α Aquilæ, _see_ Altair α Canis Majoris, _see_ Sirius α Canis Minoris, _see_ Procyon α Centauri, distance, 33, 35 luminosity and weight, 48 system of, 40, 263, 279, 280 α Herculis, 259, 272 α Lyrae, 33 α Orionis, _see_ Betelgeux α-particles, 112, 116, 117, 118 and Plate XIII (p. 113) α-rays, 111 α Scorpii, _see_ Antares Absolute temperature, 102 Absorption lines and spectrum, 51, 126 Actino-uranium, 154, 155 Adams, J. C., 18, 19 Adams, W. S., 62, 343 Age of earth, 14, 151 ff., 248 of stars, 155 ff. of sun, 180, 184 of universe, 336 Aitken, R. G., 171, 188 Altair (α Aquilæ), 33, 278 Andromeda, Great Nebula _M_ 31 in, Plate IV (p. 30), 30, 70, 71, 72, 204, 206, 208, 215, 335 age, 335 distance, 71 rotation, 206 size, 30 weight, 71, 208, 215 Andromedid meteors, 251 Angstrom unit, defined, 256 Angular momentum, conservation of, 205, 217, 232 of solar system, 232 Annihilation of matter, 185, 191, 193, 298, 331 Antares (α Scorpii), 259, 272, 273, 278 internal constitution of, 273, 291 Aquarid meteors, 251 Aristarchus of Samos, 2, 13 Aristotle, 2, 5 Arrangement of solar system, 16, 240 ff. Bode’s law, 19, 20 Copernican, 3, 5, 6, 27, 32 Ptolemaic, 2, 4, 6, 8, 31 Asteroids, 17, 226, 240, 247, 252, 348 Ceres, 17 distance of, 19, 20, 252 Eros, 34 origin of, 247, 252 Aston, F. W., 119, 154, 185 Atmosphere, of earth, 195, 343 of moon, 196 of planets, 196, 343, 344, 348 of sun, 195 Atomic nuclei, 108, 117 disintegration of, 111, 117 size and weight of, 109 structure of, 119, 139, 147 Atomic numbers and weights, 110, 287 in stellar matter, 306, 311 Atomic theory, 92, 103 Atoms, 103, 318 structure of, 106 synthesis of, 148, 149, 185 Availability of Energy, 326, 329