One of the most identifiable nebulae in the sky, the Horsehead Nebula [ http://www.seds.org/billa/twn/b33x.html ] in Orion [ http://www.astro.wisc.edu/~dolan/constellations/constellations/Orion.html ], is part of a large, dark, molecular cloud [ http://antwrp.gsfc.nasa.gov/apod/ap010923.html ]. Also known as Barnard [ http://en.wikipedia.org/wiki/Edward_Emerson_Barnard ] 33, the unusual shape was first discovered [ http://www.seds.org/messier/xtra/ngc/b33.html ] on a photographic plate [ http://www.astro.virginia.edu/~afs5z/photography.html ] in the late 1800s. The red glow originates from hydrogen [ http://periodic.lanl.gov/elements/1.html ] gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis [ http://www.dibonsmith.com/ori_s.htm ]. The darkness of the Horsehead [ http://antwrp.gsfc.nasa.gov/apod/ap061015.html ] is caused mostly by thick dust [ http://antwrp.gsfc.nasa.gov/apod/ap030706.html ], although the lower part of the Horsehead [ http://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search?horsehead ]'s neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field [ http://www-spof.gsfc.nasa.gov/Education/whmfield.html ]. Bright spots in the Horsehead Nebula [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987AJ.....93.1514Z ]'s base are young stars just in the process of forming [ http://antwrp.gsfc.nasa.gov/apod/ap070218.html ]. Light takes about 1500 years to reach us from the Horsehead Nebula [ http://heritage.stsci.edu/public/2001may/table.html ]. The above image [ http://www.noao.edu/image_gallery/html/im0057.html ] was taken with the 0.9-meter telescope [ http://www.noao.edu/0.9m/index.html ] at Kitt Peak National Observatory [ http://www.noao.edu/kpno/kpno.html ].