Horsehead Nebula, dark nebula located in the constellation Orion; designated IC 434 or B 33. It consists of a cloud of nonluminous interstellar matter resembling the outline of a horse's head and appears against the background of a bright emission nebula. The Horsehead Nebula measures about 4 min of arc at its greatest width. It is located near the belt of Orion at a distance of about 1,600 light-years from the earth.
"Horsehead Nebula." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (September 18, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/horsehead-nebula
"Horsehead Nebula." The Columbia Encyclopedia, 6th ed.. . Retrieved September 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/horsehead-nebula