A spectral line is like a fingerprint that can be used to identify the atoms, elements or molecules that are present in a star, galaxy or cloud of gas. If we separate the incoming light from a celestial source into its component wavelengths, we will see a spectrum crossed with discrete lines. Dark lines (or lines of reduced intensity) are called absorption lines, while brighter lines (lines of higher intensity) are called emission lines.

The presence of spectral lines is governed by quantum mechanics, which describes the discrete energy levels of an atom, element or molecule. Only photons with energies exactly equal to the energy difference between two energy levels can be emitted or absorbed.

In principle, we should expect all spectral lines to be extremely thin and correspond to a single wavelength. However, in practice this is not the case. According to Heisenberg’s uncertainly principle, the product of the uncertainty in the measurement of energy, ΔE, and time Δt is:

ΔEΔt ≥ h/2π

where h is Planck’s constant. The result is a natural spread of photon energies around the spectral line. The longer an excited state exists (Δt), the narrower the line width so that metastable states can have very narrow lines.

Several other effects can cause the spectral lines we observe to be become broader than we would predict due to the Uncertainty Principle. These include:

• The Zeeman effect
• Thermal Doppler broadening
• Collisional broadening
• Velocity broadening

By measuring the amount of broadening we can determine properties such as the temperature and density of a gas, and even identify the presence of a magnetic field.