Astronomy Picture of the Day . ... 31.12.2013 . ... It is visible as the dark indentation to the red emission nebula in the center of the above photograph. The horse-head feature is dark because it is really an opaque dust cloud that lies in front of the bright red emission nebula . ... The emission nebula 's red color is caused by electrons recombining with protons to form hydrogen atoms. ... Publications with keywords: Horsehead Nebula . Publications with words: Horsehead Nebula . ...