ORIGIN OF THE SOLAR SYSTEM JEANS 141 



edge of the further side of this gap was infinitesimal ; the stars were 

 scarcely more than points of light, described as "fixed stars." In 

 those days the problem of cosmogony reduced perforce to the prob- 

 lem of the origin of our own system. 



Recent research has changed all this, and the modern astronomer 

 has a very extensive knowledge of the nature, structure, and move- 

 ments of the various bodies outside our system. The cosmogonist 

 of a century ago could assert that the solar system had evolved in 

 such and such a way, and need have no fear of his theories being 

 upset by comparison with other systems. But if I put before you 

 now a theory of the origin of our system, you will at once inquire as 

 to the behavior of the 1,500 million or so of systems beyond the 

 great gap. Are they following the same evolutionary course as our 

 own system, and, if not, why not ? It may be well to consider these 

 other systems first. 



Among these 1,600 million or so of objects there are certain com- 

 paratively small classes the nature and interpretation of which are 

 still enigmatical — the planetary nebulae, the Cepheid variables, the 

 long-period variables such as Mira Ceti, and a few others. Apart 

 from these, practically all known bodies can be arranged in one single 

 continuous sequence. The sequence is approximately one of increas- 

 ing density; it begins with nebulae of almost incredible tenuity and 

 ends with solid stars as dense as iron. There is but little doubt that 

 the sequence is an evolutionary one, for the laws of physics require 

 that as a body radiates heat its density should increase, at least until 

 it can increase no further. Let us begin our survey at the furthest 

 point back to which we can attain on this evolutionary chain — the 

 nebulae. 



After the enigmatical " planetary " nebulae have been excluded, the 

 remaining nebulae fall into two fairly sharply defined classes, which 

 may be briefly described as regularly and irregularly shaped nebulae. 



The irregularly shaped nebulae comprise such objects as the great 

 nebula in Orion, and the nebulosity surrounding the Pleiades. 

 Until quite recently these irregular nebulae were supposed to be of 

 great evolutionary importance. It was noticed that they were 

 usually associated with the very hottest stars; whence arose a 

 beautifully simple cosmogony asserting that these very hot stars 

 were the immediate products of condensation of the nebulae, and that 

 their after life consisted merely of a gradual cooling until they got 

 quite cold. This cosmogony was too simple to live for long — it was 

 buried some ten years ago by the researches of Russell, Hertzsprung, 

 and others. Thanks to these researches, we now know that the very 

 hot stars associated with irregular nebulae, so far from being newly 

 born, are standing at the summit of their lives awaiting their decline 

 into old acre. 



