ASTRONOMY: C. D. PERRINE 
71 
DEPENDENCE OF THE SPECTRAL RELATION OF DOUBLE STARS 
UPON DISTANCE 
By C. D. Perrine 
Observatorio Nacional Argentino, Cordoba 
Communicated by E. B. Frost, January 7, 1918 
In the course of an investigation of the cause of the spectral differences 
of the stars, an examination of the spectra of the components of double stars 
was made. Almost at once it was seen that there was a relation other than 
the well known conclusion that in contrasted pairs the fainter component is 
generally of the earlier type. It was found that such stars are almost invari- 
ably very distant, and that the reverse generally occurs in the near pairs. 
The principal data used in this investigation were taken from the obser- 
vations of the spectra of 745 double stars made at Harvard and classified by 
Miss Cannon. 1 Seventy-eight stars were selected of which the spectra of both 
components had been determined, having a difference of brightness of at 
least half a magnitude and known to be either binary or to have common 
proper motion. 
It was noticed that the stars in which the fainter component was of the 
earlier type were, in general, closer together than those in which the com- 
panion was of a later type. The effect of distance on the separations of these 
stars was investigated by means of proper motion. The result indicated a 
greater average absolute separation in both classes than for the stars whose 
magnitudes and spectral classes are nearly the same. Incidentally, however, 
it was shown that the pairs in which the fainter component is of earlier type 
are distant, whereas those in which the fainter companion is of later type, 
with the exception of the stars both of whose components are of classes B 
and A, are much nearer. 
Of the stars selected, 24 have the fainter components of the same spectral 
type, 26 have the fainter components of later and 28 of earlier spectral type. 
The last two groups are given in detail in the tables following. 
With the exception of the pairs (12 in number) of table 1, which have 
both components of classes B and A and whose proper motions are less than 
0".05, the remainder show large differences of spectral type and large average 
proper motion. It is to be noted that 4 of the 12 stars with small /jl in this 
table, having the fainter component of later type, belong to 15 Cephei. The 
consistency in the spectral relations of this group is noticeable. 
The pairs of table 2, with two exceptions, show small proper motions. 
The proper motions of these two stars cannot be said to be large and it is fur- 
ther to be noted that their differences of spectral type are rather small. The 
1 Ann. Harvard Coll. Obs., Cambridge, 56, no. 7. 
