ANT EVOLVING UNIVEESE JEANS 237 



sphere is the range of vision of the 100-inch telescope ; each apple is 

 a nebula containing matter enough for the creation of several thou- 

 sand million stars like our sun; and each atom in each apple is the 

 size of a solar system with a diameter equal to or slightly larger than 

 that of the earth's orbit. 



Thus the arrangement of the nebulae in space reproduces on an 

 incomparably grander scale the uniform spacing of the stars in our 

 galactic system. It is natural to inquire whether the uniform ar- 

 rangement of these larger masses can not again be explained by the 

 supposition that the nebulae themselves came to birth as condensa- 

 tions produced b}'^ the gravitational instability of an earlier and even 

 more tenuous mass of uniform gas. The test of the conjecture is, of 

 course, by numerical calculation. 



The masses of two nebulae are known with fair accuracy; one has 

 3,500 million times the weight of the sun, the other 2,000 million 

 times. If all the nebulae have masses of about this magnitude, the 

 average density with which matter is spread in space must be some- 

 thing like one gramme to 10'° cubic cms. The theoretical form- 

 ulae show that instability would cause such a medium to form into 

 condensations which would be at approximately equal distances apart, 

 and that these distances would be of the order of hundreds of thou- 

 sands of light-years. While the calculated distance comes out rather 

 less than Doctor Hubble's observed distance of 1,800,000 light-years, 

 yet it is near enough to it to make our conjecture seem reasonably 

 probable. 



These nebulae provide one of the great puzzles of astronomy. The 

 theory of relativit}^ suggests that the whole universe may be expand- 

 ing, and recent astronomical observations, made mainly at Mount 

 AVilson, have suggested that it is actually doing so, and this in no 

 half-hearted way. If we may take the observations at their face 

 value, the nebulae are even now rushing away from one another at 

 almost incredible speeds. The last nebula which Mr. Humason in- 

 vestigated at Mount Wilson, at an estimated distance of about 105 

 million light-years, appears to be receding from the earth at the rate 

 of 19,700 kms a second — about 12,300 miles a second ! [Still more 

 recently, nebulae at an estimated distance of 135 million light-years 

 appear to be receding at about 15,000 miles a second.] 



Some astronomers doubt whether these apparent recessions of the 

 distant nebulae represent real motions in space or not. If they do, 

 space must have expanded quite substantially since the nebulae first 

 condensed out of the primeval gas. 



The mathematical work of Lemaitre and others has suggested 

 that the mere condensing of the primeval gas into nebulae in the way 

 just explained, would of itself suffice to cause space to start expand- 

 ing. Before the expansion started there would be approximately the 



