214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



Meteorites point to an age of the solar system, or its parent nebula, 

 close to 4,500 million years. 



THE AGES OF THE STARS 



At present there is little doubt that main-sequence ("dwarf") stars 

 depend upon the conversion of hydrogen into helium for their energy 

 source. The correlation of radius and mass, indicating central tem- 

 peratures of precisely the range required by the corresponding slow 

 nuclear reactions, can hardly be interpreted in a different manner. 

 This knowledge is so well founded that it furnishes a reliable basis for 

 the calculation of time rates of stellar evolution. 



To cover radiation losses to space, the sun has to spend an amount 

 of hydrogen very nearly equal to 1 percent of its mass in 1,000 million 

 years. Sirius, a typical star of spectrum AO quite common in the 

 galaxy, emits 13 times more energy per unit mass than the sun, con- 

 suming thus 13 percent hydrogen by weight in 1,000 million years. 

 With 60 percent hydrogen originally, the store of energy would last 

 4,600 million years. There is probably not much mixing in stars out- 

 side their central regions (35, 36, 37, 38) ; therefore, only about 25 

 percent of the fuel is available (from the central regions where the 

 temperature is high enough for nuclear reactions to proceed at a not- 

 negligible rate) , and the lifetime of Sirius becomes 1,150 million years. 

 It may then become a giant (35), and ultimately collapse — possibly 

 by throwing off its outer shell in a supernova explosion, leaving be- 

 hind a remnant which ultimately becomes a white dwarf. The success 

 in calculating "composite" models of red giants (39, 40), as well as 

 Trumpler's classification of star clusters, lends support to this concept 

 of stellar evolution. The more massive B stars will have a lifetime 

 of a few hundred million years only. This being much shorter than 

 the lifetime of the galaxy, which cannot be younger than the earth, 

 it is concluded that the early-type stars are currently replaced by new 

 stars condensing out of diffuse matter (35). Where diffuse matter 

 is no longer available, early-type stars are absent and only giants of 

 the corresponding luminosity remain, as is actually observed in glob- 

 ular clusters. Using Baade's terminology (41), Population II of the 

 globular clusters, the galactic center, and the general galactic back- 

 ground, consists of aging members born at a remote epoch ; whereas 

 Population I, connected with the diffuse matter and spiral structure 

 of the galaxy, contains young early-type stars steadily coming into 

 being and dying, in addition to the background of less massive young 

 and old stars, some of the latter existing from the very beginning of 

 the galaxy (35,42). 



The absence of normal B and A stars from the globular clusters sets 

 their age, as well as that of the galaxy, at more than 1,500 million years. 



The energy source of the giants remains a puzzle. If we take their 



