GALAXIES — SHAPLEY 139 



Probably there are dwarf galaxies in the Fornax group, but as yet 

 we have not identified them, nor have we found the Magellanic type. 

 Because of the general tendency of the universe to expand and the 

 galaxies and groups of galaxies to recede from one another, it may be 

 that eventually we shall be able to say which faint objects in Fornax 

 are members of the supersystem simply by determining their velocities 

 in the line of sight. If the suspected galaxy is really much more dis- 

 tant than the average of the Fornax group, it will show a bigger "red 

 shift" in the spectrum, a greater velocity of recession, and thus indicate 

 the larger distance and the nonmembership. 



On every expedition into remote corners of extragalactic space, it 

 is necessary to equip ourselves with information on giant and, super- 

 giant stars. The reason is, obviously, that ordinary stars in far-off 

 places are not recorded on our photographs; they are too dim. We 

 must work with the giants. It may be of interest to consider the fol- 

 lowing highly luminous stars and types of stars and see how they 

 contribute to knowledge of the metagalaxy : 



1. Supernovae 



2. S Doradus 



3. Novae 



4. P-Cygni stars and others 



1. Supernovae. — "The most energetic catastrophe in the history of 

 the world, unless it be creation itself," is how I would describe the 

 great violence of radiation and motion that accompanies the career of 

 the supernova. Simply defined, a supernova results when a star blows 

 up. Whether the disaster is caused or encouraged by head-on collision 

 with another star or another something, or by the collapse of the star's 

 structure, with the consequent atomic transformation of mass into 

 radiation, or "just happens," we cannot yet say. More observational 

 data are needed and are being obtained. The result of a supernova 

 outburst is the outpouring of light in unparalleled fashion — a spurt 

 of radiation the equivalent at times of 50 million suns and more. The 

 burst of radiation lasts sometimes several days or weeks, quieting down 

 slowly as the months go by. Wliat remains after the flare-up % Per- 

 haps a dense subdwarf star (the collapsed core of the original star) ; 

 perhaf)s a hurriedly expanding nebula ; perhaps just dust and ashes, and 

 the universe filling up with the dying glow of a radiant moment. 



Dr. Fritz Zwicky of the California Institute of Technology has 

 been the leader in recent years in the discovery of supernovae and in 

 speculations concerning them. He thinks that neutrons and neutrinos 

 play an important part in the supernova phenomenon. Certainly su- 

 pernovae play a significant part in the histoi-y of the universe. They 

 are not too uncommon. About 40 are on record, most of them dis- 

 covered in the past 10 years. Three of them appeared long ago in our 



