184 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 19 2 8 



activity. The physicists tell us that the atoms are absorbing the 

 ether waves which overtake them and are re-emitting the energy re- 

 ceived in certain definite rays. Of course only the lightest elements 

 are thrown out of the star, and the rays which we actually observe 

 are those proceeding from hydrogen, helium, and nebulium atoms. 

 This latter is a hitherto unknown gas which Bowen has recently 

 shown to be merely oxygen and nitrogen, the elements so well known 

 in our own atmosphere. They were not recognized previously 

 because such lays could not be produced in the limited confines of 

 the laboratory. 



The cause of novae outbursts, which are (he most violent of all stellar 

 activities, is still uncertain. We know mucli of the results of the 

 explosion, but the underlying cause can only be conjectured. Possibly 

 an encounter with nebulosity or another body may start the generation 

 of internal energy in large amounts near the surface of the star. It 

 can not be a deep-seated effect, for it is too sudden and ephemeral. 

 The central portions of the star are probably not affected by the great 

 display at the surface. 



Only a few years later than Tyeho's famous nova, the first periodic 

 variable star was noted. In 1596 Fabricius, a Flemish clergyman, 

 noticed that the red star Omicron in the constellation Cetus, the whale, 

 was bright for a few months and then dropped below the limit of visi- 

 bility. He doubtless thought it a nova. It was 40 years before its 

 periodic character was discovered. Continued observation showed 

 that it returned to maxima easily visible to the naked eye ever}'- 330 

 days; that it was visible about 5 months and too faint to be seen for 

 6 months; and that its rise to maximum was about twice as rapid as 

 its decline. Because of its extraordinary behavior it was called Mira, 

 the wonderful. This name it has richly deserved, for it continues to 

 show peculiarities Avhich can not be predicted and which are little un- 

 derstood. Its variations are somewhat uncertain but average about 

 six magnitudes or two hundred and fifty fold increase from minimum 

 to maximum light. 



Although it received its name largely because of the surprise in- 

 duced by its periodic character and its great range of brightness, its 

 behavior as studied by the spectroscope has been even more wonderful, 

 and it has perplexed observers since its spectrum was first observed 

 more than 40 years ago. There are several facts in connection with 

 the star and others of its class which are of special interest. Its ap- 

 parent diameter is the largest thus far measured with the inter- 

 ferometer, and its real diameter, three hundred and fifty times that 

 of the sun, is second only to that of Antares. If brought to our solar 

 system and set in place of the sun, its boundaries would extend be- 

 yond the orbit of Mars, and the earth in its yearly motion would de- 



