Three types of spectrum
Light from astronomical sources can form a continuous spectrum, an emission-line spectrum or an absorption-line spectrum. The type observed depends on the physical conditions of the emitting material and any gas through which the radiation passes.
| Spectrum | Appearance | Typical conditions |
|---|---|---|
| Continuous | A continuous range of wavelengths | Hot, dense matter, such as the dense regions within a star |
| Emission line | Separate bright lines at particular wavelengths | Hot, low-pressure gas |
| Absorption line | Dark lines within a continuous spectrum | Continuous radiation passing through cooler, low-pressure gas |
Why spectral lines form
Atoms have discrete electron energy levels. Electrons can therefore emit or absorb only photons whose energies correspond to differences between allowed energy levels.
- In an emission spectrum, excited atoms emit photons at selected wavelengths.
- In an absorption spectrum, atoms remove selected wavelengths from otherwise continuous radiation.
- For the same element, its emission and absorption lines occur at corresponding wavelengths.
The line positions in the emission and absorption examples coincide because both are set by the same energy-level differences within the atoms.