THE DISTANCES OF THE STARS. 



297 



The principle is very simple. If a body is moved to twice 

 the distance, we receive a quarter as much light from it. If 

 one candle at a distance of one foot gives one just sufficient 

 light to read by, then a 100 candle-power lamp at a distance 

 of 10 feet will be needed — and a 10,000 candle-power lamp at 

 a distance of 100 feet would be equally serviceable. If the 

 distance of a star is known, and the amount of light it gives us 

 is also known, an easy calculation tells how much light it would 

 give if it were no further aw^ay than the sun. We call this the 

 luminosity of the star, and just as a candle is taken as a 

 standard for comparing terrestrial lights, so the sun is taken as 

 a standard oE luminosity, and a luminosity of 5, say, means 

 that a star gives out 5 times as much light as the sun. 



Calculation shows us that Sirius is 48 times as luminous as 

 the suu, Procyon about 10 times, and « Centauri about twice as 

 luminous. Some of the stars are relatively very faint and give 

 out only -j-^-Q or less of the light emitted by the sun. 



There is one very interesting feature apparent among the nearer 

 stars, that the blue stars in our list are more luminous than the 

 red ones. If the stars — I mean those twenty near ones and not 

 all the stars in the sky — are arranged according to colour, the 

 luminosity progressively diminishes as we go from blue to red. 



ISTow the colour of a star is a very important feature. Most 

 stars are so faint that we can hardly detect their colour. But 

 if we look at the brighter stars we see that Sirius is blue, 

 Arcturus yellow, Aldebaran red. These differences of colour 

 mean differences of temperature. I will not enter into the 

 proof of this. It depends on the knowledge derived from 

 spectroscopic observations of the stars. The blue stars are at 

 a temperature of, say, 10,000° Centigrade, the yellow ones, like 

 the sun, at a temperature of 7500°, and the red ones, like 

 Aldebaran, at a temperature of 4000°. We all know what a 

 difference there is in the brig^htness of an electric light when it 

 is over-incandesced and when it only has enough current to 

 make the filament at red heat. We attribute the differences in 

 the luminosities of these stars very largely to the fact that they 

 are at a different temperature. No doubt there may be a 

 considerable difference in size, but perhaps the most important 

 difference is the difference in the brightness of their surfaces 

 consequent on the difference of their temperatures. 



One remarkable feature in these near stars is that no less 

 than 8 out of 20 are double stars ; for example, Sirius 

 is a double star. The bright star we see has a very faint 

 companion which can only be detected by a very large 



