ANALYSIS OF STARLIGHT — ^PAGEL 305 



Solar System. The disk stars of Population I revolve in circular 

 orbits that are confined to the central plane, and the Sun, for example, 

 is estimated to go right round a galactic circle in this plane in a period 

 of 200 million years ; whereas the stars of the halo population go round 

 in elliptical orbits that are often steeply inclined to the plane. 



The two stellar populations also differ from each other in their age. 

 The key to the age difference is that stars are formed by condensation 

 out of the diffuse clouds of interstellar matter which are concentrated 

 in the flat spiral arms, and so any stars which are young must also 

 be concentrated in spiral arms. The halo Population II, which is 

 devoid of interstellar material, is in a kind of fossilized state because 

 star formation must have stopped there at an early stage in the history 

 of the Galaxy, We imagine that the whole Galaxy itself started off 

 as a diffuse cloud of gas condensing under the effect of its own gravi- 

 tational attraction and that while it was condensing groups of stars 

 separated out of it at every stage, and indeed are still doing so at the 

 present time. Owing to the rotation of the primeval gas cloud, it 

 would have been gradually flattened out into a disk by centrifugal 

 force, but the stars that had been formed before appreciable flattening 

 had occurred would have been left behind in a spherical or spheroidal 

 bulge ; and this is more or less what we observe. 



Many stars are concentrated in more or less compact physical 

 groups known as star clusters; well-known clusters are the Pleiades 

 and the Hyades, which are near to the Milky Way and are typical 

 members of the spiral arm population or Population I; these are gen- 

 erally referred to as galactic or open clusters. One such cluster, the 

 Double Cluster in Perseus, is shown in plate 4, figure 1. 



The stars of the spherical population, or Population II, also are 

 frequently (though by no means always) found in clusters. These 

 tend to be richer in stars and more compact than the galactic clusters, 

 and they are known from their shape as globular clusters. One exam- 

 ple of a globular cluster is the system Messier 13 in Hercules shown in 

 plate 4, figure 2. 



THE COLOR-LUMINOSITY DIAGRAM AND STELLAR EVOLUTION 



Star clusters of both kinds have proved particularly helpful in 

 studying the evolution of stars in the course of time, because we can 

 suppose that all the stars in any one cluster were formed at a single 

 moment from one cloud of dust and gas, but that different clusters 

 may have been formed at different times in the past. In order to 

 understand how we interpret the observations of clusters, we shall 

 now have to go into some details both about the nature of stars and 

 al)out the kinds of observation that it is possible to make. 



Suppose that a cloud of interstellar matter with a mass of the order 

 of 10^® g. or about 1,000 times the mass of the Smi starts to condense 



