Why Emission Nebulae emit light
Emission nebula emit light due to the excitation of gas within them. To emit light gas must be
present close to a large star. Being large the star emits copious amounts of ultra-violet light. It is
this light that causes the gas to emit light.
Most high school graduates have an image that an atom contains a dense central nucleus
containing protons and neutrons. The neuclus is surrounded by a cloud of electrons occupying a large volume of
empty space. It is now understood that the electrons can only occupy certain configurations which is based on
An atom becomes excited when a photon is absorbed by an electron lifting it to a higher energy
level. In nebulae this is the absorption of a photon of ultraviolet light. The energy of the photon must either
equal or be greater than the amount of energy required to lift the electron to the higher state.
When an electron that has been lifted to at higher energy level drops back down to a lower
level it emits light at a certain frequency. It is this light that we see in emision nebulae.
You may have noticed than most emission nebulae are red. This is the result of the
dominate hydrogen gas emitting light in the red part of the light spectrum. Sulphur also emits light in the
red part of the spectrum while oxygen emits in the green region. Certain transitions within the hydrogen atom emits
light in the blue part of the spectrum.
For more information about the physics behind this you can visit the emission spectrum on Wikipedia. If you visit this page
the first image shows the emission spectrum of hydrogen. The red colouring of nebulae is the result of the
right-most emission line. This is called the hydrogen alpha emission line and more information on that can
be found the hydrogen alpha page.