In order to facilitate the discussion of the relation between colour and spectrum it is convenient to deal here with the question of the spectra of the stars.
10.
Spectra of the stars. In order to introduce the discussion I first give a list of the wave-lengths of the Frauenhofer lines in the spectrum, and the corresponding chemical elements.
| Frauenhofer line | Element | λ |
| A | 759.4 | |
| B | 686.8 | |
| C(α) | H (hydrogen) | 656.3 |
| D1 | Na (sodium) | 589.6 |
| D3 | He | 587.6 |
| E | Fe (iron) | 527.0 |
| F(β) | H | 486.2 |
| (γ) | H | 434.1 |
| G | Ca (calcium) | 430.8 |
| h(δ) | H | 410.2 |
| H(ε) | Ca(H) | 396.9 |
| K | Ca | 393.4 |
The first column gives the Frauenhofer denomination of each line. Moreover the hydrogen lines α, β, γ, δ, ε are denoted. The second column gives the name of the corresponding element, to which each line is to be attributed. The third column gives the wave-length expressed in millionths of a millimeter as unit (μμ).
On [plate III], where the classification of the stellar spectra according to the Harvard system is reproduced, will be found also the wave-lengths of the principal H and He lines.
By the visual spectrum is usually understood the part of the radiation between the Frauenhofer lines A to H (λ = 760 to 400 μμ), whereas the photographic spectrum generally lies between F and K (λ = 500 to 400 μμ).
In the earliest days of spectroscopy the spectra of the stars were classified according to their visual spectra. This classification was introduced by Secchi and was later more precisely defined by Vogel. The three classes I, II, III of Vogel correspond approximately to the colour classification into white, yellow, and red stars. Photography has now almost entirely taken the place of visual observations of spectra, so that Secchi's and Vogel's definitions of the stellar spectra are no longer applicable. The terminology now used was introduced by Pickering and Miss Cannon and embraces a great many types, of which we here describe the principal forms as they are defined in Part. II of Vol. XXVIII of the Annals of the Harvard Observatory. It may be remarked that Pickering first arranged the types in alphabetical order A, B, C, &c., supposing that order to correspond to the temperature of the stars. Later this was found to be partly wrong, and in particular it was found that the B-stars may be hotter than those of type A. The following is the temperature-order of the spectra according to the opinion of the Harvard astronomers.
Type O (Wolf-Rayet stars). The spectra of these stars consist mainly of bright lines. They are characterized by the bright bands at wave-lengths 463 μμ and 469 μμ, and the line at 501 μμ characteristic of gaseous nebulae is sometimes present.