Neutral aluminum is represented in the solar spectrum by the lines, the series that constitutes the ultimate lines in the third column elements of the periodic table.[258] The two conspicuous lines of the series in aluminum are those at 3944, 3957, and they are strengthened in cool stars,[259] in accordance with theory. They are especially mentioned as being strong in the spectrum[260] of 61 Cygni, as might be expected for a dwarf star. The series is also traced in the solar spectrum, but is too far in the ultra-violet to be studied effectively in the stars. The series lines[261] of Al+ and Al++, although they might be expected in the stars, apparently have not yet been traced in stellar spectra. SILICON (14) Four stages of the silicon atom are observed in stellar spectra. The line at 3905 is found in the coolest stars, has an observed [Pg 69] maximum[262] at , and disappears at about . This line is regarded by A. Fowler[263] as the ultimate line of the neutral atom, and on this basis an ionization potential of 10.6 volts was assigned to silicon. In view of the fact that the line appears to have a maximum within the stellar sequence, and is of temperature class II, according to King,[264] while the true ultimate line[265] of silicon is at 2881, it seems possible that 3905 is actually a subordinate line. IONIZED SILICON Ionized silicon is represented by the lines 4128, 4131, which appear at , attain maximum[266] at , and disappear at . These lines are of especial interest, as they form the characteristic feature of the “silicon stars” which occur in the early classes. The silicon stars are specially discussed[267] in Chapter XII. DOUBLY IONIZED SILICON The lines associated with the atom Si++ which appear in stellar spectra are the three at the wave-lengths 4552, 4568, 4574. These lines are first seen at , have a maximum[268] between and , and disappear at . Fowler[269] regards these lines as constituting a principal triplet; it might be expected, however, that principal lines would show a more persistent maximum. TRIPLY IONIZED SILICON [Pg 70] The atom of silicon which has lost three electrons is the most highly ionized atom of which we have certain evidence in stellar spectra. The lines at 4089, 4096, and 4116 are strong[270] among the cooler stars, and are last seen at Class . The hotter stars, such as H.D. 165052, do not display the lines of Si+++, and probably the intensity of the lines has fallen, owing to the temperature, which is above that required for the maximum of these lines. SULPHUR (16) The spectrum of neutral sulphur which has hitherto been analyzed is chiefly in the far ultra-violet,[271] and is therefore not traceable in the sun or stars.