TO KNOW THE STARRY HEAVENS 



217 



A little past the meridian in the 

 south Jupiter shines out with sixteen 

 times the brightness of a first magni- 

 tude star ; on account of its great bril- 

 liance it at once attracts the attention 

 of every observer. Nor should the 

 reader fail to notice the less conspic- 

 uous, but nevertheless equal, beauty 

 of the western skies. Here he will see 

 the bright, bluish stars, Vega and Al- 

 tair, almost setting, while the very per- 

 fect figure of the Northern Cross now 

 apparently rests in an upright position 

 on the western ground. 



A System Made Up of Five Suns. 



Even a very small telescope will 

 show that the bright star, Castor, is 

 a double sun, the brighter star of this 

 close and beautiful pair being about 

 twice as bright as the companion. The 



Figure 2. Path of the companion star about Cas- 

 tor. 



angular distance between them is now 

 a little less than six seconds and is 

 about equal to the angle subtended by 

 a line one inch long placed at a distance 

 of half a mile. Yet this is a very wide 

 separation for the two components of 

 a true double-star system. 



The companion of Castor is moving 

 around it in a great orbit of the gen- 

 eral form shown in Figure 2, a com- 

 plete revolution being probably ac- 

 complished in about live hundred 

 years. Twenty years ago it was dis- 

 covered that this companion star is it- 

 self attended by a massive but invis- 

 ible companion, which revolves around 

 it in a period of three days, while ten 

 years later the surprising discovery 

 was made that the brighter star was 

 also double, its invisible attendant 

 completing a revolution about it in a 



little more than nine days. The two 

 suns of the first pair are about 800,000 

 miles and those of the second about 

 1,000,000 miles apart. Thus when, in 

 these early evenings of December, we 

 look at this bright though very distant 

 star, we may try to picture to ourselves 

 what a wonderfully complicated sys- 

 tem it is, with its four great suns ever 

 moving about one another. 



As if this were not enough, there is 

 seen in the telescope a faint star, far 

 to the west of the pair, which is being 

 carried along with Castor as the latter 

 rapidly drifts through the depths of 

 space. Thus it undoubtedly belongs 

 to the system, so that there are here no 

 less than five great suns, forming a 

 single system together. 



The southern one of the Twin Stars, 

 called Pollux, is seen in the telescope 

 to have no less than five companions 

 within measurable distance, and at first 

 sight might promise to be almost as in- 

 teresting an object as its neighbor. But 

 Pollux is drifting over the sky at the 

 rate of two-thirds of a second a year (a 

 motion which will change its position 

 an amount equal to the apparent diam- 

 eter of the moon in the course of about 

 three thousand years), and but a few 

 careful measures are needed to show us 

 that in this motion it is leaving all of 

 its companions behind. Thus these stars 

 have no real connection with the lesser 

 Twin Star ; they are probably immense- 

 ly more distant, and they only happen 

 to lie in the same direction as seen 

 from our earth. 



A few centuries ago Castor was the 

 brighter of the Twin Stars, but Pollux 

 now exceeds it more than two times in 

 brightness. The brilliance of one of 

 the stars is thus slowly changing, but 

 the change is so slow that it has not 

 been detected by observations of recent 

 years, and we cannot tell which of the 

 stars is the variable one. 



The Variable Star, Mira. 

 The reader has probably noticed the 

 gradual brightening of this remarkable 

 star, which has now become a very 

 easily visible object. It is situated be- 

 low and to the left of Jupiter, in the 

 position B of Figure i. Having found 

 it, it will be a matter of interest to look 

 at it from time to time during the com- 



