SIZE AND AGE OF THE UNIVERSE— JEANS 



135 



Finally, the linear motions of ordinary single stars provide yet 

 another range of "pendulums." We know that the molecules in a 

 gas tend to share their energy, until finally all types of molecules, big 

 and small, light and heavy, have, on the average, the same amount of 

 energy. In the same way the stars tend to share their energy, and 

 groups of stars of different masses form a range of pendulums, the 

 most massive stars sharing their energy most slowly and the lightest 

 the most rapidly. 



Table 4 gives the average linear velocities of stars of different 

 masses as determined by Seares at Mount Wilson. We see that all 

 except the most massive stars are well on toward equipartition of 

 energ}', all having an average energy which is not very far from 

 3,750 in the units we are using. 



Table 4. — The linear velocities of stars of different masses 



These and various other "pendulums" agree in suggesting that we 

 must assign to the universe an age of 5 to 10 millions of millions of 

 years. 



THE SOURCE OF STELLAR ENERGY 



Let us now consider the state of things 5 millions of millions of 

 years ago. Observation shows that the sun is at present radiating 

 energy away at the rate of about 250 million tons a minute. This 

 time yesterday, then, it weighed 360,000 million tons more than now. 

 A million million years ago, it weighed a very great, but still calcula- 

 ble, number of tons more than now ; it was about 6 percent more mas- 

 sive than now; and, because of this, it was also a brighter star than 

 now, radiating not 250 million tons a minute, but about 300 million 

 tons a minute. After adjusting our calculations for such considera- 

 tions as this, we find that 5 million million years ago, the sun was 

 probably many times as massive as now and many times as bright. In 

 the intervening period, it has been gradually getting rid of its mass 

 in the form of radiation, until it is reduced to a mere relic of its 

 former magnificance. 



