Orion's Horsehead Nebula . ... It is visible as the black indentation to the red emission nebula seen just to the right of center of the above photograph. The bright star near the center is located in the belt of the familiar constellation of Orion . The horse-head feature is dark because it is really an opaque dust cloud which 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. ...