126 LECTUfiES 



are convenient in reference to a general description of the heavens. 

 The more conspicuous stars have individual names as Sirius, Procyon, 

 Regulus^ &c. 



The principal classification of stars, however, is in reference to their 

 magnitudes. But we must here bear in mind that the stars have no 

 assignable area or size. Tbey have no measurable disc, like the plane- 

 tary bodies. The stars of the first magnitude, when viewed with a good 

 telescope under favorable atmospheric circumstances, sharpen down 

 to a mere brilliant point. The classification is really based on 

 hrigJdness ; and stars of the first magnitude are simply the brightest 

 stars. Any one on looking at the heavens on a clear, moonless night, 

 will be able to distinguish many degrees of brightness between Sirius, 

 the brightest star in the heavens, and one that is barely visible to the 

 naked eye. It would not be difficult, perhaps, to select twenty stars 

 within these limits, which twenty different persons, without any con- 

 cert, would arrange in ihe same order. And this oaly shows how 

 quickly and how truly the eye discriminates slight shades of differ- 

 ence in brightness. Astronomers have, however, divided the stars 

 visible to the naked eye into six classes, and the telescopic stars into 

 ten additional ones, making sixteen in all. Stars of the 15th and 

 16th classes are test objects for the best telescopes which have been 

 made. It must not, however, be supposed that there is any very well 

 defined difference between the fiaintest star of the first magnitude and 

 the brightest of the second. These will not differ more from each 

 other than several of the stars of either class. 



While it is very easy to arrange the stars in the order of their 

 brightness, it is not easy to determine how much they differ the one 

 from the other. Many attempts have been made to ascertain photo- 

 metrically, (that is, by measuring in some way the quantity or intensity 

 of light,) hoio much the light of Sirius, for instance, exceeds that of 

 Aldebaran, or any medium star of the first magnitude. These efforts 

 have extended to the comparison of a star of the Gth magnitude with 

 Sirius, and, again, Sirius with the moon and the sun. Researches 

 upon this subject are attended with many difficulties, and the results 

 cannot, of course, be considered very exact, and yet they afford toler- 

 able approximations to the truth. From comparisons which have 

 been made by different methods and by different persons, it is concluded 

 that the light of Sirius is fully equal to three times that of a star of 

 medium first class magnitude ; in other words, three first class stars 

 of the brightness of Aldebaran or Regulus, united, would only make 

 one of Sirius. The comparison being restricted to medium stars 

 of their respective classes, one of the first class is equal to four of the 

 second, eight of the third, and one hundred of the sixth. 



Astronomers have agreed to place in the first class not more than 

 23 or 24 of the brightest stars ; in the second, 50 or 60 ; in the third, 

 about 300. Beyond the third class the numbers increase with great 

 rapidity. The whole number which have been accurately catalogued, 

 down to the seventh class inclusive, is about 15,000. Descending to 

 the telescopic classes, the numbers have become immensely great. 

 Struve, whose researches have been specially directed to this branch of 

 astronomy, concludes that the number of stars situated in Bessel's 



