148 
ASTRONOMY: W. 5. ADAMS 
of the stars in which they occur, we have available a means of determin- 
ing the absolute magnitudes* of stars, and hence their distances. 
The first attempt to detect such lines was made by Hertzsprung, who 
concluded that the strontium line at X 4077 gave some indication of vary- 
ing with the absolute magnitudes of the stars in whose spectra it ap- 
peared. Quite independently Dr. Kohlschiitter in the course of his 
studies of the classification of the Mount Wilson stellar spectra found 
two or three lines which appeared to vary in this way, and some results 
of an application of these lines to the determination of absolute mani- 
tudes were published in 1914. Since that time the work has been ex- 
tended greatly with the aid of the additional material available. The 
results of the investigation and of an attempt to utilize these criteria 
for the derivation of stellar distances are contained in this communica- 
tion. 
The first essential in beginning this research was an accurate classi- 
fication of the stellar spectra into the several types. This was carried 
out by the method already described (These Proceedings, 1, 481). 
Stars of the same type of spectrum but of very different absolute bright- 
ness were then compared with one another, and the relative intensities 
of the different spectral lines were examined carefully. 
To illustrate the procedure we may take as an example the two stars 
61^ Cygni and a Tauri. The parallaxes of these stars are 0."31 and 0."07, 
respectively, and their apparent magnitudes are 5.6 and 1.1. Their 
absolute magnitudes may be computed from the equation 
M = mH-5-f-5 1og7r 
in which M is the absolute magnitude, m the apparent magnitude, and 
TT the parallax. The absolute magnitudes, accordingly, are 8.0 and 0.4; 
that is, the luminosity of a Tauri is over 1100 times as great as that of 
61 ^ Cygni. A comparison of the spectra of the two stars side by side 
on a Hartmann spectrocomparator shows several points of difference. 
Of these, two are most important. The calcium line at X 4455 is very 
strong in 611 Cygni and relatively weak in a Tauri; and the strontium 
line at X 4216 is weak in 61^ Cygni and strong in a Tauri. That this 
difference in behavior depends upon physical conditions in the stars and 
is not merely accidental is made almost certain by solar investigations. 
The line X 4455 of calcium is greatly strengthened in the spectrum of 
*The absolute magnitude of a star is its apparent magnitude when reduced to unit 
distance. Tlie unit commonly employed is the distance corresponding to a parallax of 
0'!l. On this scale the absolute magnitude of the sun would be 5.5, or 4.8, if more recent, 
and probably better, values of the sun's photometric brightness are employed. 
