M42 - the Orion Nebula is a splendid diffuse nebula in the Milky Way. Located at a distance of 1,344 light years, it is the closest region of massive star formation to Earth. It is estimated to be 24 light years across and has approx. 2,000 × the mass of the Sun.
The nebula contains a very young open cluster, referred to as the Trapezium, due to an asterism of four bright stars. This cluster, which actually contains more than 300 very young stars, is responsible for the ionisation of surrounding gas, which gives rise to it's colourful nature in photographs.
The red hue is a result of recombination of Hydrogen II atoms, with a wavelength of 653nm, whilst green observed is caused by electron transistions in doubly ionised Oxygen.
Spectroscopic analysis of the nebula has revealed the prescence of many complex organic compounds in the nebula including:- Methanol, Hydrogen Cyanide, Sulphur Dioxide, Hydrogen Sulphide, Formyl Cation, Formaldehyde, Dimethyl Ether, Water, Carbon Monoxide, Acrylonitrile and Methyl Formate.
In James Clark's image above, the Running Man nebula can also be seen on the left of M42. This is a reflection nebula 1,460 light years away, and is actually comprosed of three nebulae: NGC1973, NGC1875 & NGC1977.
For more info. see the Wikipedia entry.