This is an emission nebula that encompasses all of Orion and then some. I recently heard of it in a discussion on cloudynights.com. I had never heard of it but it is HUGE! See it here: http://canopus.physik.uni-potsdam.de/~axm/images/Orion_HaHaRGB.html In the image you can see the Horsehead nebula as a very tiny spec! Doug P