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Introductory Astrophysics:
Emission Lines

There are three types of spectra:
-Continuous spectra – (white light through a spectroscope)
-Bright emission lines – (light from a heated gas)
-Dark absorption lines – (white light through a cold gas)
The “Pauli Exclusion Principle” states that the quantum numbers (n,L,m,s) are unique to each
electron in an atom
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Ion terminology in Astrophysics:
HI
HII
HIII
HIV

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Neutral Hydrogen
One electron removed
Two electrons removed
Three electrons removed

Etc…
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If these rules are not obeyed, they are “Forbidden Transitions”
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Forbidden transitions are denoted using square brackets
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Transitions onto n=2 level of Hydrogren

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Balmer continuum emission

Free-bound transitions to other levels

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Lymann/Paschen continuum

The overall result is that the spectrum of planetary nebula (for example), is very different to that of a
star
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A note on recombination:
Photons escape
Photons absorbed

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Case ‘A’ recombination
Case ‘B’ recombination

Case B results in no Lymann α photons
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( )

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‘Lf’ is the planck function for the star’s temperature, and ‘fo’ is the ionization frequency

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