Ozone.—The ozone bands which appear in solar and stellar spectra have been shown by Fowler and Strutt[244] to be of telluric origin. The maximum thermal formation of ozone occurs [245] at and 3500°, and thus its presence in giant and stars might possibly be anticipated. SODIUM (11) The ultimate lines () of the neutral atom of sodium are the lines, which lie at 5889, 5895. These are the only sodium lines which are certainly identified in stellar spectra.[246] They are first seen in the later classes, and appear to be strengthened in cool stars, in accordance with theory. Stationary sodium lines are observed[247] in Scorpii, Orionis, and other Class stars.[248] The lines are said to show an absolute magnitude effect, being strengthened in giant stars.[249] No lines of ionized sodium are found in stellar spectra, presumably because they all lie in the far ultra-violet. MAGNESIUM (12) The neutral atom of magnesium is represented in the solar spectrum by the , the , and the series, and the first triplet of the latter series constitutes the conspicuous “b” group in the green. The “b” group and the second member of the series, the triplet near 3800, are first seen[250] at , have a maximum near or , and are still strong in the coolest stars examined. The series, represented in the solar spectrum by a line at 4571, are faint ultimate lines; the strong ultimate lines[251] are the lines beginning at 2852, and are therefore outside the range of observed solar and stellar spectra. IONIZED MAGNESIUM The ionized magnesium atom gives rise to the important combination doublet at 4481 (). These lines appear in the sequence,[252] reach maximum[253] at (not at , as stated by several investigators), and are lost in the increasing strength of [Pg 68] the iron line at the same wave-length, at about . The doublet varies with absolute magnitude, and may be found to furnish a useful criterion of that quantity. It has been used by H. H. Plaskett[254] in the estimation of the temperatures of some of the stars of Class . COMPOUNDS OF MAGNESIUM Magnesium hydride.—The compound magnesium hydride, MgH₂, which has been studied in the laboratory by Brooks[255] and Fowler,[256] was detected by the latter in the sunspot spectrum.[257] It is perhaps significant that the only other hydride reported in celestial spectra is that of calcium, the next heavier alkaline earth after magnesium. ALUMINUM (13)