744 "THE POPULAR SCIENCE MONTHLY. 



components are of magnitudes four and a half and ninth, distance 

 57", p. 110. Burnham discovered that the ninth-magnitude star 

 consists of two of the tenth less than 2" apart, p. 34. 



No astronomer who haj^pens to be engaged in this part of the 

 sky unless his attention is entirely absorbed by something of spe- 

 cial interest, ever fails at least to glance at ^ Librte, which is fa- 

 mous as the only naked-eye star having a decided green color. 

 The hue is pale, but manifest.* 



The star 8 is a remarkable variable, belonging to what is 

 called the Algol type. Its period, according to Chandler, is 3 

 days, 7 hours, 51 minutes, 33'8 seconds. The time occupied by the 

 actual changes is about twelve hours. At maximum the star is 

 of magnitude five and at minimum of magnitude &"Z. 



We may now conveniently turn northward from Virgo in 

 order to explore Bootes, one of the most interesting of the con- 

 stellations (map No. 11). Its leading star a, Arcturus, is the 

 brightest in the northern hemisphere. Its precedence over its 

 rivals Vega and Capella has been settled by the Harvard pho- 

 tometry. You notice that the color of Arcturus, when it has not 

 risen far above the horizon, is a yellowish red, but when the star 

 is near mid-heaven the color fades to yellow. The hue is possibly 

 variable, for it is recorded that in 1853 Arcturus appeared to have 

 nearly lost its color. If it should eventually turn white, the fact 

 would have an important bearing upon the question whether 

 Sirius was once a red or flame-colored star. 



But let us sit here in the starlight, for the night is balmy, and 

 talk about Arcturus, which is perhaps actually the greatest sun 

 within the range of terrestrial vision. Its parallax is so minute 

 that the consideration of the tremendous size of this star is a 

 thing that the imagination can not placidly approach. Calcula- 

 tions, based on its assumed distance, which show that it outshines 

 the sun several thousand times may be no exaggeration of the 

 truth ! It is easy to make such a calculation. Dr. Elkin's paral- 

 lax for Arcturus is O'OIS". That is to say, the displacement of 

 Arcturus due to the change in the observer's point of view when 

 he looks at the star first from one side and then from the other 

 side of the earth's orbit, 180,000,000 miles across, amounts to only 

 eighteen one-thousands of a second of arc. We can appreciate 

 how small that is when we know that it is about equal to the 

 apparent distance between the heads of two pins placed an inch 

 apart and viewed from a distance of a hundred and eighty miles ! 



Assuming this estimate of the parallax of Arcturus, let us see 

 how it will enable us to calculate the probable size or light-giv- 



* Has the slipiht green tint perceptible in Sirius deepened of late ? I am sometimes dis- 

 posed to think it has. 



