                APOD: 2017 December 27 - The Horsehead Nebula

                         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 27
                                      [2]
                            The Horsehead Nebula
Image Credit: CFHT [3] , Coelum [4] , MegaCam [5] , J.-C. Cuillandre [6] (CFHT
                    [7] ) & G. A. Anselmi [8] (Coelum [9] )

Explanation: One of the most identifiable nebulae in the sky, the Horsehead
Nebula [10]  in Orion [11] , is part of a large, dark, molecular cloud [12] .
Also known as Barnard [13]  33, the unusual shape was first discovered [14]
on a photographic plate [15]  in the late 1800s. The red glow originates from
hydrogen [16] gas predominantly behind the nebula, ionized by the nearby
bright star Sigma Orionis [17] . The darkness of the Horsehead [18]  is caused
mostly by thick dust [19] , although the lower part of the Horsehead [20] 's
neck casts a shadow to the left. Streams of gas leaving the nebula [21]  are
funneled by a strong magnetic field [22] . Bright spots in the Horsehead
Nebula [23] 's base are young stars just in the process of forming [24] .
Light takes about 1,500 years to reach us from the Horsehead Nebula [25] . The
featured image [26]  was taken with the large 3.6-m Canada-France-Hawaii
Telescope [27] in Hawaii [28] , USA [29] .

         Free APOD Lectures: January 5 in NYC & January 9 in DC [30]
                 Tomorrow's picture: explosion afterglow [31]

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Site notes:
  [1] archivepix.html
  [2] image/1712/Horsehead_CFHT_960.jpg
  [3] http://www.cfht.hawaii.edu/
  [4] http://www.coelum.com/
  [5] http://www.cfht.hawaii.edu/Instruments/Imaging/Megacam/
  [6] http://www.cfht.hawaii.edu/~jcc/
  [7] http://www.cfht.hawaii.edu/
  [8] http://www.coelum.com/coelum/autori/giovanni-anselmi
  [9] http://www.coelum.com/
  [10] http://en.wikipedia.org/wiki/Horsehead_Nebula
  [11]
http://www.astro.wisc.edu/~dolan/constellations/constellations/Orion.html
  [12] ap090623.html
  [13] http://en.wikipedia.org/wiki/Edward_Emerson_Barnard
  [14] http://www.messier.seds.org/xtra/ngc/b33.html
  [15] https://en.wikipedia.org/wiki/Photographic_plate
  [16] http://periodic.lanl.gov/1.shtml
  [17] http://www.dibonsmith.com/ori_s.htm
  [18] ap061015.html
  [19] ap030706.html
  [20] ap081126.html
  [21] http://www.youtube.com/watch?v=hgTrLozRj40
  [22] https://en.wikipedia.org/wiki/Magnetic_field
  [23] http://adsabs.harvard.edu/abs/1987AJ.....93.1514Z
  [24] ap070218.html
  [25] ap090224.html
  [26]
http://www.cfht.hawaii.edu/HawaiianStarlight/AIOM/English/
CFHT-Coelum-AIOM-Jan2011.html
  [27] http://www.cfht.hawaii.edu/en/about/
  [28]
https://en.wikipedia.org/wiki/Hawaii#/media/
File:Hawaii_in_United_States_(zoom)_(US50)_(-grid).svg
  [29] https://www.cia.gov/library/publications/the-world-factbook/geos/us.html
  [30] http://asterisk.apod.com/viewtopic.php?f=28&t=37750
  [31] ap171228.html
  [32] ap171226.html
  [33] archivepix.html
  [34] lib/apsubmit2015.html
  [35] lib/aptree.html
  [36] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  [37] calendar/allyears.html
  [38] /apod.rss
  [39] lib/edlinks.html
  [40] lib/about_apod.html
  [41] http://asterisk.apod.com/discuss_apod.php?date=171227
  [42] ap171228.html
  [43] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [44] http://www.phy.mtu.edu/
  [45] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  [46] http://www.astro.umd.edu/
  [47] lib/about_apod.html#srapply
  [48] https://www.nasa.gov/about/highlights/HP_Privacy.html
  [49] https://astrophysics.gsfc.nasa.gov/
  [50] https://www.nasa.gov/
  [51] https://www.nasa.gov/centers/goddard/
  [52] http://www.mtu.edu/
