For the remaining elements, phosphorus, chlorine, fluorine, zirconium and nickel, series relations are not, as yet, available. No lines of phosphorus or the halogens have been detected in stellar spectra, but these elements have not been satisfactorily analyzed spectroscopically, and their apparent absence from the stars is probably a result of a deficiency in suitable lines. Nickel and zirconium will probably be analyzed in the near future; they are both well represented in stellar spectra, and nickel especially is probably abundant.
The relative abundances, in the stellar atmosphere and the earth, of the elements that are known to occur in both, display a striking numerical parallelism. [Table XXIX] gives the data for the sixteen elements most abundant in the stellar atmosphere. Successive columns give the atomic number, the atom, the relative stellar abundance, the relative terrestrial abundance (both for the lithosphere, hydrosphere, and atmosphere, and for the whole earth),[493] and the relative abundance in stony meteorites.[494]
[TABLE XXIX]
| Atomic number | Atom | Stellar Abundance | Terrestrial Abundance | Abundance Stony Meteorites | |
|---|---|---|---|---|---|
| Crust | Whole Earth | ||||
| 14 | Si | 5.7 | 16.2 | 9.58 | 11.2 |
| 11 | Na | 5.7 | 2.02 | 0.97 | 0.6 |
| 12 | Mg | 4.2 | 0.42 | 3.38 | 2.8 |
| 13 | A1 | 3.6 | 4.95 | 2.66 | 1.1 |
| 6 | C | 3.6 | 0.21 | .... | .... |
| 20 | Ca | 2.9 | 1.50 | 1.08 | 0.56 |
| 26 | Fe | 2.5 | 1.48 | 46.37 | 5.92 |
| 30 | Zn | 0.57 | 0.0011 | .... | .... |
| 22 | Ti | 0.43 | 0.241 | 0.12 | .... |
| 25 | Mn | 0.36 | 0.035 | 0.06 | .... |
| 24 | Cr | 0.29 | 0.021 | 0.05 | 0.29 |
| 19 | K | 0.11 | 1.088 | 0.38 | 0.10 |
| 23 | V | 0.05 | 0.0133 | .... | .... |
| 38 | Sr | 0.002 | 0.0065 | .... | .... |
| 54 | Ba | 0.005 | 0.0098 | .... | .... |
| 3 | Li | 0.0000 | 0.0829 | .... | .... |
The figures in the fifth column are derived from Clarke’s estimates of the percentage composition of the earth. The composition of the earth has been variously estimated by different investigators, and the resulting figures depend upon theories that cannot be discussed here. The order given by Clarke is based on the assumption of a nickel-iron core.
The numbers expressing the stellar abundance are percentages, calculated on the assumption that the stellar and terrestrial elements form the same fraction of the total material present. This reduces the two columns of numbers to a form in which they are directly comparable, but no great importance is attached to the absolute percentages in the third column.
The method that has here been used is subject to inaccuracy and uncertainty, especially in the estimates of the exact spectral class at which a line is first or last seen. The most that can be expected is that the results will be trustworthy in order of magnitude. It may be seen that the only element for which the stellar and terrestrial values are not of the same order is zinc. Further, it appears that when the estimates for the percentage composition of the whole earth are used in the comparison with the stellar values, the agreement is improved in the case of silicon, magnesium, aluminum, manganese, chromium, and potassium; it is about the same for calcium and titanium, is less close for sodium, and markedly poorer for iron.[495] In the stellar atmosphere and the meteorite the agreement is good for all the atoms that are common to the two, but several important elements are not recorded in the meteorite.
The outstanding discrepancies between the astrophysical and terrestrial abundances are displayed for hydrogen and helium. The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real. Probably the result may be considered, for hydrogen, as another aspect of its abnormal behavior, already alluded to;[496] and helium, which has some features of astrophysical behavior in common with hydrogen, possibly deviates for similar reasons. The lines of both atoms appear to be far more persistent, at high and at low temperatures, than those of any other element.
The uniformity of composition of stellar atmospheres appears to be an established fact. The quantitative composition of the atmosphere of a star is derived, in the present chapter, from estimates of the “marginal appearance” of certain spectral lines, and the inferred composition displays a striking parallel with the composition of the earth.
The observations on abundance refer merely to the stellar atmosphere, and it is not possible to arrive in this way at conclusions as to internal composition. But marked differences of internal composition from star to star might be expected to affect the atmospheres to a noticeable extent, and it is therefore somewhat unlikely that such differences do occur.