240 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



LOGARITHM OF SURFACE TEMPERATURE 

 4.2 4.0 3.8 3.6 



1,000 - 

 10 



100 - 



^5 AO FO GO KO 



SPECTRAL TYPE 



Figure 1. — A contracting newborn star evolves leftward in this diagram to the main 

 sequence, where a long stay should favor development of life on any planets the star may 

 have. 



ally cited because we have taken into account the change in lumi- 

 nosity of the star during its contraction. 



A star of a certain mass will arrive at the main sequence with a 

 definite spectral type and luminosity, and its character changes only 

 slightly during the long period in which the hydrogen in its core is 

 being consumed. Once the central hydrogen is exhausted, the star 

 evolves quite rapidly toward the right, to become a giant or super- 

 giant — very different in size and surface brightness from before. 



It is obvious from the table that time scales on the main sequence are 

 much longer than those of contraction. This explains why about 90 

 percent of observed stars are to be found on the main sequence. The 

 stay of a more massive star on the main sequence is shorter than that 

 of a less massive star, as it dissipates its energy much faster. Thus, 

 an star remains in this state for only a few million years, compared 

 to an M star's 100 billion. 



From this brief look at stellar life histories, it is clear that gravita- 

 tion holds a star together while nuclear interactions release the energy 

 it radiates. The third kind of reaction, chemical, does not play a 



