1. Hartmann, J.F. (1904) “Investigations of the Spectrum and Orbit of Delta Orionis.” Astrophys. J., 19:268.
2. An angstrom is the optical astronomer’s unit of wavelength. 1 Å = 10-8 cm.
3. Hartmann, op. cit.
4. Slipher, V.M. (1909) “Peculiar Star Spectra Suggestive of Selective Absorption of Light in Space.” Lowell Obs. Bull., 2:2.
5. Stars are classified according to a scheme which ranks their intrinsic luminosity which is roughly related to the star’s mass and age. The sequence O, B, A, F. G, K, M is one that proceeds from massive, luminous, young stars to smaller, less luminous, older stars.