136 
The N.Z. Journal of Science and Technology. [June 
Professor A. S. Eddington, in the monograph already referred to, proves 
conclusively that the stars we see, and those which are included in star- 
catalogues, are not by any means a representative sample of the stars 
of our cosmic system. Stars force themselves on our attention when 
they are in that particular stage in their life-history in which their 
intrinsic luminosity is greatest. Similarly, it is probable that distant 
systems, such as the spirals, are seen only when their luminosity is near 
its maximum. 
The exciting cause of the luminosity of stars, which gives them 
visibility for some hundreds of millions of years, appears to be the encounter 
of one star with another. Similarly, the origin of the light of a spiral nebula 
must be sought in the interpenetration of two cosmic systems. Such an 
encounter will give a luminosity which may persist for hundreds of 
thousands of millions of years, and must at the same time lead inevitably 
to the distribution of the material in the double spiral form. The details 
of structure resulting from such a union depend on the masses and 
dimensions of the original systems, on the distribution of density within 
each, on the extent of the interpenetration, and on the relative component 
of the original proper motions of the two systems. 
