THE DISTANCES OF THE STARS. 



299 



sun. The suu is bright and presents a disc of measurable 

 size to us. If these stars of which we have spoken could be 

 brought to the same distance they would present measurable 

 discs, in some cases a little larger and in some a little smaller 

 than the sun, but we should not find any enormous disparity. 

 And so we conclude that the sun is just one of the stars of 

 quite average dimensions, bigger and brighter than some but 

 less and fainter than others. We have only found, as you 

 perceive, a very limited number of things about the stars, 

 their sizes, masses, surface temperatures and luminosities. 

 There are many other things we should like to find : for 

 instance, " Have these suns systems of planets revolving round 

 them ? " To this we can at present give no answer ; but we 

 should presume that they may have. You may ask, " Are we 

 to suppose that these planets have life upon them ? " The 

 answer is, that we do not know, and can only guess by the analogy 

 of our own earth and the sun. 



I have confined myself to what we can discover about the 

 nearest of the stars. There are means, partly depending on 

 what we learn in this way, and partly on somewhat more 

 complicated applications of geometry and physics, but still 

 simple in principle, by which our knowledge is extended to 

 great distances in space. We find that there are many millions 

 of bodies which are in the main like the sun. Most of the stars 

 we see form a great assembly which extends to two or three 

 hundred times the distance of which I have been speaking in 

 the direction perpendicular to the Milky Way, and to 1000 

 times this distance when we come to the plane of the Milky 

 Way. We can even go beyond this, and we find clusters of 

 stars far removed from that continuous assemblage of which our 

 sun is a unit. Kecent work by Hertzsprung, Shapley and others 

 places the small Magellanic cloud at a distance 3000, the 

 cluster of w Centauri 700, and the cluster in Hercules 7000 — 

 if we take one million times the distance of the earth from the 

 sun as our unit. This last cluster probably contains 50,000 

 stars brighter than the sun and many more less brilliant. 



My lecture has been devoted to the attempt to give in 

 general terms some idea of the principles which guide astro- 

 nomers and the methods they employ rather than a statement 

 of the results they have obtained. It seems to me that the 

 mere statement of a scientific discovery is of little value 

 without some idea of the means which have been employed to 

 obtain it. It is, of course, quite impossible for anyone but an 

 expert to follow all the details, just as it is only the expert who 



