ASTRONOMY: W. S. ADAMS 
163 
be shown to the best advantage by a reproduction of the curves repre- 
senting the estimated intensity differences for the pairs of Hues used in 
the determinations of absolute magnitude. These are given in figure 1. 
The curves are based upon essentially all of the stars with observed par- 
allaxes for which we have spectral observations. Each point on the 
curves represents the mean for a considerable number of stars; and, as 
these stars differ in absolute magnitude, the corresponding intensity dif- 
ferences for the pairs of lines will differ. In types F and G the higher 
and lower luminosity values and the line differences balance one another 
so nearly that the successive values show but a gradual change, and 
the curves make but a slight angle with the horizontal axis. At about 
K3, however, the curves begin to bend abruptly, and the remaining 
types depart from the axis very rapidly. This is due to the absence of 
stars of even moderately high luminosity among those upon which the 
curves are based. 
The corresponding curves for the high luminosity stars of these types 
run nearly parallel to the horizontal axis. We find, accordingly, both 
for types K5-K9 and M, a branching of the curves which points directly 
toward the existence of a division into two distinct groups. This evi- 
dence is based upon all of the spectroscopic material available. 
In conclusion reference should be made to the necessity of adding to 
the symbols used in the Harvard system of classification for the M stars 
some character or figure which shall serve to distinguish between the 
spectral characteristics of the high and the low luminosity stars. The 
most important of these is the difference in the intensity of the hydrogen 
lines. Accordingly, though somewhat cumbersome in practice, I can 
think at present of no method which would convey the necessary in- 
formation in any better way than by adding to the classification based 
on band intensity the corresponding classification based on hydrogen 
line intensity. Thus Mb (G6) would indicate a spectrum in which the 
bands are strong but the hydrogen lines give a type of G6. On this 
basis the low luminosity M stars would be of normal type and would 
require no suffix. 
^ Ann. Obs., Harv. Coll., 28, 160. 
2 These Proceedings, 1, 481 (1915). ' 
3 These Proceedings, 2, 143 (1916). 
* Summarized by Eddington in Stellar Movements and the Structure of the Universe, pp. 
170-177. 
