144 



THE POPULAR EDUCATOR. 



of the stars. Delicate observations show us that they have 



proper motions, but it ia very difficult to determine these. We 



can, however, ascertain the motion of the Sun by observing the 



relative distances of the stars. We find that in 



one part of the sky the stars seem to be very 



gradually opening out, and getting further 



apart, while in the opposite quarter they are 



as gradually closing up, evidently showing 



that we are moving towards the former part, 



just as when we are travelling in a wood 



the trees in front seem opening out, while those 



we have passed appear to be getting closer 



together. 



Astronomers have naturally been anxious 

 to ascertain something of the shape of the 

 whole cluster of stars which constitutes our 

 system, and have employed the telescope as a sounding-line to 

 learn the depth in different directions. If the stars are scattered 

 at all uniformly in space, they will, of course, appear more 

 sparse in those parts where we 

 look through the thinnest layer 

 of them. Now when we observe 

 bhe sky on a clear night, we at 

 once notice a pale belt passing 

 found it, commonly known as the 

 Milky Way. In one part of its 

 course it divides into two branches, 

 which, after separating a little 

 way, and passing about a third 

 round the sky, again unite into 

 one. Powerful telescopes show us 

 bha,t this consists of a dense mass 

 of minute stars. The greater por- 

 tion, indeed, of those visible are 

 clustered along this line, while in 

 those parts of the sky removed 

 from it the number of telescopic 

 stars is comparatively small ; 

 hence we may reasonably assume 

 that this belt indicates to us the 

 direction in which the greatest 

 number of the stars lie, and in 

 which our cluster extends fur- 

 thest. 



From this we may form an idea 

 of our system, and it seems that 

 the best representation of it may 

 be obtained by taking a flat 

 circular body as, for example, a 

 cheese and splitting it by passing 

 a knife about one-third of the way 

 through, the two parts being made 

 to diverge a little, as shown at a. b 

 (Fig. 44). The Sun, (s) is situ- 

 ated somewhere near the centre, 

 and the split side causes the 



divided appearance of the Milky Way. One of the nebulae, 



when seen through a powerful telescope, is found to present a 



somewhat similar appearance, and is considered to be a cluster 



closely resembling our own. 

 When we look at the heavens 



on a clear night, we observe 



here and there two stars in very 



close proximity : the telescope 



further reveals to us that very 



many of these which appear to 



the naked eye as single stars, 



consist in reality of two or more 



so close together that they ap- 

 pear as one. Sir W. Herschel 



was the first to direct special 



attention to these objects, of 



which he compiled a list. He 



hoped that by very accurate 



Fig. 43. THE CONSTELLATION ORION. 



Fig. 44. SECTION OP THE MILKY WAT. 



they happened to lie in a straight line directed almost towards 

 the Earth ; that they were, in fact, merely optical couples, one 

 being an immense distance behind the other. After many 

 observations, Herschel found that their distances 

 and relative positions did vary, but instead of it 

 being, as he expected, an annual fluctuation 

 caused by the Earth's motion, it was a pro- 

 gressive change. He thus found that in some 

 cases the two stars were revolving round one 

 another in elliptical orbits, and that they were 

 physical couples, the two forming one system. 

 These he called binary stars or couples, to 

 distinguish them from optical pairs. Other 

 observers have followed up these investiga- 

 tions, and there are now upwards of 600 

 binary stars known and noted, and in many 

 cases their times of revolution have been calculated. 



One of the best examples of this class is afforded by Lyrae, 

 which is sometimes called the Double-double Star. To the naked 

 eye it appears a somewhat faint 

 star, but a telescope of very little 

 power will show it to be double. 

 When, however, a more powerful 

 instrument is employed, each of 

 these components is in turn found 

 to consist of two smaller ones, as 

 shown in Fig. 45. The lower pair 

 revolves in about 2,000 years, and 

 the upper in about half that time, 

 while the two couples take a very 

 long period to revolve around their 

 common centre of gravity. 



One remarkable feature in con- 

 nection with the double stars is 

 the fact that in some instances 

 the component stars are of differ- 

 ent colours. In R Leporis, for 

 instance, one is white, while the 

 other is a deep red ; in /8 Cygni 

 again, the colours are yellow and 

 blue ; and in 7 Andromedae, they 

 are orange and green. 



When we come to note the 

 colour of different stars, and com- 

 pare it with former records, we 

 find that in a few instances a 

 change has taken place. Thus 

 Sirius, which now shines with a 

 pure white light, is spoken of by 

 old observers as a ruddy star. 

 There are also many others which 

 exhibit changes in brilliancy, and 

 these changes seem, in most 

 cases, to be periodical. The star 

 on which this discovery was 

 made is o Ceti, called also Mira, 



or the Wonderful Star, a name it well deserves. At the 

 time of its greatest brightness it is usually of the first or 

 second magnitude, it then decreases for two or three months, 

 till it becomes invisible, and re- 

 mains so for about five months, 

 its minimum brightness being 

 about equal to that of a twelfth 

 magnitude star. It then again 

 appears, and the whole period 

 occupied by these changes is 

 about 331 days. 



Algol, or fl Persei, is another 

 variable star, remarkable for its 

 short period and rapid changes. 

 It ordinarily appears as a star 

 of the second magnitude, but in 

 a period of three and a half hours 

 it diminishes in brightness to the 



measurements of the apparent distances between them, he might 

 be able in some instances to detect a variation, and thus ascer- 

 tain their parallax, and by that their distances. The idea then 

 *vaa that these stars merely appeared close together because 



fourth magnitude, and after a few minutes begins again to 

 increase, attaining its former brilliancy in another period of 

 three and a half hours. At this it remains two days thirteen 

 hours, and then the same series of changes recurs. 



