THE DOUBLE (AND BINARY) STARS. 



By F. W. HENKEL, B.A., F.R.A.S. 



. Other Suns, perhaps, 

 With their attendant moons, thou wilt descry 

 Communicating male and female light, 

 Which two great sexes animate the World, 

 Stored in each orb, perhaps with some that live." 



—Milton ("Paradise Lost," VIII, 148-152). 



Though the Copernican hypothesis of the Earth's 

 motion round the Sun was immensely strengthened 

 by the discoveries of Galileo, and the Newtonian 

 theory of gravitation supplied the modus operandi 

 for this motion, yet the objection of the opponents 

 of Copernicus that the stars do not appear to move 

 as they should do in consequence of that motion, 

 long remained a serious difficulty. " Were the 

 Earth in motion in a mighty orbit round the Sun," 

 said they, " spectators on our planet would, without 

 difficulty observe displacements in the relative 

 positions of the stars during the course of the 

 year, the nearer stars moving more and the more 

 distant ones less, just as objects in the surrounding 

 landscape appear to move when seen by a spectator 

 in a moving carriage." But no such motions could 

 be detected, and all that could be answered was that 

 the distances of the stars are so vast that the size of 

 the Earth's orbit is but an insignificant quantity in 

 comparison. It was no doubt partly on account of 

 his difficulty in accepting this explanation that 

 Tycho Brahe was led to reject the Copernican views, 

 since his observations had not enabled him to detect 

 any " parallactic " change of place due to the Earth's 

 supposed motion, and his measurements of the 

 apparent diameters of the stars had led him to con- 

 clude that the brightest of these objects would be of 

 enormous dimensions (greater than the whole Earth's 

 orbit) were the Copernican hypothesis true. After 

 the invention of the telescope it was seen that the 

 apparent diameters of the stars were very much less 

 than had been assumed to be the case by Tycho and 

 others and up to the present time the true diameter 

 of a star has never been measured, being less than 

 the minimum visible, the disc seen by the naked eye 

 being an optical illusion, an effect of irradiation. 



Galileo, in one of his " Dialogues on the Systems 

 of the World," proposed that pairs of stars seen 

 close together in the sky should be observed and 

 their relative position noted and distance measured 

 throughout the year. If one of these objects be 

 nearer to us than the other, it will be displaced and 

 the angles and distances will change regularly in 

 that interval of time. The same idea was suggested 

 by others, but the first to carry out the suggestion 



systematically was Sir William Herschel. The 

 latter, finding many cases where two or some- 

 times more stars lie close together in the sky, 

 and supposing the brighter component was 

 nearer to our system than the fainter one, 

 made numerous measurements, expecting to find 

 regular annual variations due to this supposed 

 difference of distance — in other words, parallactic 

 displacement.* But instead of finding this Herschel 

 detected a regular progressive change of quite a 

 different nature, showing sometimes that one of these 

 bodies was describing an orbit round the other, or 

 that both were travelling together throughout space. 

 In his own words he " went out like Saul to seek his 

 lather's asses, and found a kingdom " — the existence 

 of systems (of Stars) of a different and higher order 

 from that prevailing in our own system, binary 

 and multiple stars. A distinction was thus made 

 between stars optically connected which merely lie 

 nearly in the same direction, as seen from our planet, 

 but may be as far removed from one another as they 

 severally are from our system, and stars physically 

 connected — systems consisting of two or more 

 members moving round a common centre, or in 

 nearly parallel paths in a common direction. All 

 over the sky there are to be found cases of two or 

 more stars lying much closer together than the 

 average distances of the stars from one another, and 

 perhaps as many as twelve thousand such couples 

 are known, the double stars. Some of these doubles 

 have components of nearly equal brightness (e.g., 

 the two components of a Centauri, the nearest of all 

 external celestial objects to our system, so far as is 

 yet known, and the two components of 61 Cygni, 

 the next nearest system — both of these are also 

 binaries) ; in other cases the members are of very 

 unequal magnitudes, like Sirius and its companion, 

 the former being the brightest of all the stars visible 

 to us, whilst the latter is only visible by the help of 

 the most powerful telescope, and was not detected 

 till 1862. In that year the late Alvan Clark first saw 

 it with the recently finished Chicago refractor, the 

 then largest instrument of its kind in the world. 

 Several hundred binary systems are now known with 

 more or less certainty, moving in elliptic orbits 

 round common centres, the orbits in some cases 

 being almost circular, in others very oval and 

 " eccentric." There are also systems of three or 

 more stars, the trinary and multiple stars. The star 

 £ Cancri consists of two larger and fairly close 

 members revolving in nearly circular orbits round 



■■'■ Parallax is the name given to an apparent displacement of an object due to a real displacement of the position of the 

 observer. Technically, the parallax of a star is the angular value of the semi-diameter of the Earth's orbit (which is 



greater as the star is nearer), as seen from the star. 



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