MO 



.STARS 



be, and to measure thin astronomers net themselves. 

 In 1805 Piazzi published Home result*, only to be 

 disproved. Pond and Briukley in England dis- 

 puted concerning each other's results, coming to 

 no definite conclusion. Their work, combined with 

 that of Strove at Dorpat in 1820, showed that the 

 change of place looked for called Parallax (q.v.) 

 was even smaller than had been supposed, in 

 fact inappreciable to the instruments of that day. 



Finally Bessel of Konigsberg, furnished WOT a 

 fine heliometer by Fraunhofer, found in 1840 that 

 the star 61 Cygni showed a parallax of 0"'3483, 

 since raised by modern observation to 0"'475. This 

 placed 61 Cygni at a distance = 600,000 times that 

 of the sun from the earth. Strove found about the 

 same time a parallax of 0"-26 for the star Vega, a 

 value since slightly diminished. Henderson also 

 found for a Centauri, a bright southern star, a 

 parallax of about i"-o, since corrected to 0"'75, a 

 distance equal to 4J years' journey of light. The 

 year's journey of Light (q.v.) is now used as the 

 unit in which to express star distance. The dis- 

 tance expressed thus is easily found from the 



parallax by the following equation : = d, 



where p is the parallax, as usually given, in 

 seconds of arc, and d = the distance required. The 

 number 3-202 is a constant depending on the velo- 

 city of light. Succeeding these early observers, 

 Auwers and Brunnow in Europe, and C. A. F. 

 Peters in the United States, continued this great 

 work, and in our own day Professor A. Hall, 

 of Washington, and Drs Ball, Gill, and Elkin, 

 in Britain and at the Cape of Good Hope, have 

 wrought diligently in the same field. We give 

 below a table of a few of the more interesting 

 stars, with their parallaxes and distances : 



Star. Fumllu. 



Centauri 0"-76 



61 Cygni 0"-47 



Arrt urns 0"'127 



Vega 0"-18 



Polaris 0"-076 



Aldebomn 0"-B16 



Di.Un^ln 



Uiht-uulU. 



4-8 



6-80 



26-41 



1778 



4245 



6-20 



Observer!, 

 Gill and Elkin. 

 Ball. 



C. A. F. Peters. 

 A. Hall. 

 C. A. F. Peters. 

 O. Struve. 



Since 1889 Professor Pritchard of Oxford has em- 

 ployed photography in determining stellar parallax 

 with considerable success. But very much yet 

 remains to be done in this field of research. 



From what is said above it is evident that the 

 stars are distributed through all space, as far as 

 we know it. To a cursory glance they appear to 

 be capriciously scattered, but more minute observa- 

 tion shows traces of arrangement, though we are 

 not yet able to state distinctly the law governing 

 it. The Galaxy (q.v.) or Milky Way forms a ring 

 in the heavens, entirely consisting or minute stars, 

 and as we pass away from this ring the stars 

 become much less abundant. The Herschels laid 

 the foundation for this important discovery by 

 their star-gauges, A telescope of 18 inches aper- 

 ture, 20 feet focus, and a magnifying power of 180, 

 giving a field of view 15' in diameter, was used in 

 all these. The process consisted in directing this 

 instrument to a part of the sky and counting the 

 stars in ite field. This repeated hundreds of times 

 gave a fair idea of the average numl>er of stars in a 

 circle of 15' diameter at all parts of the sky. It was 

 found that at 90 from the ring of the Galaxy this 

 number was 4-15, at 45 from it 10-36, at 15 30-30, 

 and in the ring 122-00. Sir W. Herschel considered 

 this to indicate that our sun was one of the stars 

 of the Galaxy, which extended on all sides of us as 

 a flat disc of stars. This view, however, pre- 

 supposes that the stars in this disc-like cluster are 

 somewhat uniformly distributed, a supposition 

 which has little to support it. The Galaxy may 

 better be considered as what it appears, a ring of 

 mall stars relatively near one another and vastly 



distant from us. The whole question remains still 

 unsettled, awaiting more numerous measures of 

 stellar absolute parallax than we yet possess. 



In particular part* of the sky stare gather in 

 dusters. The Pleiades, Hyades, and Pnesepe are 

 examples of loose clusters of com|>aratively bright 

 stars. Telescopes reveal marvellous groups, such 

 as that in Hercules, which contains in very small 

 space thousands of stars, spreading at the edge of 

 the mass into curious curved sprays, an arrange- 

 ment seen also in less closely packed groups. 

 These clusters are quite distinct from Nebulie 

 (q.v.), and form still a puzzle to the astronomer. 

 Tin- iiiiliceiihi- form a very curious instance of 

 a clustering mixture of nebuhe and stars. In 

 the case of all these clusters, and even of groups 

 less closely compacted, it is probable that a physical 

 connection binds the various members together. 

 The five intermediate stars of the ' Plough ' are 

 known to be moving together, in the same direc- 

 tion and with sensibly equal speed. And thi.- 

 grouping is evident in many other cases. But the 

 laws governing and producing it remain yefc 

 unknown. 



We have alluded here to star motion, called by 

 astronomers Proper Motion. This, although hardly 

 affecting the places of the stars in the sky as 

 marks of reference, is distinct and even measured 

 in the case of very many stars. An annual motion 

 of this kind amounting to 6" of a great circle of 

 the heavens is accounted large, only some three 

 or four stars exceeding this speed, which would 

 require more than 300 years to transfer the star a 

 distance in the sky equal to the moon's diameter. 

 Although so minute in apparent amount, these 

 motions are in reality of a s]ieed inconceivable by us. 

 The star Groombridge 1830 has a real velocity of 

 more than 200 miles per second, although annually 

 shifting its place in the sky by only 7" "03; 61 

 Cygni lias a proper motion of 5""12; a small star 

 1584, xxiiL h Cordoba Catalogue, has recently been 

 shown to have a proper motion of 6" "205, while 

 many others have motions of much less amount. 

 Part of these motions is due to the solar move- 

 ment in space (see SI:N ), which has been calculated 

 from them. 



Double and binary stars were first discovered by 

 Sir William Herschel, and the known number is 

 noted now in thousands, and is continually increas- 

 ing. There are stars so close as to appear single 

 to the eye, or even, in many cases, to consider- 

 able telescopic power, but which, when sufficiently 

 magnified, are seen to consist of two or more 

 separate stars. In some cases no known telescopic 

 power suffices to show the double separate, but 

 the periodical doubling of certain lines in ite 

 spectrum alone reveals its duplicity. In other 

 cases a single star may show such irregularities 

 in its proper motion as to lead to the suspicion 

 that it is attended by a dark companion, whose 

 presence is only indirectly revealed to us by the 

 effect it lion in disturbing its luminous neiglibour. 

 This is the case with Sinus (q.v.) and Procyon, 

 the companion of the former having Keen actually 

 discovered as an orb of feeble light, after Bessel 

 had pointed out the probability of its existence. 

 Other stars are also surmised to have companions 

 of this kind. Some of the double stars appear to 

 have no physical connection between their com- 

 ponents. Others undoubtedly form si/stems, of 

 two or more suns revolving in periods round their 

 common centre of gravity. These periods vary 

 from 1159 years in the case of 61 Cygni to 10'8- 

 years in that of S Equnlei. When the distance 

 from us and period of such a ' double ' are known, 

 together with the dimensions of its orbit, its mass 

 can be calculated, on the supposition that it is 

 controlled by the force of gravitation. 



