386-388] The Ages of the Stars 417 
Terrestrial evidence indicates that the age of the earth must be between 
1500 and 5000 million years, so that this is probably the time which has 
elapsed since the planets and their satellites were bom out of the sun. But 
stellar evidence demands far greater ages for the stars. We have estimated 
that the time necessary for the primaeval chaos to condense into nebulae must 
have been much more than 60,000 million years; the time for the condensa 
tions in these nebulae to form stars rotating with some approach to uniformity 
of angular velocity, such as is shewn by the sun, is of the order of 10 13 years. 
The time required for the motions of the stars to acquire the observed ap 
proximation to a final steady state of equipartition of energy is again of the 
order of 10 13 years. The distribution of the eccentricities and periods in the 
orbits of binary stars is such as to suggest that these orbits have been moulded 
by long ages of interaction with other stars, and calculation enables us to fix 
the length of the necessary time as being of the order of 10 13 years. Older 
binary systems shew a closer approach to equality of mass in their two 
constituents than is shewn by younger systems, and calculation shews that 
the time necessary to establish the approximation to equality observed in 
the older system is of the order of 10 13 years. The galactic system is a 
confused medley of stars moving nearly at random, but still shewing some 
faint evidence of ordered motion. A few of the more massive stars move in 
orderly parallel motion like flights of swans or ducks; the majority move with 
the random motion of a cloud of starlings. But it seems reasonable to 
suppose that at one time these latter also shewed the same type of ordered 
motion as is still shewn by their more massive companions. A dynamical 
investigation has shewn that the time necessary to break up the formation 
of the less massive stars would again be of the order of 10 13 years. Thus all 
available lines of evidence agree in assigning lives of the order of millions of 
millions of years to the stars. 
388 . We observe a star because it is emitting radiation. Modern physical 
theory teaches that all radiation carries mass with it, so that the emission of 
radiation necessarily diminishes the mass of a star. The rate of diminution 
of a star’s mass is readily calculated; the rate at which the sun’s mass is 
diminishing is found, for instance, to be 250 million tons a minute. 
Neither the sun nor any other star can continue radiating away its mass 
at such a rate for ever. The mass of a star fixes a limit to the total amount 
of radiation it can emit, just as surely as the volume of a cistern fixes a limit 
to the total amount of water it can emit. In the case of the star there is no 
replenishment, or at least none comparable with the amount lost. The sun 
loses 250 million tons of mass every minute, and there is no known source of 
replenishment which can supply new mass to it at even a small fraction of 
this rate. 
Calculation shews that a star whose age is 10 13 years must in the course 
of its life have emitted radiation whose total mass is many times greater than