PRINCIPLES OF ASTRONOMY.-No. VI. THE SIDEREAL HEAVENS. To minds delighting in grand and enlarged conceptions, the study of sidereal astronomy will afford unbounded scope. In conducting our readers through the solar system, we have passed over distances which, expressed in numbers, overwhelm the mind; we have introduced them to spheres compared with which our globe sinks into insignificance; still, of them we have learnt enough to convince us that, with points of difference, there were others of similarity sufficiently striking to demonstrate their relationship to the earth. We are now about to contemplate the remoter occupiers of space-the fixed stars. Our former unit of measurement is here of no avail, for the distance of the earth from the sun dwindles into a point compared with the least remote of the fixed stars, and we must assume another, namely, the velocity of light, to express, in at best but an imperfect manner, the comparative distance of the sun and the stars. It is well known that the light of the sun occupies eight and a quarter minutes in passing from him to us, travelling, that is, at the rate of 192,000 miles in a second of time; at the same rate the light of the star 61 Cygni, one of the nearest to us, would not reach the earth under ten years and a half from the time it left the star, so extensive is that space which it would have to traverse. When we survey the heavens above us on a beautiful night, and call to mind that every star whose rays strike on our vision sent forth these rays many years anterior to the time of observation, how overwhelming become our ideas of the illimitable extent of space! how exalted in our conceptions is the Divine Creator of the material universe! how insignificant the human family! "What is man that thou art mindful of him: the son of man that thou visitest him!" will be our involuntary exclamation. Though the contemplation of the extent of the universe will not originate religion, yet in a well-regulated mind it cannot fail to enlarge our views of Almighty power and grandeur, and to teach us lessons of humility and self-abasement. The fact of the time light takes to traverse space will lead to some striking deductions. The brilliant star which made its first appearance in the time of Tycho Brahe, and remained visible two years, must have sent forth its rays for a long period before it was seen, and must have been extinct several years before its creation and disappearance were announced to terrestrial astronomers; so must also that star of the fourth magnitude, discovered by Mr. Hind in the constellation Ophiuchus, April 28, 1848, where none was seen on April 5, and which vanished from the heavens before the end of the year. Some stars created centuries ago may not yet have announced their existence to our earth, their rays even now may be travelling through space; and it may be that others have ceased to exist, although we continue to refer them to their usual point in the celestial sphere. As the stars are so dissociated from our system, how is it possible to derive any knowledge of them? how can the senses report what is passing in the depths of space? Here the telescope comes to our aid; by increasing our power of penetrating into space it discloses a few of the secrets of the universe; it has shown that the boundary of creation has not yet been reached, for every increase of optical power unfolds new worlds all around us in inexhaustible supply, in numbers beyond calculation, in arrangements beyond conception varied and sublime. The star Castor appears single to the naked eye, but with a magnifying power of 250 presents the appearance given in the drawing, being separated thereby into two components, differing not much in magnitude, and distant, by angular measurement, 5". About a century ago Dr. Bradley remarked that a line drawn from Pollux, another star in the constellation Gemini, passed through the centres of these-a happy remark, which has led to valuable results. We might have supposed that this star appeared double from the one component being behind the other and much further off; but since that time we no longer find the members of the system in the same position, the smaller one having described nearly half a circle round the other at such a rate as will ensure the completion of an entire orbit in 250 years. This is a fine specimen of those binary systems, of which many hundreds are scattered over the face of the heavens, all, with novel combinations, obedient to the same law which produces the revolution of the earth round the sun. Let us revert for an instant to the method by which the revolution of one star round another may be demonstrated. Suppose a telescope to be fixed in the meridian when a double star crosses the field of view, and that near the eye-glass are inserted two fine threads, the one horizontal, the other vertical. If both stars are on the vertical wire at the same instant, the line joining them runs north and south; if both run along the horizontal wire, it will be east and west; if neither of these cases occur, but the line joining them be inclined from the vertical, and coincides with neither wire, we have the means of measuring its angle of inclination from a fixed point-as the north. If the same thing has been done at a distant epoch, and we find the angle to have varied, we know the amount and direction of the star's motion in the time which has elapsed between the two observations. By methods somewhat similar to the one indicated, laborious observers, from Sir William Herschel downward, have measured and recorded the position of many double stars, and have determined approximately the periods of their revolution, which vary, some occupying half a century and others several hundred years. Here, then, is presented to our view an entirely new order of things, o which our system affords no type. Two self-luminous and independen suns revolve round a common centre of gravity, most probably accompanied each by a retinue of planets, introducing complex and novel movements diverse from those of the solar system, and yet obedient to that wonderful law of gravitation which would thus seem to pervade the universe; and yet a binary system is one of the least complex of those to which the study of sidereal astronomy will introduce us. When we consider the extremely small space which separates the double stars, it may seem to savour of presumption to attempt a delineation of the orbit of any one in particular; but when such men as Herschel, Hind, