The New Student's Reference Work/Stars
Stars, those heavenly bodies which appear as bright points having fixed positions in the sky. The spectroscope in the hands of Fraunhofer, Huggins and others has shown that these so-called fixed stars really are gigantic and very distant suns; for they give continuous spectra crossed by dark lines, indicating, as in the case of our central luminary, a solid or liquid core surrounded by incandescent vapor.
Ptolemy and Hipparchos divided the stars visible to the naked eye into six different groups, according to their brightness. These groups are called magnitudes, and since the invention of the telescope this classification has been so extended that astronomers grade stars into 14 or 15 groups. In the first magnitude they place the 14 brightest stars, including Sirius, Vega, Arcturus etc.; in the second group are 48 stars, including the Pole Star and two Pointers; in the third 152; in the fourth 313; in the fifth 854; and in the sixth 2,010, making a total of 3,391, which are practically all the stars which can be seen without the aid of a glass. The number of stars which can be photographed, or which can be seen with our largest refracting telescopes, runs into the millions. The distances of the fixed stars are known only in a few cases, and even then very roughly. This quantity is determined by measuring the annual parallax of the star, a quantity which, except in a very few cases, is so minute as not to exceed the errors of measurement. The nearest star, so far as known, is α Centauri, whose parallax is only ¾ of a second of arc, and even this, our nearest neighbor, is at the stupendous distance of four “light-years,” i. e., at a distance such that light would require four years to traverse it.
The student should bear in mind that, although these stars are said to be fixed, most of them exhibit, in fact, a small amount of motion, say, a second or so in a year. In addition to this motion across the heavens, which is measured in angle and is called proper motion, nearly all stars exhibit a motion toward us or away from us, which is called motion in the line of sight. The measurement of this latter motion is one of the recent achievements of the spectroscope, based on Döppler’s principle. Consult Miss Clerke’s System of the Stars; Chambers’ The Story of the Stars; Lockyer’s Meteoric Hypothesis; and Sheiner’s Astronomical Spectroscopy. See Nebula, Parallax and Telescope.