COSMOGONY — JEANS 17 I 



THE IMMENSITY OF TIME 



The immensity of space is paralleled by lluil ol' time. We can 

 estimate the a^es of stars from the impression that time has made 

 upon them, just as we estimate the age of a tree from the number of 

 subdivisions of its stem, or of rings in its cross section. There are 

 three principal methods of doing this. The orbits of binary stars, 

 which are circuhir at birth, are gradually knocked out of shape by 

 the forces from passing stars. As we can calculate the rate at which 

 this process occurs, the shape of stars' orbits can be made to reveal 

 their ages. The moving clusters provide a second method. Groups 

 of bright stars such as the Great Bear, the Pleiades, Orion's Belt, 

 are often found to consist of exceptionally massive stars which move 

 in regular orderly formation through a jumble of slighter stars, like 

 a flight of swans through a confused crowd of rooks and starlings. 

 Swans, however, are conscious beings and continually adjust their 

 flight so as to preserve their formation. The swanlike stars can not 

 do this, so that their orderly formation must in time be broken by the 

 gravitational pull of other stars. When this happens, the lighter 

 stars are naturall}' knocked out of formation first, Avhile the most 

 massive stars retain their formation longest. This agrees with what 

 is observed, and as we can calculate the time necessary to knock out 

 the lighter stars, we can at once deduce the ages of those which are 

 left in. A third method of investigation rests upon a rather abstruse 

 dynamical tlieorem, which shows that after a sufficient time the 

 energies of motion of the different types of stars must tend to 

 equality, the little stars making up for the smallness of their mass 

 by the rapidity of their motion. Scares has shown that the stars 

 near the sun have nearly attained to this ideal state, and as we can 

 calculate the time needed to establish it we can again deduce the ages 

 of the stars. 



It is gratifying and significant that all three lines of investigation 

 lead to the same result: The stars are found to be some millions of 

 millions of years old, perhaps from 5 to 10 millions of millions. 

 We can not state their age with much precision, but it is the general 

 order of magnitude, not the exact figure, that is important. 



STELLAR RADIATION 



Year after year, century after century, for millions of millions of 

 3'ears, the sun radiates enough energy from each square inch of its 

 surface to keep a 50-horsepower engine continually in action; still 

 hotter stars may radiate as much as 30,000 horsepower per square 

 inch. If this energy were produced by the combustion of coal, the 

 stars would all be completely burnt out in a few hundreds or thou- 



