The O-type stars constitute the hottest normal stars, and are characterized by moderately weak hydrogen lines, lines of neutral helium (He I) and by lines of singly ionized helium (He II). The spectral type can be judged easily by the ratio of the strengths of lines of He I to He II; He I tends to increase in strength with decreasing temperature while He II decreases in strength. The ratio He I 4471 to He II 4542 shows this trend clearly.
Visible in a number of stars in this figure are two interstellar features, due to interstellar gas. The Ca II K line (labeled "K (I.S.)") is an interstellar feature, as well as the diffuse interstellar band (labeled "I.S. band") at a wavelength of about 4430 Å.
These spectra, and the spectra in the following pages (unless otherwise noted) were obtained on the Gray/Miller spectrograph on the 0.8m telescope of the Dark Sky Observatory, using the 1200 g/mm grating which gives a resolution of 1.8 Å.