My most favorite deep sky object is Horsehead Nebula in Orion Constellation. It is visible as the dark indentation to the red emission nebula in the background. The horsehead feature is dark because it is really an opaque dust cloud that lies in front of the bright red emission nebula. Like clouds in Earth’s atmosphere, this cosmic cloud has assumed a recognizable shape by chance. After many thousands of years, the internal motions of the cloud will surely alter its appearance. The emission nebula’s red color is caused by ionization i.e.,electrons recombining with protons to form hydrogen atoms, whereas the dark nebula is made of heavier gases such as oxygen etc that do not ionize.
Horsehead Nebula is 1500 light years away and is about 2 to 3 light years wide. The red hydrogen wall behind the horsehead, that is partially shown here, is about 25 to 30 light years wide.
Took this image over 20 hours (18 images of Luminance of 30 minutes each, color data of 7 images each (Red, Blue, Green) of 20 minutes each, and added some Hydrogen Alpha data (12 images of 20 minutes each).