Spectra of Different Luminosity Classes
Presence of different lines determined by Spectral Class  (temperature)
Width of  individual lines determined by Luminosity Class
“Pressure broadening”:
High density (so high pressure) frequent atomic collisions
Energy levels shifted by 2nd nearby atom  broad lines
Main sequence stars are “high” density and pressure
Supergiants are low density and pressure
Something can cause a main sequence star to expand to a large size
to form a giant or supergiant