Located in the constellation of Orion, The Monkey Head Nebula is an emission nebula. Emission nebulae are clouds of ionised gases that emit their own light at various optical wavelengths, hence the name. These nebulae vary greatly in density, ranging from millions of atoms per cubic centimetre to only a few. They occur when interstellar gas clouds, comprised dominantly of neutral hydrogen atoms, are ionised by nearby stars, most commonly B and O type stars. The vast quantities of ultraviolet light with high energy delocalise the electrons from the hydrogen nuclei, that later recombine in an ‘excited state' and then emit photons of light when they de-excite.
Other forms of nebulae include absorption nebulae, also known as ‘dark nebulae,’ which are so dense that they obscure all light from behind them, hence the name. Another intergalactic object from the nebulae family is the reflection nebula, which occurs when its gas is not sufficiently ionised by close by stars to form an emission nebula, however adequate amounts of energy are scattered to make it visible.
Many have heard of The Orion Nebula, located in the ‘sword’ of the asterism. NGC 2174 is located near Orion’s head, above Betelgeuse and covers an area larger than the full Moon. However, unlike The Orion Nebula, it cannot be viewed by the naked eye and requires binoculars at the very least to be seen.
NGC 2174 is often described by the phrase ‘stars versus mountains of gas, and the stars are winning.’ This refers to the stellar nurseries that are the vast collections of gas, plasma and dust in which the stars are formed. However, when these baby stars grow, they release huge amounts of energy and stellar winds that consume, evaporate and disperse that clouds. So yes, the stars are winning.