however, the first magnitude may be regarded as restricted to 18 or 20 principal stars; the second, to 50 or 60 next inferior; the third, to about 200 yet smaller; and thus on, the number in each class increasing rapidly as we descend in the scale of brightness. The number of stars in the first seven magnitudes, amounts, all together, to nearly 20,000. The whole number of stars visible with the best telescopes is not known; but it must amount to several millions. The number of stars distinctly visible to the naked eye, is less than is generally supposed by those who only judge from the impression made, when viewing them on a fine evening. The number thus visible, at the same time above the horizon, does not greatly exceed a thousand. All the stars visible to the naked eye, with some others, are represented on celestial globes of 12 or 18 inches in diameter.* *Students of astronomy who can have the use of a celestial globe, or celestial atlas, ought to make themselves familiar with the principal stars and constellations. To rectify the globe for this purpose, let the frame which supports it, be placed by estimation, or by the compass which is sometimes attached, so that the north and south points marked on its upper surface, called the horizon of the globe, may correspond to the north and south points of the horizon or nearly so. Then, let the brass ring in which the globe is suspended, called the meridian of the globe, be slid in its support, till the north pole of the globe, which is that situated in the constellation of Ursa Minor, is elevated above the northern point of the horizon by an arc equal to the latitude of the place. Find the day of the month on the horizon, and the corresponding point in the contiguous graduated circle will be the sun's place in the ecliptic. Find this place in the ecliptic marked on the globe, and bring it to the graduated side of the meridian. Keeping the globe in this position, set the index placed at the north pole. to 12 on the hour circle around the pole; or if the globe has a moveable brass hour circle instead of an index, bring 12 on this hour circle to the graduated side of the meridian. Then turn the globe westwardly till the index points to the hour at which the globe is to be used; or when there is no index, till the hour on the brass hour circle comes to the graduated side of the meridian. The positions of the stars represented on the globe, will then correspond to their positions in the heavens; so that if a straight line be conceived to be drawn from the centre of the globe through the places of the stars marked on its surface, they will point to the stars themselves. Kendall's Uranography and Atlas, Revised Edition of 1854, is a work adapted to give the astronomical student a satisfactory knowledge of the sidereal hea vens. 354. Relative light of stars of the different magnitudes. According to the present classification of the stars, the light of an average star of the second magnitude, is about one fourth that of an average star of the first magnitude. For the other magnitudes, the light of a star of one magnitude is regarded as about half that of a star of the next higher magnitude. There is, however, considerable variety in the brightness of stars, that are classed as of the same magnitude; especially those of the first magnitude. The light of Sirius, the brightest star in the heavens, is regarded as being from 15 to 20 times as great as some of the stars of the first magnitude; and more than 300 times as great as an average star of the sixth magnitude. 355. Distribution of the stars. The stars appear to be very unequally distributed over the heavens. This is observable by the naked eye, and becomes still more apparent by means of the telescope. There are various spaces which are faintly luminous, shining with a pale white light. Many of these, on applying telescopes of sufficient power, are found to consist of multitudes of small stars, distinctly separate, but very near to one another. These are called Nebula. The well known space called the milkyway, is of this kind; and there are some others visible to the naked eye. In some of the nebulæ or clusters, the number of stars crowded into a small space, is immensely great. According to the estimation of Sir J. Herschel, there are some which contain more than ten thousand stars in a space that would be covered by a tenth part of the moon's disc. Again, there are many spaces, some of considerable extent, in which but few stars are seen, even with the best telescopes. 356. Clusters of stars and nebula. The beautiful cluster of stars called the Pleiades, in which six or seven are readily discernible by the naked eye, exhibits within the small space they occupy, fifty or sixty conspicuous stars, when viewed with a telescope of moderate power. The constellation called Coma Berenices, is another group more diffused, and composed of larger stars. In the constellation Cancer, there is a luminous spot or nebula called Præsepe, or the bee-hive, which a telescope of moderate power resolves entirely into stars. In Perseus, is another spot crowded with stars, which become separately visible with a good telescope. Most of the nebulæ, however, require a very powerful telescope to resolve them into stars; and there are many which have never been thus resolved, they being, it is probable, differently constituted. A prominent one of this class is situated near the star in Andromeda. It is visible to the naked eye, and has, from its appearance, often been mistaken for a comet. It should be remarked that many of the most prominent objects hitherto regarded as belonging to the class of irresolvable nebulæ, have recently, by the aid of the gigantic telescope of Lord Rosse, been resolved into stars. 