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 const,ncy of their relative positions in space, and by their inappre ciable 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 have from an early age been grouped in constellations and classified according 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 ear liest catalogue which has come down to us is that in Ptolemy's "Almagest," sup posed to have been compiled by Hippar chus (150 B. c.). Ptolemy gives 1,030 stars, of which several cannot now be certainly identified. The last important catalogue before 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.