A diffuse nebula is a form of nebula, composed of large bright clouds of low density gas and dust. There are two types of diffuse nebulae: emission and reflection. They are mainly composed of ionized Hydrogen, leading to the sometimes being called H II regions. They often appear red/pink in colour and are associated with large clusters of stars. Examples of diffuse nebulae include the Orion nebula, the Lagoon nebula and the Omega nebula.
In 1610, Nicholas-Claude Fabri de Peiresc discovered the Orion nebula. It was not until around 1731 that Jean-Jacques D'Ortous De Mairan discovered M43. Several more were discovered in the following decades. These nebulae were originally believed to be star clouds or unresolved star clusters. However, the research of William Higgins in the 1860s using spectroscopic methods showed that they were composed of gas.
The two types of diffuse nebula depend on whether the light emitted from the nebula is produced when excited atoms decay or instead light is reflected. Emission nebulae produce light from the decay of excited atoms. They contain large hot stars that emit large amounts of high energy radiation. This can excite the surrounding gas, which then re-emits this radiation later when atoms return to a lower energy state. On the other hand, reflection nebulae reflect the light that interacts with dust particles. This leads reflection nebula to often have a white/blue appearance. Furthermore, diffuse nebulae can be a mixture of both emission and reflection, an example being the Trifid nebula.
These clouds of ionized gas typically have masses in the range of 100-10,000 that of the sun. Their size can vary from being less than a light year across to many hundreds, resulting in a great variation of their densities from 1 to 1,000,000 atoms per cubic centimetre. Unsurprisingly given they contain ionized gas, emission nebulae are very hot. Their average temperature is of the order of 10,000 kelvin.
Often a cloud of gas is surrounded by surrounded by O and B type stars. These emit large quantities of Ultraviolet radiation which ionzies hydrogen present within the nebula forming a cloud of protons and electrons. When these recombine to produce hydrogen, they emit light with a wavelength of 656.3 nm. This light is red and the reason may emission nebula have a red/pink colour.
A planetary nebula is an example of an emission nebula. it is comprised of a white dwarf surrounded by various ionized gas, usually other than hydrogen. This leads to other colours, such as blue caused by singly ionized helium and green caused by doubly ionized oxygen. A spectacular example of this is the Ring nebula that contains several colours.
When light is reflected off dust, it is often polarized. As the size of dust particles tends to be similar to the wavelength of visible light, blue light is scattered more than red. This results in the nebula having a similar spectra to the stars illuminating them but stronger in the blue component. Ultimately, this means they tend to appear blue. It is difficult to estimate the size, as only the portion of the cloud that is reflecting enough light to be detected can be seen.
Reflection nebulae can be determined to be a reflection nebula using spectroscopic methods; if a nebula is a reflection nebula, it should have the roughly same spectra as nearby stars.