357. Variable stars. Some stars undergo periodical changes in their brightness, and are, therefore, called variable stars. One of the most remarkable of this class of stars, is Mira, or o Ceti, which was discovered to be variable in the latter part of the 16th centu ry. When brightest, it is of the second magnitude, and continues to exhibit nearly the same appearance for about three weeks. It then decreases, and in about two months ceases to be visible to the naked eye. After remaining thus invisible for six or seven months, it again appears, and in the course of six or seven weeks, is restored to its former brightness or nearly so. These periods, and also the greatest brightness of the star, are, however, subject to some variations. The average period of all the changes is about 11 months or, more exactly, 332 days. At the times of the least light of the star, it is frequently invisible, even with good telescopes. Another very remarkable variable star is Algol, or ß Persei, which was discovered to be such, in the latter part of the last century. It is usually of the second magnitude; but, after having continued so, during a period of about 60 or 61 hours, it suddenly decreases, and is reduced in about 4 hours to the fourth magnitude. Continuing thus, about a quarter of an hour, it then increases, and in about 4 hours more, it regains its usual magnitude. The period of these changes is 2 d. 20 h. 48 m. 58.5 sec.* * According to the observation of Professor Argelander, a German Astronomer, given in the Astr. Nach., Nos. 416 and 417, the star. Ceti had its greatest brightness in the year 1840, about the 3d of October; and the star & Persei, on the 22d of December in that year, had its least brightness at 9 h. 50 min., mean time at Greenwich. With these epochs and the periods given above, the times of the There are more than twenty other stars known to be variable to a greater or less extent; some of which have but recently been discovered to be so. The periods of the changes vary from a few days to more than a year. 358. Temporary Stars. Several instances are recorded of stars suddenly appearing, some of them of great splendour, where none had before been observed; and there are several stars noted in some of the ancient catalogues, that cannot now be found. One of the most noted of these temporary stars broke forth with great brilliancy on the 11th of November, 1572, in the constellation Cassiopeia, and was attentively observed by Tycho Brahe, the celebrated Danish astronomer. It was then as bright as Sirius, and increased in splendour so as to become distinctly visible at midday. It began to diminish in December of the same year, and in March 1574, it entirely disappeared. In the years 945 and 1261, a brilliant star appeared in the same region of the heavens with that of 1572. Some have thought it must have been the same star that appeared in each of these years, and that it was, therefore, a variable star with a period a little over 300 years. On the 27th of April, 1848, Mr. Hind of London, discovered a new star of the sixth magnitude, in the Serpent Bearer, which increased in brightness for a few days, then began to wane, and disappeared in less than two years. On the 5th of April, Mr. Hind had examined that part of the heavens with care, and was certain that at that time no star as bright as the ninth magnitude existed, where this one of the sixth was found three weeks later. 359. Double Stars. Many stars which when viewed with the naked eye or with telescopes of small power appear single, are by means of those of larger power resolved into two, three, or more stars distinctly separate but very near to one another. These are called double or multiple stars. Some of these are resolvable into separate stars by a telescope of moderate power, as Castor in the twins, which consists of two stars nearly equal, both being between greatest light of the former star and least light of the latter, may be approximately determined for a few subsequent years. the third and fourth magnitudes, at the distance of 5" from each other. Many of them, however, require for their separation, a telescope of the superior class, and serve as good objects to test its perfection. The individual stars forming a double star, are mostly very unequal in magnitude; and many of them exhibit the curious phenomenon of contrasted or complementary colours, that is colours which if combined would form white light. In such instances, the larger star is usually of a ruddy or orange hue, and the smaller is blue or green. * In the beautiful double stars a Herculis, and y Andromeda, which may be separated by a telescope of moderate power, this contrast is finely exhibited. 360. Binary stars or systems. Sir W. Herschel was the first that gave much attention to the subject of double or multiple stars. He observed a large number, and noted the distances by which the individual stars were separated, and their relative positions. Continuing and repeating his observations, he found that the distance and relative positions of these component stars were subject to slow but progressive changes. After having had his attention, frequently, thus directed for more than twenty years, he at length ascertained and announced the striking and interesting fact, that several, at least, of the double stars formed systems, in which one of the individuals revolved round the other, or rather, both round their common centre of gravity. These have received the appellation of binary stars or binary systems, to distinguish them from the other double stars whose apparent proximity probably proceeds from one being situated nearly behind the other, without their having any physical connection. There are fifty or more of the double stars which are now known to form binary systems; a few of the more prominent of these are, Castor, or a Geminorum, y Virginis, Ursæ, a and Herculis, o * This probably depends on the well known optical fact, that when the retina of the eye is excited by any bright colour, a feeble light, which if seen by itself, might appear white, is affected with a tint complementary to that of the stronger |
