The properties determined by the atomic nucleus are the mass, and the isotopic and radioactive properties. The astrophysical study of these factors is as yet in an elementary stage, but it seems that all three have a bearing on the frequency of atomic species, and that future theory may also relate them to the problem of the source and fate of stellar energy. Moreover, up to the present no general formulation of the theory of the formation and stability of the elements has been possible, and it is well to keep in mind the data which are apparently most relevant to the problem—the observational facts relating to the nucleus. Probably the study of the nucleus involves the most fundamental[Pg 5] of all cosmical problems—a problem, moreover, which is largely in the hands of the laboratory physicist. The chief nuclear data are summarized in Table I. Successive columns contain the atomic number, the element and its chemical symbol, the atomic weight[1] and the mass numbers of the known isotopes,[2] the percentage terrestrial abundance,[3] expressed in atoms, and the recorded stellar occurrence. Presence in the stars is indicated by an asterisk, absence by a dash. TABLE I No. Element Atomic Weight Isotopes Percentage Terrestrial Abundance (Atoms) Stellar Occurrences 1 Hydrogen H 1.008 1.008 15.459 * 2 Helium He 4.00 4 .. * 3 Lithium Li 6.94 7, 6 0.0129 * 4 Beryllium Be 9.01 9 0.0020 — 5 Boron B 11.0 11, 10 0.0016 — 6 Carbon C 12.005 12 0.2069 * 7 Nitrogen N 14.01 14 0.0383 * 8 Oxygen O 16.00 16 59.940 * 9 Fluorine F 19.0 19 0.0282 — 10 Neon Ne 20.0 20, 22, (21) .. — 11 Sodium Na 23.00 23 2.028 * 12 Magnesium Mg 24.32 24, 25, 26 1.426 * 13 Aluminium Al 27.1 4.946 * 14 Silicon Si 28.3 28, 29, 30 16.235 * 15 Phosphorus P 31.04 31 0.0818 — 16 Sulphur S 32.06 32 0.0518 * 17 Chlorine Cl 35.46 35, 37, (39) 0.1149 — 18 Argon A 39.88 40, 36 .. — 19 Potassium K 39.10 39, 41 1.088 * 20 Calcium Ca 40.07 (40, 44) 1.503 * 21 Scandium Sc 44.1 45 .. * 22 Titanium Ti 48.1 48 0.2407 * 23 Vanadium V 51.0 51 0.0133 * [Pg 6] 24 Chromium Cr 52.0 52 0.0213 * 25 Manganese Mn 54.93 55 0.0351 * 26 Iron Fe 55.84 54, 56 1.485 * 27 Cobalt Co 58.97 59 0.0009 * 28 Nickel Ni 58.68 58, 60 0.0091 * 29 Copper Cu 63.57 63, 65 0.0028 * 30 Zinc Zn 65.37 (64, 66, 68, 70) 0.0011 *