STARS, a designation familiarly applied to all the heavenly bodies which look like points of light. This designation includes five of the planets, Mercury, Mars, Venus, Jupiter and Saturn. In scientific language the term is ap plied only to those bodies which seem fixed on the celestial sphere, and are, therefore, termed fixed stars. These are immense incandescent bodies, scattered through the immensities of space at distances which beggar all our con ceptions. In accordance with the general plan of the present work, we begin with a study of the stars as they appear to us, and then pass to the consideration of their actual constitution and situation in space. We shall, therefore, suppose the reader to scan the heavens at night while we point out certain features of the scene of grandeur then presented to his vision.
The first feature to be noticed is the great diversity in the brightness of the stars, or, to use a technical expression, in their magnitudes. These range all the way from that of the bright est stars down to the point of invisibility to the naked eye. The smallest telescope will show scores of stars too faint to be seen without its aid, and the greater the power of the instru ment the greater the number that will be brought into view, without any limit yet known.
The ancient astronomers divided the stars into six orders of magnitude, called the first, second, etc. The 15 or 20 brighter stars were classified as of the first magnitude, those a grade below them as of the second, and so on, until they came to the sixth, which comprised the faintest stars visible to the unaided eye. All these stars are now called lucid; fainter ones telescopic. In modern times the preceding sys tem of magnitudes has not only been extended to the telescopic stars, but has been arranged on a plan which gives greater precision, and enables us to designate the exact measure of the light which we receive from a star by its magnitude. The magnitudes of stars are now designated to tenths, or, to give greater precision, even to hundredths of a magnitude. Thus we have
stars of magnitudes 3.0, 3.1, 3.2, etc. These numbers are arranged on a diminishing scale of light,.so that a difference of one magnitude corresponds to the multiplication or division of light by a number very near 2.5, or, more ex actly, the number whose logarithm is 0.4. This means that a star of magnitude 3.0 is about two and a half times as bright as one of magnitude 4.0; this again about two and a half times as bright as one of magnitude 5.0, etc. The exact scale is such that a star of the first magnitude is precisely 100 times as bright as one of the sixth, while the latter is 100 times as bright as one of thellth, so that an increase of five units in the scale of magnitude corresponds to a division of the light by 100.
Careful examination will also show that the stars of each magnitude are several times as numerous as those of the brighter magnitudes. There are three or four times as many stars of the fourth magnitude as of the third; three or four times as many of the fifth as of the fourth and so on in nearly geometrical progression. But when we pass to the stars invisible to the naked eye the progression is not so rapid. Yet its common ratio does not fall below two at any point yet known. The result is that the stars of the fainter magnitudes are to be counted by millions. The faintest stars visible in the largest telescopes are about of magni tude 17 or 18.
Another familiar feature of the sky. is that the stars are not scattered uniformly, but that the brighter ones appear to be collected to a greater or less extent into constellations. There is, however, no precise dividing line between one constellation and another. The irregularity is such that the number of constellations is somewhat arbitrary. The fainter the stars the less marked is the division into constellations. The telescopic stars cannot be divided into constellations at all. See CONSTELLATIONS.