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.
