ISLAND GALAXIES — DOUGLAS 197 



greater tlie intrinsic luminosity of the star. This rehition was well 

 established lor the less distant stars, and it seemed so logical to 

 expect stars with identical characteristics to obey the same law, 

 whether near or far, that it has been ai)plied to stars in these remote 

 galaxies. From a series of photograplis the period of Jight varia- 

 tion is found, then the established relation gives the true luminosity, 

 and this, together with the apparent brightness on the photographs, 

 gives the necessary data for calculating the distance. 



It Avas by this method that Hubble determined the distance of the 

 great nebula in Andromeda (PI. 3) to be more than 900,000 light- 

 years. Another very large, bright spiral in the constellation of Tri- 

 angulum Avas found by similar means to be at about the same dis- 

 tance. It is believed, however, that the thousands of fainter spirals 

 are very much more distant. In Plates 1 and 2 are shown two of the 

 spirals in the region of the heavens designated by the constellation 

 named Coma Berenices. Here, and in the adjacent region of Virgo, 

 spiral nebulse are richly strewn on photographic plates of long ex- 

 posure, and both Hubble and Shapley have estimated for some of 

 these no less a distance than 100 million light-years. 



In spite of these tremendous distances much can be learned about 

 the island galaxies, though, of course, the farther away a galaxj' is 

 the less up to date will be the news which the light brings. Thus in 

 the case of the Andromeda nebula, approximately 1,000,000 light- 

 years distant, the rays of light which produced the image on the 

 negative of Plate 3 had been traveling through space for 1,000,000 

 years, and consequently the picture we see is not the Andromeda 

 nebula as it is to-day but as it was 1,000,000 years ago. 



Just what it is like now we can only conjecture — probably not so 

 very different from the i)icture, for 1,000,000 years is less in the life 

 of a star than 1 second of time in the average life of a man. 



The radial velocities of the brighter nebuke can be determined by 

 means of the spectroscope and show that they are moving through 

 space with great velocities. The Andromeda nebula is approaching 

 our galaxy with a velocity of 300 kilometers per second. Most of 

 the spirals, however, are receding at speeds averaging 600 kilometers 

 per second. 



There are two ways of endeavoring to find out the total mass of a 

 galaxy, and when two quite independent methods lead to results 

 which are in good agreement the astronomer feels considerable 

 confidence in tlie reliability of his calculations. The first method is 

 based upon a speculation regarding the ratio ot" luminous to non- 

 luminous matter in a galaxy and the theory that the luminosity is 

 determined by the mass. When the absolute luminosity of a galaxy 

 is known, its total mass can tliercl'ore be calculated. This method 



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