29] lib/apsubmit2015.html
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  [31] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
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  [36] http://asterisk.apod.com/discuss_apod.php?date=180206
  [37] ap180207.html
  [38] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [39] http://www.phy.mtu.edu/
  [40] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
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