STELLAR EVOLUTION JEANS. 159 



thought of as approximating rather to masses of fixed dimensions, 

 and for these the luminosity falls off as the temperature decreases. 



Our sun radiates light at a rate of about 2 ergs per second per 

 gram of its mass. Gravitational contraction, as Lord Kelvin showed, 

 could provide energy at this rate for only about 20,000,000 years, 

 and radio-active and chemical energy could only slightly lengthen 

 this period. For a giant star, radiating at 1,000 times the rate of the 

 sun, the maximum period would be only a few thousand years. This 

 period is far too short, and it is now generally accepted that, so far 

 from gravitation and known sources of energy providing the whole 

 of a star's radiation, they can provide only an insignificant fraction. 

 Energ}^ of adequate amount can originate only from subatomic 

 sources, as, for instance, from internal rearrangements in the posi- 

 tive nuclei of the atoms or from the transformation of a small frac- 

 tion of the star's mass into energy. It is a matter of simple calcu- 

 lation to show that all other stores of energy in a star can constitute 

 onty an insignificant reservoir of energy which, unless continually 

 replenished from subatomic sources, would be exhausted in, astron- 

 omically, a moment. Thus the rates of radiation and of generation 

 of subatomic energy must be practically equal, and the luminosity 

 of a star will be determined by the latter rate at any instant. 



We may now think of the evolution of the stars as represented by 

 the march of a vast army through our diagram (fig. 2), the indi- 

 viduals keeping, for the most part, within the marked belt. Each 

 individual takes his marching orders from the supply of subatomic 

 energy, and so long as we remain in ignorance of the exact source 

 and nature of this we can not be certain whether the motion of the 

 army is up or down, or even that it is all in the same direction. But 

 if we are right in conjecturing that the stars were born out of a nebula 

 of very low density, the order of march will be from low density 

 to high ; our army will be marching downwards in the diagram. Its 

 tail, except for a few stragglers, is about at absolute magnitude —4, 

 its head is lost in darkness. In the next lecture we must study the 

 incidents which may occur during the march of this army of stars. 



II. THE EVOLUTION OF STELLAR AND PLANETARY SYSTEMS. 



In the last lecture we followed up, so far as is permitted by mod- 

 ern theoretical and observational research, the train of ideas on 

 which Laplace had based his nebular hypothesis. Theoretically we 

 found that a shrinking mass of rotating gas ought in time to assume 

 a lenticular shape, after which further shrinkage would result in the 

 ejection of matter from the sharp edge of the lens. It is suggested 

 that the spiral nebula? form instances of this process, the spiral arms 

 being the ejected matter and the central nucleus the remnant of the 

 original rotating mass of gas. The spiral arms are observed to 



