The letters at the top of the diagram represent spectral types of stars, because these provide a better and more exact working classification than the names of colours. The colours which approximately correspond to the various spectral types are indicated at the bottom of the diagram. Only a very few sample stars are shewn, but all known stars are found to be concentrated around the positions of these few typical stars. Broadly speaking, there are two distinct and disconnected regions which are occupied by stars. First, and most important, is a region shaped rather like a reversed γ: the central line of this region is marked in by a continuous thick line, following a determination of its position by Redman. Second, there is a smaller region near the left-hand bottom corner of the diagram. The stars which occupy this region are very faint, and have far higher surface-temperatures than other stars of similar luminosity. We have already seen how a star’s diameter can be calculated from its surface-temperature and luminosity. This amounts to the same thing as saying that two stars which occupy the same position in the Russell diagram must have the same diameter. Thus there is a definite diameter associated with each point in the diagram, and we can map out stellar diameters in the diagram, just as we can map out heights above sea-level on a geographical map, by a system of “contour lines.” In the present case the “contour lines” prove to be a system of almost parallel curves. These lie roughly as shewn by the broken lines in fig. 22, all stars lying on any one line having the same diameter. [Illustration: Fig. 22. The Russell diagram.] This diagram throws a flood of light on the general question of stellar diameters. We see at once that stars of the biggest diameters—100 times the sun’s diameter or more—must necessarily be red stars of high luminosity. And in actual fact the stars of large diameter shewn in the table on p. 272 are all red and have very high luminosities; they are red giants. The majority of the stars in the sky lie in the belt which runs across the diagram of fig. 22 from top left-hand to bottom right-hand. This is known as the “main-sequence.” The position of this band with reference to the “contour lines” of diameters shews that main-sequence stars are of moderate diameters. The brightest of all may have twenty times the diameter of the sun, while the faintest may have only about a twentieth of the sun’s diameter, but they all have diameters which are at least comparable with that of the sun. The sample of stars from near the sun, which we have already discussed, provides many instances of main-sequence stars; we have, in order of decreasing luminosity: