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SIR FKANK W. DYSON, li.A., Y.R.S.y ON 



these great distances in the same way as an artillery officer in 

 France can locate and range an enemy position. 



It is, however, only a few of the nearer stars whose distances 

 have been measured by astronomers : the number does not 

 amouut to more than a few hundreds. Xo doubt there are still 

 many stars — say one or two thousand — within measurable 

 distance of us, that is to say, within 500 million million miles, 

 whose distances will probably be, but have not yet been deter- 

 mined. But these are only a few of the myriads of the stars 

 we see with our telescopes. Other methods are being employed, 

 and very successfully, for determining their distances. I shall 

 not speak about these, but will rather tell you something more 

 about the stars which are nearest to us. I will confine myself 

 to the stars which are not further than a million times the 

 distance of the sun from us — that is, roughly, stars within 100 

 million million miles of us. There are about twenty stars 

 known to be within this limit of distance, and if we consider 

 only those stars which are not less than 100 times as faint as 

 can be seen with the naked eye, it is probable that there are 

 still ten or fifteen more to be discovered. Let us consider, 

 then, a huge sphere whose radius is one million times the 

 distance from us to the sun. Suppose we make a model of this 

 sphere and let us take a globe the size of the earth for our 

 model. On this scale a star of the same diameter as the sun 

 would be as big as a tennis ball. Imagine, then, from 30 to 40 

 tennis balls equally scattered inside the earth; this gives a 

 picture of how near the stars are to one another. This gives us 

 a good idea of the great distances between the stars. 



These stars which are nearest to the earth differ a great deal 

 in their magnitudes or brightness as seen by us. Thus Sirius, 

 the brightest star in the sky, is one of them, and the very 

 bright star Procyon is another, and a Centauri, the nearest of all 

 the stars to us, is one of the brightest in the southern constel- 

 lations. Others are fairly bright stars visible to the naked 

 eye, but, on the other hand, a large proportion are faint and only 

 visible with telescopic aid. From some of these stars we receive 

 only ^^^^ light which Sirius gives us, and from some 



less than yo^-oo- Il^ese great differences are partly ca\ised 

 by difference of distances, but to a greater extent by intrinsic 

 differences in the amount of light given out by the stars. 



When the distances of stars are known we are able to tell 

 how far the difference in their apparent magnitude is due to 

 differences of distance and how far to real differences in intrinsic 

 brightness. 



