20 
ASTRONOMY: KAPTEYN AND ADAMS 
slowly than the fainter stars must be accepted with considerable reserve. 
Certain features of this investigation are necessarily somewhat crude, 
and the data upon which it rests were not collected with a view to such 
a question. Moreover, of two direct observational points of evidence 
one appears to be opposed to this conclusion. 
The stars denoted as c stars by Miss Maury were shown by Hertz- 
sprung to be exceedingly luminous and very distant. According to an 
unpubHshed investigation by Kapteyn and Hertzsprung these stars, of 
average magnitude 4.5, must be 300 times more luminous and 4.5 times 
as far away as the average A stars of the fifth magnitude. We should, 
therefore, expect a low velocity in their case. In reality the average 
radial velocity of 28 of these stars is 12.8 km. For the average of 
all the A stars Campbell finds 11.1 km. In considering this result it 
should be borne in mind that the effect found here is for the second 
type stars and may not apply to the A stars; also that the c stars may 
actually constitute a separate spectral class with which the A stars are 
not directly comparable. 
It is known from the researches of Hertzsprung that the variable 
stars of the d Cephei type are stars of very high luminosity. He finds 
for their average absolute magnitude the value —2.2, using for this 
purpose their parallactic motions and apparent magnitudes. The aver- 
age absolute magnitude of the 198 stars given by Campbell is -f 1.8. 
The average radial velocity, freed from the sun's motion, of 11 of the 
b Cephei variables, of average type -F8, is 9.0 + 1.2 km. The average 
radial velocity of Campbell's F stars is 14.75 km. This would give 
1.5 km. as the value for our coefficient h. 
SUMMARY OF RESULTS 
1. The radial velocities furnish a very thorough test of the theory 
of the star streams. The results found for the F, G, K, and M stars are 
in close agreement with those we should expect from the theory as de- 
rived from proper motions. 
2. The radial velocities of the stars of the smallest proper motions show 
the effects of the two star-streams with the same certainty as those of 
the other stars. The existence of the two star-streams is, therefore, 
proved at the greatest distances for which we have adequate data. 
3. The K stars behave in general like the other stars, but there are a 
few exceptional cases. These do not appear to be due to the absence 
of the second stream. 
4. For all of the spectral classes the average radial velocities show a 
regular increase with the proper motion. ^ 
