,[361][362] and that thermal ionization is the predominant factor under these conditions.
The absorbing layer is to be regarded as consisting of a mixture of all chemical elements, without any assumption as to quantity, so long as the partial pressure of each individual element is low. In other words, no account is taken, at the present stage, of the relative abundances of different kinds of atoms—the total effectiveness of the corresponding elements as absorbers. The changes in the absorption of the black body radiation by a given element with changing temperature will be the same whatever the partial pressure, provided it is low, and it is with these changes that the preliminary schematic discussion is concerned.
LOW TEMPERATURE CONDITIONS
At low temperatures all the elements will tend to be in their normal atomic state, unless they are aggregated into molecules or compounds. At temperatures of 2500°, which is about the lower limit encountered in dealing with stellar spectra, there is evidence of the existence of various oxides (CO, TiO₂, ZrO₂), of “cyanogen,” and of hydrocarbons, but most of the other possible compounds appear either to be dissociated or to be in very low concentration. Probably the normally polyatomic gases such as oxygen, nitrogen, and sulphur, are to some extent present in the molecular state. Even at atmospheric pressure all the metals are vaporized at 2500° excepting tantalum and the platinum metals, which boil at about 2800° under a pressure of 760 mm; at lower pressures the temperature of vaporization is, of course, lower. The metallic molecule appears normally to be monatomic, so that it will give its line spectrum unless it is in combination. The fact that silicon, the most refractory substance, excepting carbon, with which we have to deal, gives its line spectrum in the coolest stars known, indicates that all the elements may be considered to be gaseous in stellar atmospheres.
ULTIMATE LINES
The absorption spectrum given by the reversing layer when it is at a low temperature will consist of the lines given most readily by the atom in its normal state. The energy transfers which move an electron from its normal orbit to another correspond to the “ultimate lines,” and these lines will therefore be especially outstanding in the spectra of the coolest atmospheres. They are of such importance, from theoretical and from practical standpoints, that a list of them is reproduced here. Successive columns of the table give the atomic number and atom, the ionization potential, the wave-lengths of the ultimate lines, and an indication of their observed occurrence in stellar spectra. An asterisk denotes that the line has been observed, and a dash indicates that it has not been recorded.
It may be remarked that the ultimate lines of sodium, potassium, lithium, rubidium, and caesium are in the visible region—a fact which is utilized in the laboratory flame tests used in qualitative analysis.[363] The brilliancy of the flame colors obtained in the Bunsen burner, at the temperature of about 1500°C., is a striking elementary illustration of the readiness with which the atom in its normal state will take up and re-emit the frequency corresponding to the ultimate lines (second pair for K, Rb, Cs).
[TABLE XVII]
| Atom | Ionization Potential | Wave-lenght | Stellar Ocurrence | |
|---|---|---|---|---|
| 1 | H | 13.54 | 1215 | - |
| 2 | He | 24.47 | 584, 557 | - |
| 3 | Li | 5.37 | 6707 | * |
| 4 | Be | ? | 2349 | - |
| 5 | B | ? | 2498, 2497 | - |
| 6 | C | ? | 2479 | - |
| 7 | N | ? | - | - |
| 8 | O | 13.56 | 1306, 1304, 1302 | - |
| 9 | F | ? | - | - |
| 10 | Ne | 16.7 | - | - |
| 11 | Na | 5.12 | 5896, 5890 | * |
| 12 | Mg | 7.61 | 2852 | - |
| 13 | Al | 5.96 | 3962, 3944 | - |
| 14 | Si | ? | 2882 | - |
| 15 | P | ? | 2553, 2536 | - |
| 16 | S | 10.31 | 1915, 1900 | - |
| 17 | Cl | ? | - | - |
| 18 | A | ? | - | - |
| 19 | K | 4.32 | 7699, 7665 | * |
| 20 | Ca | 6.09 | 4227 | * |
| 21 | Sc | ? | 4247, 3652 | * |
| 22 | Ti | 6.5 | 5065, 5040, 5014 | * |
| 23 | V | ? | 4331, 4333 | * |
| 24 | Cr | 6.75 | 4290, 4275, 5254 | * |
| 25 | Mn | 7.41 | 4034, 4033, 4031 | * |
| 26 | Fe | ? | 2756, 2749 | - |
| 27 | Co | ? | 3454, 3405 | * |
| 28 | Ni | ? | 3415 | * |
| 29 | Cu | 7.69 | 3274, 3248 | * |
| 30 | Zn | 9.35 | 2139 | - |
| 31 | Ga | 5.97 | 4172, 4033 | * |
| 37 | Rb | 4.16 | 7948, 7800 | * |
| 38 | Sr | 5.67 | 4607 | * |
| 40 | Zr | ? | 4496, 4392 | * |
| 42 | Mo | ? | 3903, 3864, 3798 | * |
| 47 | Ag | 7.54 | 3383, 3281 | * |
| 48 | Cd | 8.95 | 2288 | - |
| 49 | In | 5.76 | 4511, 4102 | * |
| 50 | Sn | ? | 3262 | ? |
| 55 | Cs | 3.88 | 8943, 8581 | - |
| 56 | Ba | 5.19 | 7911 | - |
| 57 | La | ? | 3949 | * |
| 79 | Au | 8.72 | 2676, 2428 | - |
| 80 | Hg | 10.39 | 2537, 1850 | - |
| 81 | Tl | 6.08 | 5350, 3775 | - |
| 82 | Pb | 7.38 | 4058, 3684 | * |