As the name suggests, an emission nebula emits or "produces" radiation, normally electromagnetic covering a range of wavelengths. For such a process to occur, the nebula needs to be provided with a supply of energy: a nearby star, or group of stars can do the trick. Clearly, there will be regions of space containing high concentrations of atoms, but without a convenient power source. Such nebulae may well be undetectable.
 
 

We will consider the emission process in IC 434 and NGC 2024 in the context of the Hydrogen atom. Remember that an atom of H consists of one proton and one electron (ignoring the isotopes Deuterium and Tritium).
 

NGC 2024

At the temperatures found in space, even within a nebula, most H atoms have their electron in the most energetically stable state, the GROUND STATE. For the H atom, this state lies at –13.6 eV (with respect to "infinity" as the zero reference point).

According to quantum mechanics, the bound electron in the H atom cannot have a continuous range of values for its total energy. Instead, its energy is quantized: only certain values of E_TOTAL are allowed. We can picture the allowed energy values on an energy "ladder diagram", as shown in the diagram below:
 
 
 

If, however, the H atom is in proximity to an energy supply the atomic electron can absorb energy and become excited to one of the higher energy levels. If the energy supply is in the form of photons, the incoming photon energy must exactly match the energy difference (D E) between the initial and final energy levels. If this is so, the electron is "raised" to a higher energy level and then rapidly (< 10^-7 s) de-excites, cascading down the energy levels. Each transitionfrom one level to a level of lower energy emits a photon in a random direction. The animation, below, shows this idea in action:

 

The transition from n = 3 n = 2 is the dominant transition in H gas, and produces photons with l = 656.3nm. This is in the red region of the spectrum: this is why IC 434 glows predominantly red.

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• Transitions from any level down to level 1 (the ground state) produce UV photons (the so called Lyman series)
• Transitions from a higher level down to level 2 (first excited state) produce photons in the visible region of the EM spectrum (the Balmer series).
• Any transitions from n = 4, 5 etc. down to n=3 give rise to IR photons.

A moment’s thought should now make you wonder what would be the situation if the H atoms were much closer to a hot star, which is irradiating them with copious amounts of photons ?
 
 

Many of these photons will lie in the Ultraviolet region of the EM spectrum (look at a Black body curve for an object with a temperature of about 20000K to see this). UV photons have a higher energy than visible light photons. It requires a minimum of 13.6eV to ionise a Hydrogen atom in its ground state. This corresponds to a UV photon with a wavelength of about 91nm.
 
 

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In a so-called H II REGION, there is such a high flux of UV radiation with wavelengths less than 91nm (and therefore energies greater than 13.6eV) that most of the hydrogen gas is ionised. You could visualise such a region as consisting of a collection of protons (hydrogen nuclei) surrounded by a "sea" of ionised, free electrons.

The diagram gives an overall picture of both these types of region. Notice that the H II region contains neutral He atoms: it takes UV photons with energy considerably higher than 13.6eV to ionise the strongly bound He atom.
By chance, some of the protons and electrons can recombine to re-form a neutral H atom. The re-formed H atom may have its electron in one of the higher energy levels: as the electrons cascade down to lower levels radiation is emitted. Of course, in a radiation-dense environment, the neutral H atom may be ionised again, and the process repeats itself.
 
 

So in an H II region, hydrogen atoms absorb high energy UV photons from the star and re-emit them as lower energy photons as the atom de-excites. This process is called fluorescence.
 
 

The hot stars in IC 434 and NGC 2024 produce this effect. So our emission nebulae consist of both H I and H II regions, emitting radiation.
 
 

Other examples of H II regions are the Orion Nebula and the Lagoon Nebula (in the constellation Sagittarius). Both are powered by very hot (O and B-class) stars.