This image is the result of single night’s session, from about 11pm until 4am.

The Monkey Head Nebula (so named because, with a bit of imagination, that’s what it resembles) is in the constellation Orion. It is designated NGC 2174 and is a cloud of gas surrounding a star cluster, known as NGC 2175. Like many such nebulae, new stars are being born here as the gas and dust in the nebula condenses. The gas cloud is predominantly ionised hydrogen, with the energy for ionisation coming from the ultra-violet radiation from the new stars. It is about 6,400 light-year away. 

It covers an area of sky larger than the full Moon, and can just be seen with binoculars as a faint grey patch. Being faint, the nebula was not discovered until 1877, by the French astronomer Édouard Stephan (of Stephan’s Quintet fame).

Note on colours:

This image has been taken with narrow-band filters, each sensitive only to the bright emission of a particular element (where H is the symbol for hydrogen, O for oxygen and S for sulphur). The images have then been assigned to the red (S), green (H) and blue (O) channels of the final image. Such “SHO” images are therefore false colour. If our eyes were sufficiently sensitive, we would see it as predominantly red, due to the strong emissions from the Hydrogen gas clouds.