The opacity is greatest at the central wavelength _o, and decreases as we shift from the core of the line to the wings. As the opacity increases, we achieve a of two-thirds for a shorter and shorter path length, and we see less far into the medium (photons can travel less far before being absorbed, so those that escape are emitted from the highest regions of the stellar atmosphere). The line center thus come from higher (cooler) regions of the atmosphere, while the profile wings come from deeper (hotter) zones.

[NMSU, N. Vogt]

There are three primary mechanisms for line broadening: natural, Doppler, and pressure (collisional) broadening. For the H line in the solar spectrum, natural broadening contributes 2 × 10^-4 Angstroms, Doppler broadening 0.4 Angstroms, and pressure broadening 1 × 10^-4 Angstroms.

Natural broadening:
We apply Heisenberg's uncertainty principle to the uncertainty in the orbital energy and the electron lifetime in an upper level.

Doppler broadening:
In thermal equilibrium, the atoms in a gas move randomly with a velocity distribution which can be modeled as a Maxwell-Boltzmann function (between v and v + dv for a mass m of atoms).

Pressure (collisional) broadening:
Electron orbitals can be perturbed by collisions with neutral atoms or by a close encounter with the electric field of an atom. Individual collisions combine to produce an overall collisional broadening in the line, with pressure broadening based on the statistical sum of numerous close passes near to ions. The effect scales with the time interval between encounters.

Remember that pressure broadening scales with n, the density of the medium, and so can become much stronger in high density environments. In low density environments, the line is optically thin and the equivalent width EW is proportional to n. At intermediate values of n, the core of the line saturates (becomes optically thick) but the wings remain optically thin and can deepen (EW goes as the square root of log(n)). For high densities, pressure broadening can deepen the wings and EW scales with the square root of n. For example, absorption features in luminous giant stars are narrow due to their low atmospheric densities, while pressure broadening widens the lines in denser Main Sequence star atmospheres where collisions are more common.