EEAL MAGNITUDES OF STARS. 



Since then, the sun placed beside the star o Centauri, both 

 being at the same distance from the observer, would appear less 

 bright than the star in the proportion of 22 to 51, or 1 to 2, it 

 follows, that the star a Centauri is a sun more than twice the 

 superficial magnitude of ours, supposing that its luminous surface 

 has the same lustre. 



35. Sir J. Herschel found that the lustre of the dog-star is four 

 times that of a Centauri ; but according to what has been given 

 in the table in page 182, the distance of the dog-star called 

 Sirius, from our system is 896087 times that of the sun. Now 

 from these data, it will be easy to calculate the relative magni- 

 tudes of our sun and the dog- star. 



Since the light received from the dog-star is four times that 

 received from a Centauri, while the light received from the latter 

 is 21956 million times less than that received from the sun, it 

 follows, that the light received from the dog-star is 5489 million 

 times less than the light received from the sun. Let us now 

 imagine the sun removed to the distance of the dog-star, and let 

 us consider what would then be the light received from it. The 

 distance of the dog-star being 896087 times that of the sun, the 

 sun removed to that distance, would shine with a light less than 

 its present light, in the proportion of the square of 896087 to 1, 

 which is in the proportion of 802972 millions to 1. But from 

 what has been stated above, it appears that the dog-star at the 

 same distance, shines with a light less than the sun in the pro- 

 portion of 5489 millions to 1 . It follows, therefore, that the sun 

 and the dog- star, being placed at the same distance from the 

 observer, the lustre of the dog-star would exceed that of thesun 

 in the proportion of 802972 to 5489, or 146| to 1 ; * from which we 

 arrive at the surprising conclusion, that, supposing the surface of 

 Sirius to have a lustre equal to that of the surface of the sun, its 

 surface must be 146| times greater than that of the sun, so that 

 this stupendous globe of light would have a diameter 12*09 times 

 greater than that of our sun ; and, since the diameter of the latter 

 is 882000 miles, that of Sirius would be 10,663380 miles. 



36. Since no telescope, however great might be its power, has 

 ever presented a fixed star with a sensible disk, it might be 

 inferred that, for the purposes of stellar investigations, the im- 

 portance of that instrument must be inferior to that which it may 

 claim in other applications. Nevertheless it is certain, that in no 

 department of physical science has the telescope produced such 



* Sir John Herschel makes the proportion 63*02, which is certainly 

 incorrect, that being the ratio of the brightness of Sirius to that of a 

 Centauri, and not that of Sirius to the sun ; see Herschel's Astronomy, 

 p. 553, edition 1849. 



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