298 



SIR FKAXK W. DYSON^ M.A.^ F.E.S._, ON" 



telescope. These two stars revolve round one another in about 

 50 years. Procyon has a faint companion which gives only 

 4 Q as much light, and these go round one another in 40 

 years, tj Cassiopeiae has a companion about 60 times as faint 

 as itself, and they go round one another in 230 years. The 

 companiou of a Centauri is very bright, and they revolve about 

 one another in 81 years. 



When we know the distances of these stars from us we are 

 able to calculate their distances from their companions, and we 

 find that the distance of Sirius from its companion is 21 times 

 the distance of the earth from the sun, that of Procyon 15, of 

 « Centauri 23 times, and so on. AVe can use this knowledge 

 to find out another very important fact about the stars, for the 

 time which stars take to revolve about one another depends on 

 their distance apart and the strength of the pull which their 

 mutual gravitation exercises. This pull is proportional to the 

 masses of the stars, and in this way we find that the mass of 

 Sirius is times that of the sun, that of a Centauri twice, and 

 of some of the other stars something at least as great as 4 of 

 the mass of the sun, and so we establish the fact that these stars, 

 at any rate, are not very different from the sun in the quantity 

 of matter that they contain. 



When we know the distance we can also determine something 

 about the rate at which the stars are moving. If we know the 

 distance of an aeroplane which is flying perpendicularly to the 

 line joining us to it, the measurement of its change of angular 

 position at once enables its velocity to be determined. In the 

 same way the knowledge of the distance of a star gives us 

 means to find in part the star's velocity. As the spectroscope 

 enables us to determine how fast a star is approaching or 

 receding from us, we are enabled to determine completely the 

 A elocities of a number of stars. We find, then, that these are 

 quite comparable with the velocity of the sun, which is moving 

 with a velocity of 11 or 12 miles per second in the direction of 

 the bright star a Lyrse. 



These are various particulars in which the stars resemble 

 the sun. They are, roughly speaking, of the same kind of mass, 

 their luminosity varies a good deal, and the velocities with which 

 they travel are quite comparable with that of the sun, may be 

 a little more or a little less. One other thing in which they 

 resemble the sun, though I shall give you no detail of this, is 

 that they consist of the same chemical elements. I have gone 

 through these particulars in order that you may see the general 

 lines of argument of the proof that the stars are bodies like the 



