504. HARLOW SHAPLEY 
from all other types of variables, which almost without exception 
are giants in luminosity. 
6. In view of (a) the irregularities of the light variations, (6) 
the apparent immersion of the variables in nebulosity, and (c) the 
spectroscopic evidence for the irregular churning about of this 
nebulous matter, it seems reasonable to believe that the varia- 
tions in brightness result from collision or friction with the irregular 
nebulosity in which the variables are involved. The encounter 
of star with nebula is at present the best, though perhaps not an 
entirely satisfactory, explanation of the cause of galactic novae; 
and it is the only hypothesis that has been suggested to account 
for temporary stars in the rapidly moving spiral nebulae. 
7. Long-exposure spectrograms of the Orion nebula, using the 
too-inch reflector and a rapid focal-plane spectrograph, have 
recently shown in the bright-line spectrum the presence of hydrogen, 
nebulium, helium, carbon, and nitrogen; they also show a faint 
continuous spectrum in all parts of the nebula. 
8. The bearing of the foregoing observations on the question 
of geological climates becomes obvious when we note the following 
points: (a) The condition that causes variation of a star in the - 
Orion nebula must also gravely affect the atmosphere surround- 
ing any attendant planet. (6) The sun is moving with a velocity 
of 20 kilometers a second through a region of space, large sections 
of which are known to be occupied by diffuse nebulosity (most of 
it probably much less dense than that in Orion). (c) The observed 
variation of the friction variables in Orion is generally from 20 
to 80 per cent of the total light, and sometimes appears rapidly 
oscillatory, sometimes secularly progressive, sometimes a dis- 
continuous brightening or dimming. (d) A change of 20 per cent 
in the solar radiation, if maintained for a considerable period of 
years, would sufficiently alter terrestrial temperature to bring on 
or remove an ice sheet; an 80 per cent change, unless counteracted 
by concurrent changes in the terrestrial atmosphere, would com- 
pletely desiccate or congeal the surface of the earth. 
October, 1920 
