COSMOGONY—JEANS 159 
ple one that the origin of a star’s heat is the star’s mass. He lives by 
transforming his mass into radiation; we can estimate his present age 
by noticing how much of him is left, and another calculation, based 
on the same datum tells us how much longer a life he may expect. 
The interval from top to bottom of the evolutionary ladder, about 
two hundred million million years, is the total life of a star, and 
stars differ one from another mainly in being merely higher or 
lower on the ladder, younger or older. 
The ages of the stars are not the same thing as the age of the 
universe, nor even are they necessarily comparable with that age. 
The stars may be likened to icebergs coming down from the North 
and melting as they drift into tropical waters. We can estimate 
the ages of the icebergs within our vision, but we can not say for 
how long the stream of icebergs has been drifting down from pole 
to equator nor for how long new icebergs will continue to form 
and come down to replace those that pass southward to their 
doom. Over the polar regions where the icebergs are born a veil 
of fog is drawn, and we do not know how to look behind that veil. 
But the problem of the ages of those stars which are now in being 
is a comparatively simple one, and for all practical purposes these 
constitute the universe for the astronomer and cosmogonist alike. 
To each star can be assigned a total span of life of the order of a 
hundred million million years followed by darkness and _ possibly 
ultimate extinction; to our sun we can assign a past life of about 
seven million million years, so that as regards time, although not 
as regards magnificence, the greater part of his life is yet to come. 
The ages which we must now attribute to our sun and the other 
stars are many hundreds of times longer than was, until quite re- 
cently, thought probable or even possible. This extension of the 
time scale will call for a rearrangement of ideas in many depart- 
ments of cosmogony and astronomy. Many of the questions involved 
are of a highly technical nature, but one is comparatively simple as 
well as of great interest. Of the various theories which have been 
put forward to explain the origin of our earth and the other planets, 
the so-called tidal theory seems (to the present writer at least) to offer 
enormously more advantages and to be open to far fewer objections 
than any of the others. According to this theory, our sun, some time 
in the past in his voyage through space, must have encountered a 
star more massive than himself traveling on a course which came so 
near to his own that enormous tides were raised on his surface, tides 
of such colossal height that the tops of the tidal waves lost all con- 
tact with the underlying parts and started on independent careers 
of their own as planets. When submitted to mathematical treat- 
ment this theory shows itself able to account for the main features of 
