PRESIDENTIAL ADDRESS SECTION A. 39 



fluctuations and the absolute magnitude has been found. If, 

 therefore, a very faint star in a very distant star cluster exhibits 

 these light variations, the period of the variability can be used for 

 placing the star in its proper position in the absolute magnitude 

 scale. 



In a series of remarkable and brilliant papers Dr. Shapley 

 has pursued this and kindred methods very thoroughly in order to 

 find the distance of globular star clusters, and concludes that the 

 brightest stars m them correspond to the giant stars of greatest 

 luminosity which are found in our local star field. 



The distances he derives for the globular clusters vary from 

 20,000 to 200,000 light years. The light from the farthest cluster 

 must therefore have been on its way 180,000 years earlier — and the 

 cluster as we now see it is that much younger — than the light from 

 the nearer cluster in order to arrive at the earth at the same time. 

 If we can assume that the clusters are of the same age then we 

 can say that 180,000 years makes little difference in the appearance 

 of a star cluster, as except for its smaller size and fainter stars — 

 which are merely the effect of greater distance — the clusters look 

 much alike. Considerations such as these bring home to us the 

 immensities of space and the eons upon eons of time we must allow 

 for the evolution of stellar systems. It seems probable that the 

 distances separating the individual stars in the nucleus of a 

 cluster, where they appear so crowded together as to be almost 

 irresolvable, is much the same as the distances separating us from 

 the bright stars we know in our immediate vicinity. 



Dr. Shapley concludes that the diameter of the cluster Messier 

 No. 3 is such that light would take 470 years to traverse it, and 

 light travels at the rate of 186,000 miles a second; its distance he 

 concludes to be of the order of 250,000 million million miles 

 (13,900 parsecs or 45,000 light years) equivalent to a parallax of 

 0000072", and this is one of the nearer clusters ! The most remote 

 cluster known is about five times as far away. 



There is still another method of estimating the distances of 

 remote stars. From time to time bright stars suddenly appear in 

 the sky in places where no bright star was previously seen. These 

 are called "blaze stars" or "Novae"; they appear to be due to 

 some tremendous cataclysm which causes the star to burst into 

 flame with explosive violence, so violent that the star is transformed 

 into a gaseous nebula and subsides into a faint object very rapidly. 

 When these "blaze stars" appear in the milky way they are com- 

 paratively near to us, and appear sometimes as bright as Sirius, 

 the brightest star in the sky. The distances of some of these have 

 been measured, and from their known distances and apparent mag- 

 nitudes their absolute magnitudes have been found. At the 

 standard distance of 32-6 light years they would appear as bright 

 as Venus at maximum brilliancy and would easily be seen in full 

 sunshine. 



The bright Nova in Aquila, which Mr. Watson, of Beaufort 

 West, was one of the first to discover three years ago, attained an 

 absolute magnitude of —4-9, a figure arrived at from trigono- 



