This nebula has been made extremely famous thanks to the efforts of the Hubble space telescope but it has long been a target of interest to amateur astronomers like myself. It is relatively easy to locate (just below the Easternmost star of Orion's belt - Alnitak) and one of the few objects in the sky that truly resembles it's namesake. However it's popularity as an imaging target does not say anything about the level of difficulty involved in effectively photographing it!
The image above is a composite of four and a quarter hours worth of exposures through an eight inch telescope but there are just not that many photons to be collected! So what are we looking at? Why is it so dark?
The first clue is its location: just below the belt of Orion... we know of some other nebulae just below the belt of Orion:
This is M42 (the Orion Nebula) and M43 (the Running Man Nebula) and we can immediately see a number of features that are also present in the Horsehead image at the top of this post. First and most obvious is the red glow in the heart of M42, this deep red colour also forms the backdrop to the Horsehead and is the characteristic glow of Ionised hydrogen gas. That is hydrogen atoms that are being so battered with ultra-violet light - produced by the young energetic stars being born within the nebula itself - that they are stripped of their electrons. When the hydrogen atoms recombine with their dissociated electrons they emit red light. It is this process that lights up many nebulae in the sky. We can also see pervasive blue regions. This blue is not so obvious in the Horsehead image but the big fuzzy blob in the lower left corner shows hints of it (it would show more if I hadn't over-exposed it in order to reveal the dark nebula). This blue will lead us to the "darkness"... When white light passes through regions of gas and dust the different colour wavelengths that make up the white light react by diffracting around the particles of dust, red light tends to pass through in more or less the same direction it was going in the first place while blue light gets scattered and reflected all over the place, this is the same process that makes sunsets red and the day time sky blue (its called Rayleigh scattering), so where we see this blue colour we know we have enough dust to scatter starlight into its constituent colours; but what if there was more dust? More matter between us and the stars lighting up the dust and the gas? Well then you get the dark lane at the top of M42, enough dust completely blocks visible light and gives us the Dark Nebulae that Edward Emerson Barnard was so fond of cataloguing.
So now we have all the information needed to understand the Horsehead image: the horse's head is literally the shadow cast by dense billows of dust between us and the pervasive glow of Ionised hydrogen beyond. But its not just a shadow, look at the base of the horse's neck and you can see billows in the dust, faint hints of variation in the darkness, pillow like structures of dark brown. Faint though it is we can see the dust itself. But we are not done yet! I didn't just show you the Orion nebula picture because its pretty and gives me a chance to show you another of my pictures, I showed you it because the Horsehead nebula and the Orion nebula are both part of a much larger structure called the Orion Molecular Cloud, this enormous structure encompasses the entire constellation of Orion and when imaged by better photographers than me is simply staggering: https://apod.nasa.gov/apod/ap161204.html
