310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



diagram for the nearest galactic cluster, the Hyades, which are ex- 

 ceptional in that we can estimate their distance rather accurately 

 by a trigonometrical method based on their group motion. 



We have seen that if the stars are in equilibrium and uniform 

 throughout, the hot stars should be much brighter than the cool ones. 

 Astronomers have a peculiar habit of plotting the temperature scale 

 backward, with the hot stars of spectral types O and B on the left 

 and cool stars of spectral types K and M on the right, and we see 

 that most of the stars do indeed lie on a line going from top left to 

 bottom right in the diagram, which is known as the "main sequence" 

 or "dwarf sequence." 



There are however, a few stars definitely not on the main sequence. 

 These stars are bright, about 100 times as bright as the Sun, and 

 cool (that is, as hot as the Sun or cooler), and it follows from this 

 that they must be very much larger, having something like 10 times 

 the solar radius of 700,000 km. These stars, of which there are many 

 well-known examples like Arcturus and Capella, are therefore called 

 "giants," in contrast to "dwarfs" like the Sun which lie on the 

 main sequence. According to current theories of stellar evolution 

 developed in the last 10 years by Allan Sandage, Martin Schwarz- 

 child, Fred Hoyle, and others, a star is believed to remain on the 

 main sequence for a long period of time in the earlier part of its 

 life history, and the length of its lifetime on the main sequence 

 depends on how bright it is. A hot star will last only for a few 

 million years, while a relatively modest star like the Sun, which is 

 much more sparing in using up its nuclear fuel resources, is estimated 

 to be able to last for over 10,000 million years. When a certain pro- 

 portion of the hydrogen in its central regions has been transmuted into 

 helimn, the star is no longer able to remain in a steady state on the 

 main sequence, but swells up to form a red giant; and this state of 

 affairs is reached much sooner by a bright, hot star than by a faint 

 cool one. In a young cluster, therefore, the main sequence will 

 stretch upward to include stars of considerable brightness, and the 

 upper limit where the main sequence terminates will gradually travel 

 downward in the diagram in the course, of time; that is, with 

 increasing age, the stars that have just left the main sequence will 

 gradually become fainter and so the age of a cluster can be judged 

 from observation of its color-luminosity diagram. The age of the 

 Hyades is estimated in this way to be a few hundred million years. 



Wlien we look at the corresponding diagram for a typical globular 

 cluster, like the one shown in figure 2, we see the main sequence 

 terminating much lower down, at about three or four times the 

 Sun's brightness. Most of the bright stars in globular clusters, and 

 in the halo Population II generally, are red giants, and the bright 

 blue stars are completely absent. It is concluded from this that 



