The number of stars visible to the naked eye is about six thousand, but telescopes show tens of millions. It is customary to divide the stars into classes, called magnitudes, according to their apparent brightness. By a system of photometry, or light-measurement, they are grouped into stars of the first, second, third, etc., magnitude. With the naked eye no stars fainter than the sixth magnitude are visible, but very powerful telescopes may show them down to the eighteenth magnitude. Each magnitude is about two and a half times brighter than the next magnitude below in the scale. A first-magnitude star is about one hundred times brighter than one of the sixth magnitude. But, in reality, the variation of brightness is gradual, and for very accurate estimates fractions of a magnitude have to be employed. There are about twenty first-magnitude stars, but they are not all of equal brightness. A more accurate photometry assumes a zero magnitude, very nearly, represented by the star Arcturus, and makes the ratio 2.512. Thus a star, nearly represented by Aldebaran or Altair, which is 2.512 times fainter than the zero magnitude, is of the first magnitude, and a star, nearly represented by the North Star, which is 2.512 times fainter than the first magnitude, is of the second magnitude. Counting in the other direction, a star, like Sirius, which is brighter than the zero magnitude, is said to be of a negative magnitude. The magnitude of Sirius is—1.6. There is only one other star of negative magnitude, Canopus, whose magnitude is—0.9. But for ordinary purposes one need not trouble himself with these refinements. [Illustration: =Schiaparelli’s Chart of Martian “Canals.”= ] The stars are divided into five principal types, according to their spectra. These are: I. White stars, having a bluish tinge, in which the spectrum is characterised by broad dark bands, due apparently to an extensive atmosphere of hydrogen, while there are but few lines indicating the presence of metallic vapours. About half the stars whose spectra have been studied belong to Type I. II. Yellowish-white stars, resembling the sun in having their spectra crossed with a great number of lines produced by metallic vapours, while the hydrogen lines are less conspicuous. These are often called solar stars, and they, too, are very numerous. III. Orange and slightly reddish stars, whose spectra contain mostly broad bands instead of narrow lines, the bands being situated toward the blue end of the spectrum, whence the prevailing colour, since the blue light is thus cut off. Only a few hundred of these stars are known, but they include most of the well-known variable stars. IV. Small deep-red stars having dark bands absorbing the light of the red end of the spectrum. Less than a hundred of these stars are known.