THE STARS IN ACTION" — JOY 187 



conclusion was verified b}' Yogel, who observed the star with the 

 spectroscope and found that it was actually moving in an orbit in ac- 

 cordance with the theory. Two years later Goodricke discovered the 

 variability of 8 Cephei with an even smaller range (0.7 magnitude) 

 of variability and deduced a period of 5 days 9 hours. He recognized 

 it as a different kind of star, but he was not able to explain the cause 

 of its variation, and we can do only slightly better to-da5^ In the 

 same year he also observed ft Lyrse, whose period is 12 days 22 hours 

 and range 0.6 magnitude. It is also an eclipsing star, but the stars 

 are so elongated toward each other that a variation is produced, 

 independently of eclipse, by the fact that sometimes we see the stars 

 endon and sometimes broadside. His friend Pigott discovered the 

 variability of rj Aquilse in that same year. 



Goodricke died in 1786 at the early age of 22 after discovering 

 three variables which are to-day the type stars of their classes. 

 Pigott later added two more, R Coronte and K Scuti, and he also 

 discovered two comets. Those who know the difficulty of the obser- 

 vation of variable stars of small range can not but admire the skill 

 and persistence of these two young men who, on their own initiative, 

 without instruments and without precedent, opened up in a sys- 

 tematic study the field of variable-star astronomy, the importance of 

 which is being more clearl}" recognized as time goes on. Had Good- 

 ricke lived to the age proverbially allotted to the astronomer, what 

 other fundamental discoveries might not have been his ! Pigott con- 

 tinued his observations for a time after his friend's death, but later 

 was lost to astronomical histor3\ 



The eclipsing stars of which Algol is the prototype are not intrinsi- 

 cally variable at all, but are binary stars whose orbits happen to be 

 so placed that the components eclipse each other at certain times 

 when viewed from the earth. They are always close together and their 

 periods are usually short, from a few hours to a few days. Study of 

 the light and velocity changes of such stars makes it possible to deter- 

 mine their masses, densities, absolute dimensions, surface brightness, 

 and the distances between them; and these are the onh^ stars for 

 which these data may be directly calculated. About 300 eclipsing 

 stars are known, most of which are found among the hottest and 

 brightest stars. They represent the early stages of double-star exist- 

 ence, when the two stars are just being formed from a single body. 

 They are often elongated toward each other by tidal effects, and in 

 some cases may actually be in contact. 



The Cepheids are named from Goodricke's star 8 Cephei, which 

 "was the first known and the brightest of its class. They are single 

 stars of great intrinsic brightness. They are the supergiants of the 

 stars. Study of their light changes shows that they rise from mini- 



