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ASTRONOMY: H. N. RUSSELL 
I. The Individual Stars. — Existing methods of investigation have 
already put at our disposal a great mass of information regarding the 
physical characteristics of the stars — mass, density, luminosity, color, 
spectrum, temperature, and so on. The central problem of stellar 
astronomy may be formulated as follows: From the existing data, and 
from all further data which may be secured by methods new or old, 
to deduce a theory of stellar evolution, that is, of the changes in the 
temperature, density, brightness, spectrum, and other observable char- 
acteristics of a star with the progress of time, and of the dependence of 
these changes upon those factors which are invariant for a given 
system, such as mass, angular momentum and chemical composition. 
Such a theory must satisfactorily represent the observed properties of 
the general run of the stars, and the relative abundance of the different 
types, and must be capable of extension so as to account for the excep- 
tions to the usual rules. 
Among the subsidiary problems whose solution is bound up with that 
of the main problem are {a) that of the evolution of binary systems, 
whether by fission, tidal action, or otherwise; {h) that of the causes and 
mechanism of variable brightness among the stars; {c) that of the source 
of the energy which the stars- radiate into space in such enormous 
amounts. 
These problems of stellar astronomy are mainly physical in character, 
though some phases, such as {a) are mainly dynamical. 
//. The Galactic System. — ^The great majority, if not all, of the vis- 
ible stars appear to belong to an assemblage limited in space, either by 
regions nearly void of stars, or by absorbing material which conceals 
whatever may be immersed in it. Within this galactic system we may 
investigate the distribution of the stars in space, and its variation for 
stars of different spectral type, absolute magnitude, etc., the motions of 
the stars (including the Sun) , and the phenomena of preferential motion 
(^star-streaming') in certain directions, and the dependence of these 
,motions upon spectral type, absolute magnitude, etc.; and the asso- 
ciation of the stars into sub-groups, or clusters, and the motions of 
these clusters. 
All these studies lead up to a single ultimate problem, which may be 
defined as the representation of the present positions and motions of the 
stars as a stage in the history of a dynamical system (whether in a 
steady state or not) and the deduction of the presumable history of the 
system in the past and the future. Among the subsidiary problems con- 
nected with this are {a) the existence, character, distribution and gravi- 
tational influence of possible dark or absorbing matter in space; {h) the 
