Page:Collier's New Encyclopedia v. 09.djvu/81

STAR 55 STAB catalogue berore the invention of the telescope was that of Tycho Brahe, who redetermined with still greater accuracy the positions of 1,005 stars. Of modern catalogues, Argelander's "Sternverzeich- niss" is the largest, enumerating more than 300,000 down to the 9th magnitude, all situated between the pole and 2° S. of the equator. Benjamin Apthorp Gould, an American astronomer, extended this catalogue to the Southern Hemisphere at the Observatory of Cordoba, South America. The large majority of stars are of con- stant magnitude, but there are certain, known as variable stars, which vary in brightness and vary periodically. Algol, in the constellation Perseus, fluctuates between the 2d and 4th magnitudes in a period of about three days. Mira in the constellation Cetus is such another star, being usually invisible to the naked eye, but bursting out at intervals of 11 months with the brilliancy of a second or third magnitude star. There are many others. At times, stars have sud- denly appeared where none were known before. The most remarkable of these are the one described by Tycho Brahe (1572), that observed by Kepler (1604- 1606), and the star of 1866 which blazed out suddenly in Corona Borealis. Near the place of the first a small telescopic star now exists, and the last was entered as a 9th-magnitude star in Argeland- er's catalogue. The spectrum of the 1866 star as examined by Huggins consisted of a continuous spectrum crossed not only by the usual dark absorption lines, but also by bright lines which cor- responded in part with the spectrum lines of hydrogen. The general similarity of the stellar spectra to the solar spectrum is a convincing proof, if any further were needed, that our sun is a star, and that the stars are suns with a probable retinue of accompanying planets. According to the nature of their spectra, stars have been grouped under four types, Type I., of which Sirius, the brightest known star, is a good represen- tative, is characterized by a continuous spectrum with a very few absorption lines crossing it ; in type II. the spectrum is crossed by numerous fine lines, as in the sun's spectrum ; in type III., to which a Orionis and a Herculis belong, fluted spaces begin to appear; and in type IV., which includes the red stars, fluted spaces only exist. Here there is an evi- dent gradation from the spectrum of few absorption lines to that broken by gaps. According to the accepted theory of spec- trum analysis, the former spectrum indi- cates a higher temperature than the latter, so that here is an evidence of a de- velopment in time of each individual star. As a star is cooling, its spectrum passes through all these types, and thus our sun is at a later stage in its life history than Sirius, though it may not be really any older, since it is probably much smaller than Sirius, whose development will therefore be slower. Our sun can- not be compared in size or splendor to some of its distant compeers, not a few of which must present peculiarly com- plicated systems. Thus the telescope re- veals many stars to be really double. One of the most interesting of these is Sirius, from the irregularity of whose movements Peters and Auwers had con- cluded that it was accompanied by an attendant star or at least a large planet; and in 1862 such a companion was dis- covered by the younger Clark in the very direction in which theory had predicted it to be at that time. Its great minute- ness renders detection hopeless except in very favorable circumstances. The mo- tions of Procyon indicate a similar doubleness, but its companion has not yet been seen. Triple and multiple stars forming one system are also not un- common. For instance, u Herculis, re- cognized as a double star by Herschel, is really a triple star, the small Herschel- lian companion having been resolved into two in 1856 by Alvan Clark. An inter- esting optical property of these binary systems is that the color of the one com- ponent is frequently the complement of that of the other. A very remarkable double star is 61 Cygni, famous as the first star whose distance from the sun was calculated directly from its parallax, i. e., from the angular distance between its positions as viewed from opposite ex- tremities of the earth's orbit. The most probable mean of the various results gives about .5 of a second for this paral- lax. This, however, is not the nearest star; it is nearly twice as far away as a Cen- tauri, which according to Henderson has a parallax of 0".91, corresponding to a distance of 226,000 times the radius of the earth's orbit, or more than twenty millions of millions of miles (20 X 10"). In other words, light, which travels at the rate of 236,000 miles per second, takes nearly three years to pass from the nearest fixed star to us. The paral- lax of at least half the stars of the first magnitude is probably less than one-tenth of a second, so that their average dis- tance is greater than two million radii of the earth's orbit. Though, to the naked eye, stars have probably occupied the same relative positions from the earliest of historic times, the telescope reveals that they are not strictly fixed. Many have appreciable proper motions, among others 61 Cygni, as was before noticed.