FRINGE OF THE SUN JAMES 185 



state of tenuity — so diffuse, in fact, as to be millions of times more 

 rare than any gas we can possibly obtain by experimental physical 

 means. 



It was shown that the probable source of energy which excited 

 these tenuous condensations to emit the unknown spectra lines was 

 possibly the intensely hot stars which, according to the Harvard 

 (Draper) stellar classification, are known as the Type "O" (Wolf- 

 Rayet) stars, or the Type "B" (Orion or helium) stars, the temper- 

 atures of which may be anything between 35,000° and 100,000° C. 



These high-temperature stars are found in close proximity to the 

 nebulae that exhibit nebulium lines; the type "O" sometimes forms 

 the nucleus of the planetary nebulae, while the type "B" is gener- 

 ally connected with the irregular nebulae "without form or void" 

 similar to Orion. 



The emission of a certain variety of intense energy from these 

 stars, combined with the wide spacing of the atoms in a nebula, is 

 the cause of the characteristic nebular lines, for the conditions postu- 

 lated thus allow of relatively forbidden transitions of the satellite 

 electrons of the nebular atoms. As is well known, the atom may be 

 pictured as consisting of a nucleus of positive electrical charge, while 

 to balance the system there are satellite electrons which are charged 

 negatively, the electrons rotating in certain possible orbits round 

 the nucleus. The transition of an electron from one orbit to another 

 is possible. Some transitions, however, are forbidden in the normal 

 state of the atom and are only possible under very special and 

 and stringent conditions. This is termed a relatively forbidden 

 transition. "VVlien an electron makes a forbidden move, owing, maybe, 

 to the bombardment of the atomic system by a high-speed particle, 

 or radiation from an outside source, it may also radiate energy in 

 the form of light of a certain quality or wave length. 



The evidence produced by Bowen indicates then that the N^ and Nz 

 lines are due to out-of -the-ordinary electronic movements in a gaseous 

 atom, the gas being doubly ionized oxygen^ that is, oxygen that has 

 lost two electrons from its normal complement of six. The A 4363 

 line is also probably due to oxygen in the same state, while the lines 

 in the ultraviolet may emanate from single ionized oxygen. Still 

 other lines may be attributed to ionized nitrogen. These elements 

 which produce the "nebulium" spectra are the main constituents of 

 our earthly atmosphere, and as Sir Arthur Eddington has so de- 

 lightfully put it, "The light that never was on land or sea is just 

 common air." Truly, as another writer has said, nebulium itself 

 has vanished into thin air * * * ! 



The mighty nebulae are very remote from the tiny planet, Earth. 

 But near our home mystery still prevails. Our own star, the Sun, 



