CURTIS: MODERN THEORIES OF SPIRAL NEBULAE 227 



4. If the spirals are separate universes, similar to our galaxy 

 in extent and in number of component stars, we should observe 

 many new stars in the spirals, closely resembling in their life 

 history the twenty-seven novae which have appeared in our 

 own galaxy. Over a dozen such novae in spirals have been 

 found, and it is probable that a systematic program of repeti- 

 tion of nebular photographs will add greatly to this number. 

 A comparison of the average magnitudes of the novae in spirals 

 with those of our own galaxy indicates a distance of the order 

 of 10,000,000 light-years for the spirals. Our own galaxy at 

 this distance would appear lo' in diameter, the size of the larger 

 spirals. 



5. A considerable proportion of the spirals show a peripheral 

 equatorial ring of occulting matter. So many instances of this 

 have been found that it appears to be a general though not uni- 

 versal characteristic of the spirals ; the existence of such an outer 

 ring of occulting matter in our own galaxy, regarded as a spiral, 

 would furnish an adequate explanation of the peculiar distribu- 

 tion of the spirals. There is considerable evidence of such oc- 

 culting matter in our galaxy. 



An English physicist has cleverly said that any really good 

 theory brings with it more problems than it removes. It is 

 thus with the island-universe theory. It is impossible to do 

 more than to mention a few of these problems, with no attempt 

 to divine those which may ultimately be presented to us. 



While the data are too meager as yet, several attempts have 

 been made to deduce the velocity of our own galaxy within the 

 super-galaxy. It would not be surprising if the space-velocity 

 of our galaxy, like those of the spirals and the Magellanic Clouds, 

 should prove to be very great, hundreds of miles per second. 



Further, what are the laws which govern the forms assumed, 

 and under which these spiral whorls are shaped? Are they 

 stable structures; are the component stars moving inward or 

 outward? A beginning has been made by Jeans and other 

 mathematicians on the dynamical problems involved in the 

 structure of the spirals. The field for research is, like our sub- 

 ject matter, practically infinite. 



