The faint companion Sirius _B_ is one of the most interesting stars in the sky. It is of nearly the same colour and spectral type as Sirius _A_, but emits only a ten-thousandth part as much light. After allowing for the slight difference in surface-temperature, we find that its surface is only ¹/₂₅₀₀th, and its diameter ¹/₅₀th of that of Sirius _A_. Yet Sirius _A_ weighs only three times as much as Sirius _B_, although having 125,000 times its volume. It is not Sirius _A_ but Sirius _B_ that is remarkable; the average density of matter in the latter is about 60,000 times that of water, the average cubic inch containing nearly a ton of matter. Fig. 18 shews the sizes of the two components of Sirius drawn to the same scale as fig. 17. _B.D. 12° 4523 and Innes 11 h. 12 m., 57·2°._ Two stars, as to the physical state of which nothing is known, except that they are very faint, emitting ¹/₁₄₀₀th and 1/10,000th of the sun’s light respectively. _Cordoba 5 h. 343 and τ Ceti._ Two faint stars, both of reddish colour, emitting ¹/₆₀₀th and a third of the sun’s light respectively. [Illustration: Fig. 19. The System of Procyon, with the Sun for comparison.] _The System of Procyon._ This is a binary system, similar in many respects to Sirius. The main star, Procyon _A_, is of the same general type as the sun, but weighs 24 per cent. more, and emits 5½ times as much light. Its surface-temperature is about 7000 degrees, and its diameter is 1·80 times that of the sun. The faint companion, Procyon _B_, is so faint that nothing is known as to its physical condition except that it emits only 1/30,000th of the light of the sun. Its weight is 39 per cent. of the sun’s weight. Fig. 19 shews the sizes of the two components of Procyon on the same scale as before. Next in order, as we recede from the sun, come eight very undistinguished stars, every one of which is redder and fainter than the sun, none of them having a surface-temperature higher than 5000 degrees, and none of them emitting more than a quarter of the sun’s light. After these we come to: _The System of Kruger 60._ This is a binary system in which both components are small, red and dim. [Illustration: Fig. 20. The System of Kruger 60, with the Sun for comparison.] The brighter component, Kruger 60 _A_, has a surface-temperature of 3200 degrees, and emits ¹/₄₀₀th of the light of the sun. Its diameter is a third, and its weight a quarter of the sun’s; so that its substance must be packed about 7 times as closely as that of the sun. The fainter component, Kruger 60 _B_, has a similar surface-temperature but emits only 1/14,000th of the sun’s light. Its diameter is a sixth, and its weight a fifth of the sun’s; so that its substance must be packed about 40 times as closely as that of the sun. The system is illustrated in fig. 20.