We know that these two stars should contrast significantly, Betelgeuse being reddish and Rigel being bluish to the naked eye. The above spectra bring out the differences quite well. Note the broad spectrum of Betelgeuse, brightening up at longer wavelengths than Rigel, and then extending into the Infra Red. Rigel's is brightest in blue and dims out by 7000 Angstroms. Since Betelgeuse is a red giant at an advanced age, its spectrum is much more complex than for younger stars like Sirius; narrow lines like H-alpha and H-beta are broadened, and it even shows molecular lines.