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NATURE 



[August 20, 1891 



of stellar systems, in which the components are in some cases of 

 nearly equal magnitude, and in close proximity, and are re- 

 volving with velocities greatly exceeding the planetary velocities 

 of our system. 



The K line in the photographs of Mizar, taken at the Harvard 

 College Observatory, was found to be double at intervals of 

 fifty-two days. The spectrum was therefore not due to a single 

 source of light, but to the combined effect of two stars moving 

 periodically in opposite directions in the line of sight. It is 

 obvious that if two stars revolve round their common centre of 

 gravity in a plane not perpendicular to the line of sight, all the 

 lines in a spectrum common to the two stars will appear alter- 

 nately single or double. 



In the case of Mizar and the other stars to be mentioned, the 

 spectroscopic observations are not as yet extended enough to 

 furnish more than an approximate determination of the element? 

 of their orbits. 



Mizar especially, on account of its relatively long period— 

 about 105 days — needs further observations. The two stars art- 

 moving each with a velocity of about fifty miles a second, prob 

 ably in elliptical orbits, and are about 143 millions of miles 

 apart. The stars, of about equal brightness, have together a 

 mass about forty times as great as that of our sun. 



A similar doubling of the lines showed itself in the Harvard 

 photographs of j8 Aurigse at the remarkably close interval of 

 almost exactly two days, indicating a period of revolution of 

 about four days. According to Vogel's later observations, each 

 star has a velocity of nearly seventy miles a second, the distance 

 between the stars being little more than seven and a half mil- 

 lions of miles, and the mass of the system 47 times that of the 

 sun. The system is approaching us at the speed of about sixteen 

 miles a second. 



The telescope could never have revealed to us double stars of 

 this order. In the case of ;3 Aurigrc, combining Vogel's distance 

 with Pritchard's recent determination of the star's parallax, the 

 greatest angular separation of the stars as seen from the earth 

 would be 1/2CO part of a second of arc, and therefore very far 

 too small for the highest powers of the largest telescopes. If 

 we take the relation of aperture to separating power usually 

 accepted, an object-glass of about 80 feet in diameter would be 

 needed to resolve this binary star. The spectroscope, which 

 takes no note of distance, magnifies, so to speak, this minute 

 angular separation 4000 times ; in other words, the doubling of 

 the lines, which is the phenomenon that we have to observe, 

 amounts to the easily measurable quantity of twenty seconds of 

 arc. 



There were known, indeed, variable stars of short period, 

 which it had been suggested might be explained on the hypo- 

 thesis of a dark body revolving about a bright sun in a few days, 

 but this theory was met by the objection that no such systems of 

 closely revolving suns were known to exist. 



The Harvard photographs of which we have been speaking, 

 were taken with a slitless form of spectroscope, the prisms being 

 placed, as originally by Fraunhofer, before the object-glass of 

 the telescope. This method, though it possesses some advant- 

 ages, has the serious drawback of not permitting a direct com- 

 parison of the star's spectrum with terrestrial spectra. It is 

 obviously unsuited to a variable star like Algol, where one star 

 only is bright, for in such a case there would be no doubling of 

 tic lines, but only a small shift to and fro of the lines of the 

 blight star as it moved in its orbit alternately towards and from 

 our^ system, which would need for its detection the fiducial 

 positions of terrestrial lines compared directly with them. 



For such observations the Potsdam spectrograph was well 

 adapted. Prof. Vogel found that the bright star of Algol did 

 pulfate backwaifls rnd forwards in the visual direction in a 

 period correfjci ding to the known variation of its light. The 

 explanation wliidi 1 nd been suggested for the star's vniinbility, 

 that it was partially eclipsed at regular inteivals of 68-8 hours 

 by a dark companion large enough to cut off nearly five-sixths 

 of its light, was therefore the true one. The dark companion, 

 no longer able to hide itself by its obscureness, was brought out 

 into the light of direct observation by means of its gravitational 

 effects. 



Seventeen hours before minimum, Algol is receding at the 

 rate of about 24J miles a second, while seventeen hours after 

 minimum it is found to be approaching with a speed of about 

 28J miles. From these data, together with those of the varia- 

 tion of its light, Vogel found, on the assumption that both 

 stars have the same density, that the companion, nearly as large 



NO. IJ38, VOL. 44] 



as the sun, but with about one-fourth his mass, revolves with a 

 velocity of about fifty-five miles a second. The bright star, of 

 about twice the size and mass, moves about the common centre 

 of gravity with the speed of about twenty-six miles a second. 

