The stars have been divided into groups called constellations (q.v.) from the earliest times. The several stars belonging to the same constellation are distinguished from one another by Greek letters, beginning the alphabet with the brightest; and when these are not sufficient, by Roman letters and by numbers. Many of the most brilliant stars have special names. They are also divided according to their brightness into stars of the first, second, third, etc., magnitudes—a division which is necessarily somewhat arbitrary. The smallest stars discernible by a naked eye of ordinary are usually called stars of the fifth magnitude; but an unusually sharp eye can discern those of the sixth and even seventh magnitude. All below are telescopic stars, which are ivided in a very undeter mined way down to the twentieth magnitude. Sir J. Hersclidi has determined that the light of Sirius, the brightest of all the stars, is 324 times that of it mean star of the sixth magnitude. By processes of photometric observation and reasoning, it is concluded that the intrinsic splendor of a Centauri is more than twice that of our sun, and that of Sirius 394 times. Amona. stars of the first magnitude in the northern hemisphere are usually reckoned Aldebaran (in Taurus), Arcturus (in Bootes). Atair (in Aquila). Betelgeux (in Orion), Capella (in Auriga), Procyon (in Canis Minor), Regulus (in Leo). Vega (in Lyra). In the southern hemisphere are Achernes (in Eridanus), Antares (in Scorpio). Canopus (iu Argo), Rigel (in Orion), Sirius (iu Canis Major), Spies (in Virgo), and cc Centauri and a Crueis that have no special names.
No apparent magnitude, in the proper sense of the word, has yet been observed in any star. In the best and most powerfully magnifying telescopes; even the brightest stars of the first magnitude appear, not with small discs as all the planets do, but as luminous points without any visible diameter, and always the smaller the better the tele scope. We are therefore totally ignorant of the real size of the fixed stars; nor could it be determined though we were sure of their distances, for the apparent diameter is an essential element fu t he calculat ion. We cannot, then, say whether the greater brilliancy of one star, when compared with another, arises from its greater nearness, its greater size, or the greater intensity of its light. It is certain that all the fixed stars are self luminous. By the spectroscope several facts regarding their physical constitution have been made out; there are great differences in their spectra; the existence of several' known elements is considered demonstrated. Sirius, e.g., contains hydrogen, sodium, and magnesium.
The number of the stars is beyond determination. Those visible by the naked eye amount only to a few thousands. Stars of the first magnitude are usually reckoned at 15 to 20, of the second at 50 to 60, of the third about 900, of the fourth at 400 to 500, of the fifth at 1100 to 1200. But in the following classes, the numbers increase rapidly, so
that stars of the sixth and seventh class amount to above 12,000. Stars are most dense in that region of the heavens called the Milky Way, which is mostly composed of stars of the eleventh and twelfth magnitudes. W. Herschel observed 116,000 stars pass the field of his telescope in a quarter of an hour, while directed to the densest part of the Milky Way.
That the fixed stars are not really immovable, as their name would imply, is seen in the phenomenon of double or multiple stars which are systems of two or more stars that revolve about one another, or rather about their common center of gravity. As they cau be seen separate only by means of a telescope, and in most cases require it very powerful one, their discovery was possible only after the telescope was iuventetl. Galileo himself dicovered their existence, and proposed to make use of them in determining the yearly par allax of the fixed stars. After a long lapse of time, Bradley, Maskelyne, and Mayer again directed attention to the phenomena of double stars; but nothing important was made out respecting them till the elder Herschel made them the subject of a protracted series of observations, which led to the most remarkable conclusions as to their nature. The united observations of Struve, Savary, Encke, South, and especially those of Herschel the younger, continued for four years in the southern hemisphere at the cape of Good Hope, have raised the number of observed double, or rather multiple, stars to more than 6,000, of which the greater part arc binary, or composed of two, but many are triple, some quadruple, and a few even quintuple, or consisting of five stars. The distance between the stars composing these systems is always apparently small (varying from less than 1° up to 32"); but apparent nearness does not always counitnte a double star, for two really distant stars are not unfrequently so nearly in the same line, as seen front the earth, that they appear to be close together. In real multiple stars, the individuals are not only comparatively near to one another, but they revolve around one another. Among stars of the first three magnitudes, every sixth is a multiple star; among the smaller stars, the proportion is much less. In some cases, one of the stars is much lar ger than the other, as in the star Rigel in Orion, and in the polar star; but oftener the connected stars are nearly equal in luminous power. The two members of double stars are mostly of one color, but a difference of color is observed in about one-fifth of the whole number. In many of these cases, the one color is the complement of the other, and it is possible that the color of the smaller star may be subjective, arising from the action of the other upon the eye.