VARIABLE STAES ROBINSON 



125 



days to about 2 years. Furthermore, the range of variation in 

 brightness is, in general, much less in the case of Cepheids than in 

 some Mira-type stars where the brightness at maximum is 50,000 

 times that at minimum. Astronomers believe, however, that Ce- 

 pheids are related to long-period variables because a correlation 

 between redness and period, which has been noted among Cepheids, 

 can also be extended to the long-period variables. The Cepheid 

 variables and those of long period are generally believed to be 

 pulsating — that is, both 

 classes may owe their 

 changes in brightness part- 

 ly or wholly to periodic 

 expansions and contrac- 

 tions of the gases compos- 

 ing them. There are some 

 astronomers, however, who 

 believe that the variations 

 in luminosity of the Ceph- 

 eids are due to rotation 

 and to their peculiar 

 shapes. It is claimed that 

 they are pear-shaped and 

 eventually break up into 

 two stars. Each theory has 

 its objections, and much 

 more work remains to be 

 done before the matter can 

 be considered as settled. 

 Figure 1 illustrates how a 

 Cepheid may divide into a 

 binary system. 



The question, " Wliat are 

 Cepheid variables ? " is 

 hardly more interesting than the question, " What are they good for, 

 astronomically? " In fact, solving the latter question has served 

 astronomical advancement to a far greater extent probably than any 

 other research which has been pursued in many years. It happens 

 that in certain star clouds and clusters many Cepheid variables are 

 found. In studying the variables in the Magellanic Clouds, Miss 

 Leavitt, at Harvard, discovered that a relation exists between the pe- 

 riods of variation and the apparent brightness of these stars : The 

 longer the period, the brighter is the star — the so-called period-lumi- 

 nosity relation. Since all the stars in the Magellanic Clouds are at 

 very nearly the same distance from the earth, it can also be demon- 



FlGDRB 1 



Binary System 



. — The 



transition of a binary system 

 from a Cepheid 

 According to the Jeans theory, the transition 

 from the Cepheid stage may be the result of con- 

 traction. Thus Ri and Ro decrease to R'l and 

 R's, respectively. In the first case the sum of 

 Ri and Rj is greater than the distance from 

 A to B, and in the second it is less. 



