SYMPOSIUM OX RADIOACTIVITY. 53 



mination of the lines on the part of the eminent investigators, 

 particularly in England, chiefly due to the presence of impurities 

 due to barium (for radium) and hydrogen and xenon (for the 

 emanation), it is nevertheless established that there are two char- 

 acteristic and distinct spectra for radium and for the emanation. 

 There seem to be no lines common to the tv\'o spectra. But these 

 lines, with all those impurities eliminated, do not correspond to 

 unknown lines in celestial spectra. Hence, if radium or its 

 emanation are present in the sun and stars, the conditions are not 

 suitable for the development of their luminosit}- to an appreciable 

 extent. 



The close connection of heHum with radium is very remarkable, 

 and may be of significance here. It seems to be established with 

 almost perfect certainty that the a rays, which are spontaneously 

 emitted by radium with an average velocity of some six thousand 

 miles per second, actually consist of atoms of helium. Further- 

 more, a quantity of the emanation which at first gives its charac- 

 teristic spectrum begins in a few days to show the helium spec- 

 trum. Hence helium is also developed by the emanation. 



Helium is present in the sun, as has been known since its 

 discover}' there in 1868, more than a quarter of a century before 

 it was found on the earth. Its appearance on the sun is peculiar 

 in that it is seen only as a bright line at the edge of the sun with 

 the spectroscope, and does not produce dark lines, as do the other 

 elements which appear as bright lines when viewed at the sun's 

 limb. Helium is also found in large prominences viewed at the 

 edge of the sun wiuh the spectroscope, but it does not rise as high 

 in the solar atmosphere as do hydrogen and calcium vapors. 



If the sun were one hundred times as far away from us as it 

 actually is. we should probably be unable to detect the bright 

 radiations of helium in its spectrum ; they would be totally invisi- 

 ble at the distance of the nearest fixed star. 



There is a certain class of stellar spectra, however, of which 

 helium is the principal characteristic (in addition to hydrogen, 

 which is found in even,- celestial spectrum with hardly an excep- 

 tion). These stars are called helium stars, or stars of the Orion 

 t)'pe, and they are found chiefly in or near the Milky Way. \Vhen 

 a series of diflferent stellar spectra are arranged in a sequence 

 merely by the similarity of spectra and gradual change from one 

 type to another, without involving any theory of stellar evolution, 

 these helium stars are placed by all investigators at the beginning 



