CHAPTER XIII 
DIFFUSE MATTER IN SPACE 
The Temperature of Space. 
256. The total light received by us from the stars is estimated to be 
equivalent to about 1000 stars of the first magnitude. Allowing an average 
correction to reduce visual to bolometric magnitude for stars of types 
other than F and G, the heat received from the stars may be taken to 
correspond to 2000 stars of apparent bolometric magnitude 1-0. We shall 
first calculate the energy-density of this radiation. 
A star of absolute bolometric magnitude 1-0 radiates 36*3 times as 
much energy as the sun or 1-37.10 35 ergs per sec. This gives M5.10" 5 ergs 
per sq. cm. per sec. over a sphere of 10 parsecs (3-08.10 19 cm.) radius. The 
corresponding energy-density is obtained by dividing by the velocity of 
propagation and amounts to 3-83.10“ 16 ergs per cu. cm. At 10 parsecs 
distance the apparent magnitude is equal to the absolute magnitude; 
hence the energy-density 3-83.10~ 16 corresponds to apparent bolometric 
magnitude 1-0. 
Accordingly the total radiation of the stars has an energy-density 
2000 x 3-83.10- 16 = 7-67.10- 13 ergs/cm. 3 
By the formula E — aT 4 the effective temperature corresponding to this 
density is 
3°-18 absolute. 
In a region of space not in the neighbourhood of any star this 
constitutes the whole field of radiation, and a black body, e.g. a black 
bulb thermometer, will there take up a temperature of 3°T8 so that its 
emission may balance the radiation falling on it and absorbed by it. This 
is sometimes called the “temperature of interstellar space.” 
It is possible, however, for matter which has strong selective absorption 
to rise to very much higher temperature. Attention was called to the 
possible astrophysical importance of this effect by C. Fabry*. Radiation 
in interstellar space is about as far from thermodynamical equilibrium as 
it is possible to imagine, and although its density corresponds to 3°-18 it 
is much richer in high-frequency constituents than equilibrium radiation 
of that temperature. It is convenient to exhibit this by stating for each 
wave-length À an equivalent temperature T K such that the actual density 
Astrophys. Journ. 45, p. 269.