ASTRONOMY: W. S. ADAMS 
159 
the intensities of the bands. Two conclusions may be drawn at once 
from these results: First, that the Harvard system of classification, in 
which the M type stars are all included in one group on the basis of the 
presence of the bands, fails entirely to discriminate between the spec- 
tral pecuHarities of the high and the low luminosity M stars; and sec- 
ond, that the intensity of the hydrogen lines in the M stars probably 
varies with the absolute magnitude, the brighter stars having the stronger 
hydrogen lines. 
A method of determining the absolute magnitudes of stars from the 
characteristics of certain of their spectral lines has been described in a 
previous communication.' The essential feature of this method is the 
use of the two lines X 4216 of strontium and X 4455 of calcium, the in- 
tensities of which appear to be connected directly with the intrinsic 
brightness of the stars in whose spectra they occur. The intensities of 
these lines relative to other lines in the spectrum are estimated, and a 
numerical relationship is established between these intensity ratios and 
absolute magnitude by means of a selection of stars of known parallax. 
In this way the following formulae applicable to stars of types G8-K4 
have been derived. M is the absolute magnitude, and A the intensity 
ratio for each pair of lines. 
4216 4455 4455 
4250 4462 4494 
M=-1.6A+4.7 M = +1.6A+5.1 M = +2.3A-0.3 
It is this set of formulae which has been used in the case of the M stars 
of high luminosity. The average type of these stars was found to be 
G7, which is sufhciently near the limits of the group to admit of the ap- 
plication of the corresponding equations. Summarized briefly the re- 
sults for the high and the low luminosity stars are as follows : 
Average No. oj Average 
Spectmm Stars M Range of M 
High luminosity G7 48 +1.4 - 1 . 0 to + 3 . 4 
Low luminosity Ma 10 +10.3 +9.8 to +10.7 
Of the high luminosity stars only two, a Orionis and Boss 660, have 
negative values of the absolute magnitude, and only five stars have 
values exceeding 2.0. The remaining 41 stars have magnitudes ranging 
between 0.0 and 2.0. It is clear, accordingly, that on the basis of abso- 
lute magnitude determinations the M stars fall into two clearly defined 
groups, separated by an interval of about 7 magnitudes within which 
no intermediate values have been found. 
The spectroscopic evidence, therefore, confirms the h3rpothesis of 
Hertzsprung and Russell that the M type stars are divided into two 
