Stars higher above the main sequence in the HR diagram (refer back to Fig. 1.6) have higher luminosity and lower photospheric pressures. Indeed, pressure effects are often referred to as luminosity effects. Those pressure effects that are large enough to see at classification dispersion are used to assign the luminosity class to the spectrum. Generally speaking, spectroscopic pressure effects are more subtle than temperature effects. On the other hand, while temperature spans a factor of ten or so from O to M stars, pressure ranges over six orders from dwarfs to supergiants, and so even subtle effects are quite measurable in stellar spectra.

Continuum measurements across the Balmer jump can be used with the A and F stars to measure the electron pressure. A comparison of lines formed by neutral atoms to lines of ions gives a measure of electron pressure through the ionization equilibrium. Lines strong enough to show wings (including hydrogen) often have a wing strength that is dependent on the pressure through the van der Waals and Stark broadening. In each case there is also a temperature dependence, so this chapter is closely tied to Chapter 14. Either the temperature must be established securely before executing the pressure analysis or a simultaneous pressure–temperature solution must be made. There are also empirical relations between photospheric pressure and spectroscopic parameters such as macroturbulence.