Constitution and Evolution of the Stars 103
rocks in which they occur. The latter can often be defined
in geological terms, and hence we can date the various
geologic periods, finding a good general agreement with
the geological order of succession. The oldest minerals
so far studied are found in rocks of Middle Pre-Cambrian
age. Specimens from Europe, Africa and America agree
in giving ages of between a thousand and twelve hundred
millions of years. These individual crystals have been in
the rocks for all this time. The Earth, as a planet, must
be older. The speaker, from consideration of the whole
amount of uranium and lead in the Earth’s crust, showed
last year that its age is apparently less than eight billions
of years, and probably something like four billions. If,
as seems most probable, the planets were produced by
eruptions from the Sun, under the tidal influence of a
passing star, the Sun itself must have been already formed
at that remote epoch.
But we may go further. Life already existed on the
Earth in Pre-Cambrian times, and it is a moderate estimate
to say that the process of organic evolution has lasted for
a billion years. During all this time the Sun can never
have been one stellar magnitude brighter or fainter than
it is now, for in the first case, its heat would have raised
the oceans to the boiling point, and in the second, they
would have frozen solid—and either of these catastrophes
would have put an end to evolution and to all terrestrial
life. Now the Sun is a typical dwarf star, and there is
good reason to believe that it is now well advanced in
cooling and was once much brighter and hotter than it is
now—of Class F, at least, though perhaps not of Class A.
At such a time it must have been at least two magnitudes
brighter than at present. Yet in the whole of geological
time it has probably decreased half a magnitude or less.