Stellar spectra from class O to class M2 show how spectral lines vary with stellar temperature. In the spectra of hot stars we see weak Balmer lines and helium lines. The Balmer lines are strongest about A0, but are very weak in cool stars. The two ultraviolet lines of ionized calcium are strong in cooler stars, while sodium and titanium oxide (TiO) bands are strong in spectra of the coolest stars. Note that these synthetic spectra have been extended past the short wavelength limit for the human eye, about 420 nm, to show the H and K lines of calcium.