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