292 The Evolution of Binary Systems [ch. xi 
Table XXIII. Binary Systems classified by Spectral Type (Aitken). 
Type of Star 
Spectral Class 
Number 
Mean 
Period 
Mean 
Eccentricity 
f 0 to B 8 
48 
26'76 days 
0-189 
Spectroscopic 
Binary 
B 9 to A 3 
A 5 to F2 
36 
10 
13'35 „ 
32-76 „ 
0-187 
0-252 
F 5 to 00 
13 
267-4 „ 
0-129 
0 5 to M 4 
8 
152-9 „ 
0-196 
0 to B 8 
0 
B 9 to A 3 
13 
98‘9 years 
0-529 
Visual 
A 5 to F2 
9 
100-6 „ 
0-512 
Binary 
F 5 to tr 0 
30 
78-7 „ 
0-478 
G 5 to K 2 
12 
86-0 „ 
0-432 
K 5 to M 4 
4 
126-7 „ 
0-402 
In the spectroscopic binaries it is impossible to say whether the eccen 
tricity increases or decreases with advancing spectral type; it is at any rate 
safe to conclude that there is no marked correlation between spectral type 
and eccentricity. As regards periods, the later type spectroscopic binaries 
have periods which are distinctly longer than those of earlier type, but there 
is no very marked correlation extending throughout the table. 
Visual binaries shew a complete absence of correlation between period 
and spectral type, but an appreciable correlation between eccentricity and 
spectral type in the sense of a decrease in eccentricity accompanying an advance 
in spectral type. 
Aitken’s classifications, given above, include only those binary orbits for 
which the data are thoroughly well determined. We must be very cautious in 
assuming that these orbits are representative of binary orbits in general, or in 
supposing that the characteristics we have noticed would prevail in a random 
selection of binaries. 
The spectral lines of early type stars are singularly ill-defined, so that the 
velocities in early type spectroscopic binaries can only be measured accurately 
when they are very large, the Doppler effect now becoming so marked that 
the breadth and fuzziness of the lines is of little account. As a consequence, 
early type binaries in which the periods are long, and the velocities con 
sequently small, are apt to be excluded from the table. This results in a 
tendency for the periods of early type binaries entered in the table to be 
short in comparison with the periods of late type binaries which are unaffected 
by this restriction. This consideration may account for some or all of the 
correlation between period and spectral type shewn in the table for spectro 
scopic binaries. 
Visual binary systems of classes 0, B and A are in general bright but 
remote from us, so that the two components must be remote from one another