A nebula is a vast interstellar cloud consisting of dust and gas (hydrogen, helium, nitrogen, oxygen, among others) that can span well into hundreds of light years and are typically thousands of light years away. It's common for nebulas to be associated with stars or star clusters. In fact, many nebulas are regions of active star formation.
There are three types of nebula; emission nebulas, reflection nebulas, and dark nebulas. Any given nebula may be one of, or a combination of, the different types. See below for more information.
An emission nebula is a type of nebula that actually emits its own light rather than just reflecting the starlight around it. This is made possible by the presence of large, hot stars that give off very intense ultraviolet radiation (significantly more than our sun). This radiation energizes the surrounding gas causing it to glow with very specific colors (or wavelengths). A similar process occurs inside fluorescent light bulbs!
There are many emission frequencies but only three or four are relevant to astrophotographers. The most luminous element in most emission nebulas is hydrogen, which glows intensely in the deep red (656nm) (called Hα or H-alpha) but also has a fairly prominent emission line in the blues (486 nm) (called Hβ
or H-beta). This is followed by oxygen, which glows with a teal color (501nm) (called [OIII], O3 or Oxygen-III) and is the reason why the great Orion nebula appears blue/green through a telescope. Lastly is sulfur, which also glows deep red (672nm) (called [SII], S2, or Sulfur-II) and tends to be fairly faint compared to hydrogen and oxygen but nonetheless imparts a rich amount of contrast to an image.
When imaging emission nebulas, it is preferable to use narrowband filters. These filters isolate the very specific emission wavelength (see above) of each the element being images (hydrogen, oxygen, and sulfur). As a result, almost all light pollution is blocked, and the star field is much dimmer, which creates an image with spectacular contrast; much more than would be possible if a narrowband filter was no used.
A reflection nebula, as the name implies, it a type of nebula that reflects light from nearby stars. This happens when the stars in the region of the nebula, don’t give off enough radiation to create an emission nebula (see above) but are still bright enough for the surrounding dust and gas to be visible.
Reflection nebulas are visible in the entire visible light spectrum (broadband) so must be imaged with a color camera or a monochrome camera with filters (red, green and blue). Because they are broadband targets, light pollution is a real problem if imaging in the city as it overpowers the light from the nebula. Because of this, a dark sky out from the city is necessary.
A dark nebula is an interstellar dust cloud that doesn’t emit or reflect light but instead blocks much of the visible light from the background stars (LBN 86 is a good example of this). These clouds take on all sorts of shapes, sizes, and degrees of opacity and can be seen right throughout the milky way region of the sky (although not exclusively
in that region). In fact, some are big enough to be seen with the naked eye such as the coal sack nebula located adjacent to the southern cross (a dark sky is required). As is common with all nebulas, dark nebulas may also have a reflection and/or an emission component such as the Chameleon Molecular Cloud.