Star
one of the self-luminous bodies which surrounds our solar system on all sides. They are distinguished from the planets by their flickering light, by the comparative constancy of their relative positions in space, and by their inappreciable diameter even when viewed by the most powerful optical instruments. The number of stars visible to the naked eye is estimated at about 5,000; and these in constellations and classified according have early been grouped to their brightness or magnitude. Those belonging to the first six magnitudes are visible to the naked eye; but the telescope reveals myriads which are distinguished down to the 16th magnitude. The earliest catalogue which has come down to us is that in Ptolemy's "Almagest," supposed to have been compiled by Hipparchus (150 в. c.). Ptolemy gives 1,030 stars, of which several cannot now be certainly identified. The last important STAR STAR catalogue before the invention of the As a star is cooling, its spectrum passes telescope was that of Tycho Brahe, who through all these types, and thus our sun is at a later stage in its life history Brahe, who through all these types, and thus our redetermined with still greater accuracy sun is at a later stage in its life history the positions of 1,005 stars.
Of modern than Sirius, tho though it may not be really catalogues, Argelander's "Sternverzeich- any older, since it is probably much 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, 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 constant 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 ing 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 star. many others. At times, stars have suddenly 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 Argelander'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, absorption lines , but also by bright lines which corresponded 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 representative, 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 evident gradation from the spectrum of few absorption lines to that broken by gaps.
According to the accepted theory of spectrum analysis, the former spectrum indicates a higher temperature than the latter, so that here is an evidence of a development in time of each individual star. slower. Our sun cansmaller than Sirius, whose development will therefore be slower. Our sun canwill therefore be slov not be compared in size or splendor to some of its distant compeers, not a few of which must present peculiarly complicated systems. Thus the telescope reveals many stars to be really double. really doul a
One of the most interesting of these is Sirius, from the irregularity of whose movements Peters and Auwers had concluded that it was accompanied by an attendant star or at least a large planet; and in 1862 such a companion was discovered by the younger Clark in the very direction in which theory had predicted it to be at that time. Its great minuteness renders detection hopeless except in very favorable circumstances. The motions of Procyon indicate a similar indicate doubleness, but its companion has not yet been seen. Triple and multiple stars forming one system are also not uncommon. For instance, u Herculis, recognized as a double star by Herschel, is really a triple star, the small Herschelian companion having been resolved into two in 1856 by Alvan Clark. An inter-' Clark. An inter- ' esting optical property of these binary systems is that the color of the one component 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 extremities of the earth's orbit. The most probable mean of the various results gives about .5 of a second for this parallax.
This, however, is not the nearest star; it is nearly twice as far away as a Centauri, 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 miles (20 × 10¹²).
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 parallax of at least half the stars of the first magnitude is probably less than one-tenth of a second, so that their average distance 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 ap appreciable proper motions, among others 61 Cygni, as was before noticed.
STAR Its annual motion is 5".2, which is exceeded, however, by that of Groombridge, 1830, a star of the 7th magnitude, whose proper motion is 7". As a general rule, the brightest stars have the greatest proper motion, and stars below the 3d magnitude move only a few seconds n 100 years.
This motion in the heavens cannot be taken even as an approximate estimate of the velocity with which any star is moving, since only the portion of the real velocity which is resolved perpendicular to the line of sight is measurable. Spectrum analysis has, however, given us a means of measuring the velocity along the line of sight. The physical meaning of the dark absorption lines which cross the spectra of the sun and stars has been pointed out under SPECTRUM ANALYSIS.
They indicate the absence of certain rays of definite refrangibility and wave length from the solar or stellar light which reaches us. Since each star spectrum has its own peculiar lines, these must be due to absorption by by the star's own outer envelope or atmosphere. Now, exactly as ocean waves will appear shorter or longer than they really are according as the observer is sailing against or with them, so will the wave length of each individual ray be shortened or lengthened according as we approach or recede from the source of light, and each absorption line in the spectrum, corresponding as it does to a ray of definite wave length, will be displaced toward the violet or red respectively. By comparison with the spectrum of the substance to which a determined portion of these lines corresponds, this displacement may be measured, and from its relation to the particular wave length the ratio of the relative velocity of approach or recession of the star to the velocity of light can be easily calculated. Stars which show periodic variations of magnitude are called variable stars. Among these are Mira and Algal. Up to 1920 about 750 of these stars were known. Their average period of variation is about 300 days and some are 1,000 times brighter at maximum than at minimum. It is believed that these are the giants among the stars. At their maximum some are 150 times brighter than the sun.