Mr Eddington, The Distribution of the Stars, ete. 351 
: 
__ The Distribution of the Stars in relation to Spectral Type. By 
Professor A. S. EppinGton, Trinity College. 
[Read 24 November 1913] 
plane is not shown equally by the different spectral classes. 
Type B is the most condensed, and the others follow in the order 
A, F, G, K, M, ie. the sequence coincides with the usually 
accepted order of evolution. Formerly it seemed probable that 
this result was due to a progression in the average distance of 
these classes of stars, for, on the hypothesis that the stellar system 
is of oblate form, the greater the distance the greater will be the 
concentration to be expected. This explanation fitted in well with 
certain direct evidence as to the luminosities of the ditferent 
spectral types. Recent determinations by Boss and Campbell 
of the average distances of the stars of different spectral types 
negative this explanation in a most decided manner. It appears, 
for instance, that the M stars are on the average more remote and 
more luminous than Type A. We have to return to the view 
that there is a real difference in the distribution of the spectral 
types. Apparently the stars have been formed mainly in the 
galactic plane; the earliest type with their small velocities have 
not strayed far from it; the latest type with their large velocities 
have had time to become much more uniformly dispersed. 
There is an outstanding question of great difficulty. In 
parallax investigations it is found that the M stars are the faintest 
of all the types; in statistical discussions of proper motions, etc., 
they are found to be the brightest except Type B. Similar 
difficulties occur with the other types. Russell has put forward 
the theory that Type M consists of two divisions, one being the 
very earliest and the other the latest stage in evolution. Against 
this it may be urged that both divisions of Type M are charac- 
terised by very high velocities in space; this seems to indicate 
a close relation between them. Further, as far as statistical 
investigations are concerned, Russell’s theory inverts the usually 
accepted order of stellar evolution. The result arrived at in the 
previous paragraph would thus have to be reversed,—the stars as 
formed are fairly uniformly dispersed and have high velocities; 
afterwards they lose their velocities and become concentrated to 
the galactic plane. This is not so intelligible as the previous 
conclusion. 
It is well known that the concentration of stars to the galactic 
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