2 
The Astronomical Survey of the Universe [ch. i 
will be its final end ? If the heavenly bodies can no longer be regarded as 
having been created merely to minister to man’s pleasure and comfort by 
illuminating the earth, what purpose, if any, do they serve? And if life, and 
human life in particular, can no longer be supposed to be the central fact 
which explains everything, what is its relation to the magnificent, stupendous, 
almost terrifying, universe revealed by astronomy ? 
To these obstinate questions observational astronomy provides no answer. 
Her task is limited to a mere description of the universe, which others may 
interpret if they can. Another science, cosmogony, provides material that 
may help to this end, which those who essay to interpret the universe can 
only disregard at their peril. Cosmogony studies the changes which the play 
of natural forces must inevitably produce in the objects discovered by the 
astronomer; it tries to peer back into their past and to foresee their future, 
guided always by the principle that the laws of nature have moulded the 
present out of the past, and will in the same way mould the future out of the 
present. Taking as its starting-point the still picture presented by astronomy, 
it attempts to create a living cinematograph film which will exhibit the uni 
verse growing, developing and decaying before our eyes. The sequence of 
events depicted in this film will be false unless the relation of each picture to 
the succeeding one is that of cause to inevitable effect. 
Between observational astronomy and cosmogony there intervenes a third 
science, or branch of science, namely, cosmical physics. Cosmogony proceeds 
on the supposition that the matter of which the universe is constituted behaves 
as directed by natural laws, so that a knowledge as to the particular kind of 
matter with which we are dealing is a prerequisite to knowing what particular 
laws this matter will obey. As the laws of a liquid are different from those of a 
gas, a liquid star will behave differently from a gaseous star, and before we can 
predict the behaviour of a star we must know the state of the matter composing 
it. Cosmical physics attempts to provide the necessary information by de 
ducing, with such precision as is possible, the physical nature and structure of 
astronomical bodies, and of cosmical matter in general, from the observations 
of the astronomer. 
The ultimate object of cosmogony, and of the present book in particular, 
is to construct a sequence of pictures which will provide a contribution to 
wards answering the questions of whence and whither by revealing the past 
and future of the ever-changing universe. But before attacking the main 
problem we must study the physical constitution of the bodies with which we 
are dealing, and as a preliminary to this we shall survey the universe revealed 
by observational astronomy. Cosmical physics will occupy the first five chapters 
of our book, but the present chapter will merely describe the picture which 
observational astronomy provides—for cosmical physics to interpret, and for 
cosmogony to extend into the past and future.