+-------------------+------------+-----------+ | Star |Diameter in | Diameter | | |terms of sun| in miles | +-------------------+------------+-----------+ |Antares | 450 |390,000,000| |α Herculis | about 400 |346,000,000| |ο Ceti (at max.) | 300 |260,000,000| |Betelgeux (average)| 250 |216,000,000| +-------------------+------------+-----------+ All these diameters have been measured directly by the interferometer. On the scale used in figs. 17 to 20, in which the sun is about the size of a sixpence, the circle necessary to represent ο Ceti would be as large as the floor of a good-sized room, while the second star of the system (for ο Ceti is binary) would be the size of a grain of sand. We may obtain some idea of the immense size of these stars by noticing that every one of their diameters is larger than the diameter of the earth’s orbit, so that if the sun were to expand to the size of any one of them we should find ourselves inside it. These stars must be exceedingly tenuous. Antares, for instance, occupies 90,000,000 times as much space as the sun, so that if its substance were as closely packed, it would weigh 90,000,000 times as much as the sun. Yet, in actual fact, it probably has only about 40 or 50 times the sun’s weight, the difference between this number and 90,000,000 arising from the difference between the densities of Antares and the sun. On the average a ton of matter in the sun occupies considerably less than a cubic yard; in Antares it occupies rather more space than the interior of Saint Paul’s Cathedral. Yet a detailed study of stellar interiors shews that we can attach but little meaning to an average of this sort. It is quite likely that matter at the centre of Antares is packed nearly (but not quite) as closely as matter at the centre of the sun (p. 291 below). The huge size of Antares is probably due mainly to an enormously extended atmosphere of very tenuous gas, and there is not much point in striking an average between this and the compact matter at the centre of the star.