49 
). - 12°, 1172 
49-5 i] Stellar Spectra 49 
A star, however, is not a mass of matter at uniform temperature. Heat 
continually flows from its interior to its surface. Since heat always flows in 
the direction in which the temperature is falling, just as water flows in 
a direction in which the height is falling, a star’s temperature falls as the 
surface is approached. Energy which has been emitted in some internal 
layer of high temperature, and has the constitution appropriate to that high 
temperature, passes to outer space through surface-layers of lower temperature. 
These layers absorb radiation which is of a wave-length suitable for absorption— 
D. 35°, 4001 
i.e. radiation of those particular frequencies which represent jumps of the 
atoms from one possible state to another—but, on account of their lower 
'ephei 
temperatures, they do not emit an equal amount of radiation of the same 
wave-lengths. There is now no longer a statistical balance between the 
absorption and emission of the radiation of these particular wave-lengths, 
Irionis 
['nun 
lus 
but definite deficiencies in the radiation of the wave-lengths in question, 
and, when the radiation reaches outer space, these deficiencies reveal them 
selves by dark lines crossing the spectrum at the places corresponding to 
these special wave-lengths. These dark lines are called “absorption-lines.” 
The outer layer we have just described, the seat of the absorption by which 
these lines are produced, is called the “ reversing-layer ” of the star; the 
inner layer which produces the light which undergoes this absorption is called 
the “ photosphere.” 
leminorum 
We should accordingly expect the spectrum of a star to consist of the 
ocyon 
continuous bright spectrum already discussed, modified by the presence of 
absorption-lines. Such absorption-lines were first observed in the spectrum 
of the sun by Fraunhofer in 1814. 
pella 
The “ flash spectrum ” which appears at a total eclipse of the sun provides 
proof that the above explanation of the dark lines is the true one. After the 
photosphere has been covered by the advancing moon, the only light which 
reaches the earth is that emitted by the reversing layer, and the spectrum of 
cturus 
this light is found to consist of bright lines on a dark background, these lines 
having precisely the same wave-lengths as the Fraunhofer dark lines. This 
itelgeux 
proves that the atoms of the reversing layer are themselves emitting radiation 
of the wave-lengths in question; the Fraunhofer lines appear relatively dark 
merely because the reversing layer emits less radiation of these wave-lengths 
than it extracts from the light of the photosphere. 
Cygni 
A bsorption-Lines. 
Ceti 
51. Since Fraunhofer first observed absorption-lines in the spectrum of the 
sun, it has been found that not only the spectrum of the sun, but practically 
all stellar spectra are of the general type suggested by theory, consisting, 
that is to say, of a continuous bright background with absorption-lines 
superposed. 
J 4