 The system of the two stars, which are about 3^ millions of 

 miles apart, considered as a whole, is approaching us with a 

 velocity of 2*4 miles a second. The great difference in lumin- 

 osity of the two .stars, not less than fifty times, suggests rather 

 that they are in different stages of condensation, and dissimilar 

 in density. 



It is obvious that if the orbit of a star with an obscure com- 

 panion is inclined to the line of sight, the companion will pass 

 above or below the bright star, and produce no variation of its 

 light. Such systems may be numerous in the heavens. In 

 Vogel's photographs, Spica, which is not variable, by a small 

 shifting of its lines reveals a backward and forward periodical 

 pulsation due to orbital motion. As the pair whirl round their 

 common centre of gravity, the bright star is sometimes ad- 

 vancing, at others receding. They revolve in about four days, 

 each star moving with a velocity of about fifty-six miles a second 

 in an orbit probably nearly circular, and possess a combined 

 mass of rather more than two and a half times that of the sun. 

 Taking the most probable value for the star's parallax, the 

 greatest angular separation of the stars would be far loo small to 

 be detected with the most powerful telescopes. 



If in a close double star the fainter companion is of the white- 

 star type, while the bright star is solar in character, the com- 

 posite spectrum would be solar with the hydrogen lines unusually 

 strong. Such a spectrum would in itself afford some probability 

 of a double origin, and suggest the existence of a companion 

 star. 



In the case of a true binary star the orbital motions of the 

 pair would reveal themselves in a small periodical swaying of 

 the hydrogen lines relatively to the solar ones. 



Prof, Pickering considers that his photographs show ten stars 

 with composite spectra ; of these, five are known to be double. 

 The others are : t Persei, i Aurigoc, 5 Sagittavii, 31 Ceti, and 

 fi Capricorni. Perhaps /3 Lyrse should be added to this list. 



In his recent classical work on the rotation of the sun, Duner 

 has' not only determined the solar rotation for the equator but 

 for different parallels of latitude up to 75°. The close accord- 

 ance of his results shows that these observations are sufficiently 

 accurate to be discussed with the variation of the solar rotation 

 for different latitudes which had been determined by the older 

 astronomical methods from the observations of the solar spots. 



Though I have already spoken incidentally of the invaluable 

 aid which is furnished by photography in some of the applica- 

 tions of the spectroscope to the heavenly bodies, the new power 

 which modern photography has put into the hands of the as- 

 tronomer is £0 great, and has led already, within the last few 

 years, to new acquisitions of knowledge of such vast importance, 

 that it is fitting that a few sentences should be specially devoted 

 to this subject. 



Photography is no new discovery, being about half a century 

 old ; it may excite surprise, and indeed possibly suggest some 

 apathy on the part of astronomers, that though the suggestion 

 of the application of photography to the heavenly bodies dates 

 from the memorable occasion when, in 1839, Arago, announcing 

 to the Academic des Sciences the great discovery of Niepce and 

 Daguerre, spoke of the possibility of taking pictures of the sun 

 and moon by the new process, yet that it is only within a few 

 years that notable advances in astronomical methods and dis- 

 covery have been made by its aid. 



The explanation is to be found in the comparative unsuitability 

 of the earlier photographic methods for use in the observatory. 

 In justice to the earlier workers in astronomical photography, 

 among whom Bond, De la Rue, J, W. Draper, Rutherfurd, 

 Gould, hold a foremost place, it is needful to state clearly that 

 the recent great successes in astronomical photography are not 

 due to greater skill, nor, to any great extent, to superior instru- 

 ments, but to the very great advantages which the modern 

 gelatine dry plate possesses for use in the observatory over the 

 methods of Daguerre, and even over the wet collodion film on 

 glass, which, though a great advance on the silver plate, went 

 but a little way towards putting into the hands of the astronomer 

 a photographic surface adapted fully to his wants. 



The modern silver-bromide gelatine plate, except for its 

 grained texture, meets the needs of the astronomer at all points. 

 It possesses extreme sensitiveness ; it is always ready for use ; 



