For the constellations of the Zodiac, see Zo misc. The several stars belonging to the same constellation are usually distinguished from one another by Greek letters, beginning the alphabet with the brightest: and when these are not suf ficient by Roman letters and by numbers. Many of the most brilliant stars have special names. They are also divided according to their bright ness into stars of the first, second, third, etc. magnitudes. a division which is necessarily some what arbitrary. The smallest stars discernible by the unaided eye are usually called stars of the fifth magnitude: but an unusually sharp eye can discern those of the sixth and even sev enth magnitude. All below are telescopic stars, which are divided clown to the twentieth magni tude. The quantity of light given by a star of any magnitude is taken as 2.512 times as great as the quantity given by a star one magnitude fainter. This number is called the 'light-ratio,' and it is so chosen that a diminution of five magnitudes corresponds to a division of stellar light by just 100 (2.512 = eloo ). In other words, 100 average stars of the sixth magnitude should give as much light as one of the first.
But the whole matter of stellar photometry is subject to some uncertainty. According to the Harvard Photometry the following are the brightest stars in the order of lucidity: Sirius, Canopus*, Arcturus. Capella, Vega, a Centauri*, Rigel. Procyon. Eridani*, /3 Centauri*, a Orionis, Altair, Aldebaran, a Crucis*. Antares, Pollux, Spica, a Piscis Austria*, Regulus. Those marked with an asterisk are not visible in our northern latitudes. No real magnitude in the proper sense of the word has yet been observed in any star. In the best and most powerfully magnifying tele scopes, even the brightest stars of the first mag nitude appear, not with small disks as all the planets do, but as luminous points without any visible diameter, and always the sum Iler the better the telescope. 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 in the calculation. We cannot, then, say whether the greater brillianey of one star, when compared with another. arises from its greater nearness, its greater size, or the greater intensity of its light. But it is certain that the fixed stars are self-luminous. The num ber of the stars is beyond determination. Those visible to the unaided eve in the Northern Hemi sphere amount only to about 3000. Stars of the first magnitude visible north of 35° south decli nation are reckoned by II eis at 14, of the second at 48, of the third at 152. of the fourth at 313, of the fifth at S54, and of the sixth at 2010. But in the following classes the numbers increase rapidly, so that it is impossible to say how many there are. Recent photographic observa tions seem to indicate that if the exposure of the sensitive plate in the telescope was continued long enough the entire plate would be light struck. If this be so we must conclude that the entire heavens would be found covered with stars if we were able to see the faintest ones in ex istence.
That the fixed stars are not really immovable, as their name would imply, is seen in the phe nomenon of double or multiple stars, which are systems of two or more stars that revolve about one another, or rather about their common cen tre of gravity. As they Ca n be seen separate only by means of a telescope, and in most cases require a very powerful one, their discovery was possible only after the telescope was invented.
Galileo himself discovered their existence and proposed to make use of them in determining the yearly parallax of the fixed stars. After a long lapse of time. Bradley, Maskelyne. and ?layer again directed attention to the phenomena of double stars, but nothing important was made out respecting them until the elder Herschel made them the subject of a protracted series of obser vations, which led to the most remarkable con clusions as to their nature. At present over 10,000 stars are known to be double: and in many cases more than one companion accom panies the principal star. The apparent angular distance between two stars must be less than about thirty seconds in order that they may be counted a double. The theory of probabilities renders it almost certain that this vast number of double stars'eould not exist unless there were some real physical connection between the component num. bers of a double. Still, there is always a possi bility of duplicity being merely apparent, the two stars lying nearly in the same direction in space, but at widely different distances from the earth. Such apparent doubles are called optical doubles, and those in which a real physical con nection has been demonstrated by observed grav itational rotation of the component members are called binary stars. Of these only about 250 are known. In some cases one of the components of a double star is much larger than the other, as in the star Rigel, in Orion, and in the polar star; but very often the connected stars are nearly equal in luminous power. The two mem bers of double stars are mostly of one color when the two components are nearly equal; but a difference of color is often observed when the components are widely different in size. In many of these eases the one color is the complement of the other. It was Sir W. Herschel who first advanced the view, which has been confirmed since, that double stars are connected systems of two or more stellar bodies revolving in regular orbits around one another, or rather round their common centre of gravity. Their motions are found to follow the same laws as prevail in the solar system, and the orbits are elliptical. These distant bodies are therefore subject to the New tonian law of gravitation. The period of revolu tion has, in many cases. been computed; the shortest is estimated at years; others are set down at hundreds. In cases where the paral lax is known the size of the orbits can be deter mined, and thus the astronomer is able to assert in regard to the double star a Centauri that the orbit described by the two components about each other is 24 times as large as that of our earth around the sun. Even the masses of these stars have been calculated as being together 2, that of our sun being I. It is a consequence of these revolutions that some stars are now seen double that formerly seemed single, and vice versa. If the plane of revolution has its edge presented to the earth the stars will seem to move in a straight line, and at times to cover one another. In addition to these double stars that can be seen to revolve and change their relative positions there are others of which the components are so close together that even our most powerful telescopes fail to separate them. These doubles are known to exist from peculi arities in their spectra. They show a doubling of the spectral lines that must be caused by a duplicity in the source of light, and where this doubling is shown to occur periodically we must conclude that the two sources of light are re volving in some way or other.