32 Eridani, yellow and blue-green—“magnifici, superbi,” according to Secchi. β Capricorni (close to the splendid naked-eye double α Capricorni), orange-yellow and blue. γ Crucis, orange-yellow, companion fifth magnitude, rather distant. γ Leporis, companion crimson. There is also a third faint star, forming a triple group. β Capricorni, like Antares, besides its visible companion, has a close invisible one, only known by the shifting of lines in the spectrum, and this is not an uncommon case. β Crucis consists similarly of two bright stars and a spectroscopic companion, and so also θ Eridani. κ Velorum and α Pavonis have spectroscopic companions only, one revolving in a period of 116½ days, the other of only 11¾ days, and the period of μ¹ Scorpii is counted in hours!—34 hours 42 minutes. The brief periods of these spectroscopic binaries[6] are in striking contrast with those of many visual binaries, such as ζ Sagittarii with a period of 19 years and γ Centauri with 150 years; and this is what one would expect, since the stars must be comparatively far apart and their orbits ample for them to be visible separately. Sometimes the stars of a pair or of a group are known to be moving together through space, though no movement of revolution round a common centre has yet been detected, probably because it is very slow. [6] A “binary” is a system of two stars which are known to be comparatively close together and influencing one another’s movements. A “double star” may be a binary, or the two stars may really be very far apart and have no connection, merely happening to lie one nearly behind the other. It is an interesting fact that most of the spectroscopic binaries, which are such close and rapid pairs, are found among the blue and white stars, the numbers steadily decreasing as we pass through the yellow to the red stars. A significant fact about visual binaries is that companions which differ much in colour invariably differ much in brightness also. This is probably to be explained by supposing that one was from the beginning much larger than the other, and that there is consequently a difference in the rapidity with which each runs through its life-changes. Where the two are alike in colour and spectrum, like the two solar stars of α Centauri and the two Orion stars of α Crucis, it is found that they are also nearly equal in mass, so they keep the same pace and grow old together.