FREE BOOKS

Author's List




PREV.   NEXT  
|<   69   70   71   72   73   74   75   76   77   78   79   80   81   82   83   84   85   86   87   88   89   90   91   92   93  
94   95   96   97   98   99   100   101   102   103   104   105   106   107   108   109   110   111   112   113   114   115   116   117   118   >>   >|  
es its light to affect the retina of the eye less quickly than does that of its brighter companion, and, in consequence, the reversal of its apparent motion with the swinging of the telescope is not perceived so soon. The third-magnitude star eta has a companion of magnitude ten and a half, distance 90", p. 340 deg.. The star beta, of the second magnitude, and reddish, is variable to the extent of half a magnitude in an irregular period, and gamma, of magnitude two and a half, has an eleventh-magnitude companion, distance 162", p. 285 deg.. [Illustration: MAP NO. 20.] Our interest is revived on turning, with the guidance of map No. 20, from the comparative poverty of Pegasus to the spacious constellation Cetus. The first double star that we meet in this constellation is 26, whose components are of magnitudes six and nine, distance 16.4", p. 252 deg.; colors, topaz and lilac. Not far away is the closer double 42, composed of a sixth and a seventh magnitude star, distance 1.25", p. 350 deg.. The four-inch is capable of splitting this star, but we shall do better to use the five-inch. In passing we may glance at the tenth-magnitude companion to eta, distance 225", p. 304 deg.. Another wide pair is found in zeta, magnitudes three and nine, distance 185", p. 40 deg.. The next step brings us to the wonderful variable omicron, or Mira, whose changes have been watched for three centuries, the first observer of the variability of the star having been David Fabricius in 1596. Not only is the range of variability very great, but the period is remarkably irregular. In the time of Hevelius, Mira was once invisible for four years. When brightest, the star is of about the second magnitude, and when faintest, of the ninth magnitude, but at maximum it seldom exhibits the greatest brilliance that it has on a few occasions shown itself capable of attaining. Ordinarily it begins to fade after reaching the fourth or fifth magnitude. The period averages about three hundred and thirty-one days, but is irregularly variable to the extent of twenty-five days. Its color is red, and its spectrum shows bright lines, which it is believed disappear when the star sinks to a minimum. Among the various theories proposed to account for such changes as these the most probable appears to be that which ascribes them to some cause analogous to that operating in the production of sun spots. The outburst of light, however, as pointed out by Scheiner, sh
PREV.   NEXT  
|<   69   70   71   72   73   74   75   76   77   78   79   80   81   82   83   84   85   86   87   88   89   90   91   92   93  
94   95   96   97   98   99   100   101   102   103   104   105   106   107   108   109   110   111   112   113   114   115   116   117   118   >>   >|  



Top keywords:

magnitude

 

distance

 

companion

 
variable
 
period
 

constellation

 

double

 

irregular

 
capable
 

variability


magnitudes
 

extent

 

occasions

 

quickly

 

brilliance

 

exhibits

 

greatest

 

seldom

 
reaching
 

fourth


averages

 

attaining

 

Ordinarily

 

begins

 

retina

 

maximum

 

remarkably

 

Hevelius

 

hundred

 

faintest


brightest

 

invisible

 
Fabricius
 

affect

 

analogous

 

ascribes

 

probable

 
appears
 
operating
 

production


Scheiner

 
pointed
 

outburst

 

spectrum

 
twenty
 
observer
 

irregularly

 

bright

 

theories

 

proposed