SURVEY OF THE PROBLEM
3
/
showing strongly
tars; it is rather
¡nsity favourable
ng the possibility
ssary to account
ume that in the
e not connected
f fixed composi-
lown wards. Its
pressure; hence
l entirely on the
led; and these
gravity holding
'ed into it from
ere must adjust
'face conditions
lurface and the
a conventional
er unit area; it
arly significant
e of the stellar
ve must expect
ofold sequence,
iensional order,
ional order was
qjes. But the
due to Adams
ctra transverse
riterion follows
e parameter g ;
one time sup-
ial distribution
T e and g into
coincidence of
3 . Consider now the connection between our three pairs of parameters
—g and T e ; Draper Type and Absolute Magnitude criterion; M and R —
any pair defining a unique state of the star. The connection of the spectral
criteria with g and T e is a problem of great importance in which much
recent progress has been made; but it is not a problem of the stellar
interior and lies outside the main lines of our investigation. As regards
the connection of g and T e with M and R, the connection of g needs no
comment; the main question is, How is T e , or equivalently the rate of
outflow of radiation, determined by the mass and radius of the star?
That is the central problem of this book. Various branches of inquiry
will diverge from it; but it supplies the continuous thread in the dis
cussion, so long as we are studying the stellar interior.
This is essentially a problem of the stellar interior and not of superficial
conditions. The sun does not radiate 6.10 4 * * * * * 10 ergs per square centimetre per
second because its photosphere is at 6000° C.; its photosphere is main
tained at 6000° because 6.10 10 ergs are streaming through it. It is under
the temperature gradient in the interior that the radiant stream gathers
way; the surface layers cannot dam the flow since their capacity for
storing energy is insignificant; they can only adjust themselves to let it
through. Qualitatively the radiant stream is greatly transformed in
passing through the last few thousand kilometres of the star, and the
actual waves that spread through space are born in the photospheric
layers; but quantitatively it is one continuous stream passing from the
interior into outer space.
The intensity of this outward flow of energy through the interior
depends on two factors, the one helping and the other hindering. Heat
flows from a higher to a lower temperature, and the cause of the flow
within the star must be a gradually increasing temperature from the surface
to the centre. The hindering factor is the obstruction opposed by matter
to the transmission of this stream of heat. We shall find that in a star
the heat is transmitted almost entirely by radiation, and the obstruction
to the flow of radiation is the opacity or absorption coefficient of the stellar
material. Our problem is, therefore, firstly to find the distribution of
temperature inside a star so as to determine the temperature gradient
urging the flow; secondly, to determine the opacity of matter under the
physical conditions prevailing in the interior.
when we con-
by considering
>y stars of the
; third way of
tes is obtained
•ameters.
4 . Here at the outset we must deal with a criticism urged by Nernst,
Jeans and others. It has been argued that this procedure for calculating
the outward flow of radiation is necessarily doomed to failure, because
the star’s output of heat energy is determined by entirely different con
siderations. The supply of heat replenishing that which the star radiates
into space must come from the conversion of other forms of energy ; and
1-2