54 
The Light from the Stars [ch. ii 
the colour-index and bolometric correction do not depend solely on a stars 
spectral class. As regards colour-index, the deviation is shewn in the following 
table given by Seares*. 
Type 
Colour-index 
dense diffuse 
T (dense) - T(difruse) 
BO 
-0-32 
B 5 
-0i7 
A 0 
o-oo 
F 0 
0-38 
N 
GO 
0-72 
0-86 
470 
G 5 
0-83 
1*15 
890 
K 0 
0-99 
1-48 
1020 
Kh 
1-26 
1-84 
850 
MO 
1-76 
1-88 
250 
The temperature difference in the last column is somewhat provisional, 
being based on somewhat uncertain data, but it suffices to indicate the extent 
to which the spectra of stars of types G and K depend on the densities as 
well as on the effective temperatures of the stars, 
Estimates of Effective Temperatures. 
54. The effective temperature T e of a star has been defined to be such 
that the radiation emitted per second from each square centimetre of the star’s 
surface is aT e 4 , this being the amount of radiation that w r ould be emitted from 
a perfect radiator at temperature T e . 
We have found four ways of estimating the effective temperature of a 
star: 
(1) From a comparison of a stars observed angular diameter with the 
total radiation received from it (§41). 
(2) From the shapes of the energy curves obtained by analysing the 
distribution of energy in the star’s spectrum (§ 45). 
(3) From the colour-index, which gives, in a general way, the ratio of 
photographic to visual light emitted by a star (§§ 47, 53). 
(4) From the bolometric correction, which gives a measure of the ratio of 
a star’s total radiation to its emission of visual light (§ 48). 
That these four methods give fairly accordant values for the effective 
temperatures of stars of different spectral types, is shewn in the following 
table which is given by Russell, Dugan and Stewartf. 
* Astrophys. Journ. lv. (1922), p. 198. 
f Astronomy , p. 753.