40 PRESI1)E>-TIAL ADDRESS SECTION A, 



By investigating all the known scattered helium stars, 

 Charlier finds that they form a bun-.shaped cluster haying- 

 dimensions of the order of 0,000 light years Ly 2,000 light 

 years, and that our Sun is about 500 light years aAvay from 

 the centre of this system. Helium stars are also shown by 

 the spectroscope to have very small velocities or to be nearly 

 stationary in space, so that when a displacement is found 

 for a helium star, this must be chiefly the reflex of the solar 

 motion, and so the distance of the star can be deduced from 

 this displacement. 



It has been found possible with modern instruments of 

 exceedingly powerful light grasp to detect the presence of 

 helium stars in globular clusters. This enables the distance 

 and dimensions of the cluster to be at once deduced, on the 

 assumi:)tion that all helium stars are of the same (uder of 

 absolute magnitude. The conclusion is arrived at that such 

 a cluster as the Hercules cluster is no less than 100,000 light 

 years away, and is really a stellar system almost approaching 

 in dimensions to those attributed to the Milky Way. These 

 tremendous figures must be accepted Avith reserve until it 

 has been conclusively proved that the stars of these compact 

 clusters differ in no respects from non-cluster stars. The 

 chief use of such powerful instruments as the new 100-inch 

 reflector of the Mount AVilson Observatory will be to 

 investigate the spectra of the stars in such clusters, with a 

 view to increasing oiir knowledge of these very distant 

 sj'stems. 



Much has been learnt from the study of the behavioui; 

 of variable stars. One particular class of variable star— -the 

 Cepheid variable — has recently attracted much attention. 

 It is now considered by many astronomers that the Cepheid 

 variable — in which the*^ rise to greatest light is fairly rapid 

 imd the fall more leisurely — is due to the pulsations of a 

 gaseous star obeying the Unvs of a perfect gas. From this 

 it has been deduced that the period of vibration of the mass 

 of gas varies directly as the density, and hence iipon the 

 absolute magnitude of the star. Many such variables have 

 been found in the smaller Magellanic Cloud, and have led 

 io an estimate of the distance of this sys-fem, viz.. (iO, 000 light 

 years. The law connecting the period of variation with the 

 absolute magnitude has been tested for all known Cepheid 

 variables, and has been found to be consistently true. Hence 

 the determination of the distance of a Cepheid variable is a 

 simple deduction from its period of variati(m and its apparent 

 mean magnitude. . . 



One fruitful result of the spectroscopic examination of 

 stars, combined with an investigation into their absolute 

 mao>nitudes, has led to the discovery of two types of stars— 

 the oiants and the dwarfs— and has considerably inodihed 

 our Tdeas as to the order of stellar evolution. Briefly, the 

 stars can be divided into six principal spectral classes 

 {omitting some minor classes), viz., helium stars^ hydrogen 

 stars, calcium stars, solar stars, and stars with fluted spectra. 

 These are referred to as B-, A-, Y-, (i-, Iv- and M-type stars. 



