1884.] 
Gaseous Heavenly Bodies. 
539 
only fixed stars of great mass ever reach that temperature 
which corresponds to the emission of a bluish white light, 
there lesults the following proposition : 
The duration of the transition from a reddish to a bluish 
light is always very small in comparison to the duration of 
the subsequent return from a bluish to a reddish light. 
It therefore we have two fixed stars, A and B, which simul- 
taneously reach the culminating point of their heat-radiation, 
A possessing a relatively large and B a relatively small mass, 
A will at. first outstrip B in its development, and reach the 
culmmatmg point of its temperature of eradiation earlier 
than B. . Subsequently it will again lose the advance which 
it had gamed. During the very protracted period of cooling- 
down A will again be overtaken by B in the process of 
lefrigeiation, so that B will reach the end of this period 
^ ian ^ ^ as . a temperature of eradiation 
than B, and not only attains this temperature earlier than B, 
but letains it longer, A will at all times have a higher tem- 
perature of eradiation than B. 
If therefore two such stars, which simultaneously reached 
the culminating point of their heat-radiation, are, for the 
sake of brevity, designated as “ coeval ” stars, we arrive at 
the following proposition : — 
Of two coeval stars, the one whose light approaches in 
colour nearest to the red extremity of the speCtrum has the 
smaller mass. 
. ^ b°th stars have a relatively low age and are at equal 
distances from the earth, the red star, in spite of its smaller 
mass, may appear as the brighter of the two, inasmuch as 
it is not yet as far removed from the culminating point of its 
heat-eradiation, or brightness, — and inversely, from the 
greater brightness of the redder star we may infer the com- 
paratively low age of both. As in double stars the brighter 
of the two is generally designated as the principal, the fol- 
lowing proposition would follow on the supposition of the 
equal age of both : — 
If the colour of the principal star lies nearer the red ex- 
tremity of the prism than that of its companion, the prin- 
cipal is the one possesing the smaller mass, and both stars 
are still — if we may use the expression — in a relatively 
youthful stage. 
During the contraction of a gaseous cosmic body the 
parts belonging to the superficial stratum approach the 
centre ; and as the speed of this subsidence can never ex- 
ceed the speed of a freely falling body, we may deduce an 
inferior limit for the duration of the nebular spot period 
2, N 2 
