Page:The American Cyclopædia (1879) Volume XV.djvu/324

 312 STAR Inn'jf known which had suddenly acquired new brilliancy. When first observed by astrono- mers with this abnormal lustre it was shining as a star of the second magnitude. Examined by Huggins and Miller with the spectroscope, its light revealed a startling state of things in those remote depths of space. The usual stel- lar spectrum, rainbow-tinted and crossed by dark lines, was seen to be crossed also by four exceedingly bright lines, the spectrum of glow- ing hydrogen. Either the star was actually " in flames " at the time, that is, surrounded by burning hydrogen, or else some cause had raised the hydrogen around the star to a state of intense heat, but without actual combustion. The greater part of the star's light manifestly came from this glowing hydrogen, though it can scarcely be doubted that the rest of the spectrum was brighter than before the out- burst, the materials of the star being raised to an intense heat. The maximum brightness of the star exceeded that of a tenth magnitude star nearly 800 times. After shining for a short time as a second magnitude star, T Corona (as the star was called thenceforth) diminished rapidly in lustre, and it is now between the ninth and tenth magnitudes. The stars are not absolutely at rest, though many years pass before the motion of any star can be detected. Halley, comparing the observed places of Arc- turus, Aldebaran, and Sirius with the places assigned by the Alexandrian astronomers, found reason to believe that these three stars are ap- proaching the ecliptic. This surmise was con- firmed by the elder Cassini, who observed that Arcturus had shifted southward 5' in latitude since the time of Tycho Brahe. Bradley made observations to give means for detecting stellar motions, and before long astronomers ' began to recognize many instances of measurable mo- tion. In 1783 Sir W. Herschel took up the idea that the stellar motions are in part due to a proper motion of the sun himself. Tobias Mayer had suggested this idea in 1771, but comparing Romer's observations with his own could find no evidence in its favor. Herschel was more successful. From the motions of seven stars, as estimated by Maskelyne, he de- duced the inference that the sun is moving toward a point in the constellation Hercules in right ascension 257. From a more exact in- quiry, using Mayer's list of proper motions, he was led to place the point toward which the sun is moving (or, as it is called, the " apex of the solar way ") near the star Hercules. In 1805, using Maskelyne's catalogue of the prop- er motions of 36 stars (published in 1790), he set the apex in right ascension 245 62' 30" and N. declination 49 38'. Bessel in 1818 expressed his agreement with Tobias Mayer, in regarding the evidence as insufficient for determining the direction of the sun's motion ; but since then Madler, Argelander, O. Struve, and Sir G. B. Airy have dealt with the prob- lem, with results confirming the views of Sir "W. Herschel in a very remarkable way, con- sidering the imperfect evidence available in Herschel's time. Nevertheless it is notewor- thy that, although the balance of the stellar motions indicates the real existence of a proper motion of our sun toward Hercules, yet on any of the usually accepted theories of stellar dis- tribution, the stellar motions accounted for by the sun's motion do not form nearly so large a proportion of the observed stellar motions as they should do. The present writer has shown by a simple geometrical method that they should constitute one half of the total ; or rather, that the sum of the squares of the ob- served displacements should be reduced one half on making the proper correction for the effects due to the sun's motion. The real re- duction, instead of being one half, is between 2*3- and ^y. This does not throw any doubt on the fact of the sun's motion, but it renders altogether untenable the commonly accepted theories as to stellar distribution. The mo- tions hitherto mentioned are apparent motions of the stars on the celestial sphere. Motions of recession or of approach would of course not be indicated in this way ,* nor would they produce any appreciable change in a star's brightness. This is easily perceived when we consider that motions of recession or of ap- proach would be of the same average order as thwart motions. What thwart motions may be in actual amount we do not know, but we do know what proportion they bear to the distances of the stars they respectively apper- tain to. Thus if a star were displaced 10" in a year (and no star has yet been observed to have so large a proper motion), the actual dis- tance traversed in one year would be to the star's distance as sin. 10" to 1, or as 20,626 to 1. A corresponding motion of recession or approach would therefore diminish or increase a star's brightness in one year by T6 ^ 23 part, and the brightness would be diminished or in- creased only by y^g- part in 103 years. Such a change would be quite inappreciable even if the observation of irregular variations of stel- lar brightness did not prevent us from placing any reliance on apparent changes of brightness as indications of distance. It might then ap- pear hopeless to attempt to determine whether the stars have motions of recession or ap- proach ; but spectroscopic analysis affords a means of dealing with this problem which has been successfully applied by Huggins and Vo- gel, and may hereafter be widely extended. If a star is changing its distance from us, light waves of any given order proceeding from the star must reach the observer with their length increased if the star is receding, and decreased if the star is approaching. On comparing, then, any known line in a stellar spectrum with the corresponding line in the spectrum of the terrestrial element, any shift of the line which can be detected will indicate recession if toward the red end of the spectrum, and approach if toward the indigo end. Applying this method, Huggins has recognized motions