Astrophotography by Rob


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 energy levels.

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.


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