308 VINCENT E. SMITH 



ment ^^ which would have an appreciable additive effect over 

 a long period of time. But perhaps one of the most crucial, 

 because the most cosmic, evidences in this regard is the phe- 

 nomenon of the expanding universe. 



To approach the evidence for an expanding universe, it might 

 be initially observed that the distances of the nearer stars, 

 with respect to a terrestrial observer, can be determined from 

 the various angles at which their light strikes the earth in the 

 course of the earth's annual movement about the sun. From 

 the angles involved, distances can be computed by simple 

 trigonometry. But for more distant objects this change of 

 angle (parallax effect) is so small that a different method 

 must be used, and fortunately another tool is at hand. This 

 tool is furnished by the stars called Cepheid variables which 

 undergo periodic changes in their visible radiation, rapidly 

 increasing in luminosity and then fading back into their original 

 brightness. A correlation exists between the brightness of a 

 star and its period of pulsation; the longer the period the 

 brighter the star. The phenomenon of Cepheid variables, 

 named from the star Delta Cephei, the first laiown example 

 of such a pulsating star, enables us to know the absolute 

 luminosity of the star in question, and when this is compared 

 with apparent brightness, the distance of a Cepheid variable 

 can be determined. ^^ 



By invoking the periodic law for Cepheid variables, Edwin 

 P. Hubble showed that distant nebulae, such as the Andromeda 

 nebula, once believed to be part of the Milky Way, are actually 

 distant galaxies ^^ — in the case of Andromeda, two million light 

 years away. Moreover, this challenge to the older conception 

 of a nebula led to the view that the universe is expanding. 



^* Although this irregularity in perihelion is discernible in the case of Mercury 

 and is explained by relativity mechanics, it is believed to exist, in a degree too 

 small to be observed, in the case of the other planets. 



^^ An explanation of Cepheid variables will be found in A. Eddington, The Ex- 

 panding Universe (Ann Arbor, 1958) pp. 7-8. 



^"For Bubble's work, see his The Realm of the Nebulae (Oxford, 1936). 



