122 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1932 



energy are interchangeable, the light radiated in one second weighs 

 more than 2,000,000,000,000 tons. It represents one class of stars 

 known as '' variables," on account of their variation in brightness. 

 Among the variables, S Doradus is probably related to the subclass 

 which the astronomer calls novae. 



Within the earliest records of man the first star observed to vary 

 in brightness was indeed a nova in the constellation Scorpius and 

 was discovered by the Greek astronomer, Hipparchus, 134 years be- 

 fore Christ. This star, which appeared very suddenly as a dazzling 

 bright object where one had not before been seen, aroused the curiosity 

 of Hii^parchus and undoubtedly prompted him to compile the first 

 catalogue of star positions, by reference to which the appearance of 

 any new star, or the disappearance of any of those which he had 

 already observed and catalogued, could be established. 



The outstanding peculiarity of novae is that they manifest a sud- 

 den outburst in brightness and then gradually fade away, becoming 

 invisible to the unaided eye after a few weeks. The second known 

 star of this kind, a nova in Cassiopeia, was observed first by Tycho 

 Brahe, a Danish astronomer, in 1572. When Tycho first saw it, 

 this star was among the very brightest in the sky ; within a few days 

 it grew still brighter so that it could be seen even in daytime without 

 the use of a telescope. When the planet Venus is at its brightest, 

 it is possible to observe it in the daytime, provided we know in what 

 part of the sky to look, but Tycho's star is said to have been even 

 brighter, notwithstanding the fact that Venus approaches us as 

 closely as 26,000,000 miles, whereas most novae are probably hun- 

 dreds of millions times as distant. 



In recent years a more careful watch has been kept over the stars ; 

 in fact at Harvard, the whole sky is photographed many times in the 

 course of a j^ear's work. Also more people know the constellations 

 and recognize very quickly the presence of any new bright star 

 where previously none had appeared. As a result, known examples 

 of novae have increased fairly rapidly in recent years. The bright- 

 est of the recent ones appeared in 1918 in the constellation of Aquila. 

 In less than a week the normal brightness of this star increased 

 nearly 70,000 times, and on June 8 it was exceeded in brightness 

 by only one other star, Sirius. 



Aside from the telescope, no instrument has been more useful 

 to the astronomer than the spectroscope. In its simplest form the 

 spectroscope is nothing more than a finely polished glass prism 

 through which light from a star, or from any other source, is allowed 

 to pass. In passing through the prism the blue rays are always 

 bent more than the red, depending upon wave length, so that ordi- 

 nary " white light " is separated into all the colors of the rainbow. 



