Page:The American Cyclopædia (1879) Volume XV.djvu/325

 STAR 313 cen S of recession and approach ranging from 10 m. to nearly 50 m. a second. Some of the stars have proper motions in the same direction and at the same apparent rate. Madler, noticing this peculiarity in the constellation Taurus, was led to surmise that the centre round which all the stars are moving lies in that constella- tion, and he assigned Alcyone, the principal star of the Pleiades, as the centre in question. Beyond the observed community of motion in Taurus there was not any direct evidence for this theory ; and this observed phenomenon as held by astronomers to afford but weak vidence for a theory of importance. Yet Mad- ler's views were described in every text book of astronomy, in terms which would have been cely justified if there had been an over- helming mass of evidence in their favor, and astronomers had been practically unanimous accepting them. In point of fact, even the ne piece of direct evidence which seemed to pport Madler's theory is found on examina- on to have no weight whatever. It is true that if there is a centre around which all the stars are moving, the stars lying toward that centre should exhibit a community of proper otions, and the stars in Taurus do exhibit the uliarity ; but unfortunately for the theory, the same feature exists in other parts of the heavens. A map constructed by the present riter, showing all the stellar proper motions yet satisfactorily determined, exhibits many ch cases, and some of them are more remark- le than the case of the stars in Taurus. One igular instance of this "star drift" is ob- rved in the constellation Ursa Major, in hich the stars /?, y, tf, e, and are all travel- g in the same direction and at the same rate. s these are bright stars, it appeared to the riter that they would afford an instructive t of the theory of star drift, if their motions recession or approach could be determined, is was effected by Huggins a year after the rheory of star drift had been enunciated, and was found that, as the theory required, the ve stars had a common motion in the direc- >n of the line of sight, and that they are all "ing at the rate of about 17 m. a second m the solar system. The inference fairly educible from this fact, that these stars form single system or family travelling together rough space, is interestingly confirmed by e fact that all five belong to the same order. See SPECTRUM ANALYSIS.) Although many )eculations were broached respecting the con- ^itution of the sidereal heavens from the ear- iest ages of astronomy, the first to enter on " e systematic study of the subject, combining bservation with theory, was Sir TV. Herschel. "itchel, it is true, had theorized carefully and undly, but his labors were not extended yond a few points of detail; and though right of Durham made some observations for the purpose of determining the structure
 * >f the milky way, yet the telescope he used

only one foot in focal length) was far too small to give any really satisfactory results. At the beginning of his labors Sir TV. Herschel took as the basis of his conceptions the belief that our sun is a member of a system of suns, scattered with a certain general uniformity throughout a region of space having a defined figure, possibly determinable if only a telescope could be constructed powerful enough to reach the limits of the system in all directions. To effect this, he devised his system of " star gau- ging by counting." It is clear that the further the sidereal system extends in any given direc- tion, the greater will be the number of stars lying toward that direction, since the distribu- tion is supposed (in a general sense) uniform ; and therefore, if the same telescope, with un- changed power, were directed toward every part of the heavens in turn, then by counting the number of stars brought into view in these different directions the relative extension of the system along those visual lines could be determined in other words, the shape of the star system. Let it be noticed that this plan of star gauging required that one and the same telescope should be applied to different parts of the heavens ; it assumed a general uniformi- ty of distribution within the limits of the sys- tem ; and it required that the telescope should penetrate to those limits. Recognizing these points, we shall not fall into the mistake made by many (including Arago and the French astronomers generally, Smyth, and others, and repeated in almost all the text books) of con- founding this method of star gauging with the method devised by Sir TV. Herschel when a long experience had convinced him that the assumptions on which he had based the for- mer method were unsound. While he still supposed these assumptions sound, however, he deduced as the result of applying his first method the inference that the sidereal sys- tem is shaped like a cloven fiat disk. (See GALAXY.) But gradually his observations showed him that special laws of aggregation exist within the star depths. He saw, first, that the milky way is not produced by the combined lustre of stars scattered like those around us, but extending to enormous dis- tances. Next he perceived that the stars form- ing the richer parts of the milky way are not arrayed along great ranges in distance, but really spread more richly within limited and roughly globular regions. In the same paper (all the passages we quote are from the " Phil- osophical Transactions") he wrote as follows: " On a very slight examination it will appear that this immense starry aggregation [the milky way] is by no means uniform. . . . By refer- ring to some one of these clustering aggre- gations in the heavens, what will be said of them will be much better understood than if we were to treat of them in a general way." He selects the great double clustering aggrega- tions in Cygnus, which form such conspicuous star clouds on clear summer nights. Here, he says, " the stars are clustering with a kind of