ASTRONOMICAL MAGNITUDES AND DISTANCES. 295 



only twenty stars of the first magnitude, that is, of the first degree of 

 brightness. It is demonstrable that Sirius is a hundred and sixty times 

 as bright intrinsically as our sun. From a comparison of Sirius with 

 the moon, and the moon with the sun, it is found that sunlight is 6,000 

 million times brighter than that of Sirius. But since Sirius is 1,000,- 

 000 times more distant than our sun, if brought as near us, the intensity 

 of its light would be increased in the ratio of the square of 1,000,000 

 or 1,000,000 millions. This quantity, divided by 6,000 millions, would 

 show Sirius to be at least a hundred and sixty times as bright intrinsi- 

 cally as our sun. 



It is fair, however, to suppose that the chief cause of difference of 

 brightness of stars is difference of distance. If so, we then have the 

 means of approximating to the distance of even telescopic stars. Her- 

 schel estimated that stars of the first order emit, on an average, light 

 one hundred times as intense as those of the sixth ; hence the latter 

 must be at ten times the distance, the intensity of light varying in- 

 versely as the square of the distance. Sirius is 98,000,000 of millions 

 of miles from us; stars of the sixth magnitude are therefore about 

 1,000 millions of millions distant, and their light reaches us only after 

 a flight of 169 or 170 years. 



The space-penetrating power of a telescope depends upon the ratio 

 between its aperture and the pupil of the eye. Herschel estimated that 

 his four-foot reflector penetrated 194 times as far as the naked eye ; 

 and, as the faintest stars visible to the naked eye are ten times as dis- 

 tant as the brightest, it follows that the faintest stars visible in the 

 field of Herschel's telescope are 1,940 times as distant as Sirius or 

 Alpha Lyrce. In other words, Sirius would still be visible if removed 

 to about 2,000 times its present distance. With Lord Rosse's six-foot 

 reflector Sirius would still be \'isible at 3,000 times its present distance. 

 If, therefore, this brightest star in the heavens should suddenly shoot 

 away from us with the velocity of light, it would remain visible to the 

 naked eye twice 170 years, since 170 years would be consumed in its 

 flight and 170 more in the return of the light; and with Lord Rosse's 

 telescope it could be traced 100,000 years longer (twice 3,000 times 170 

 years). This is the lowest estimate, too, for Sirius is many times brighter 

 than the average star of the first magnitude. If intrinsically only four 

 times as bright, it could still be seen by the telescope so infinitely far 

 into space that light from it could reach us only after the lapse of 

 100,000 years. 



But we need not pause even at these distant telescopic suns. Be- 

 yond the stars that merely dot with points of light the telescopic field 

 of view, hovering on the outskirts of this inconceivable space swept by 

 the far-fathoming line of telescopic vision, are discerned fainth'-defined 

 masses too distant to be resolved into stars, whose hazy light is gath- 

 ered from congeries of suns, the individual blazing brightness of which 

 is reduced in our view to the fleecy films of unresolvable nebulae. Even 



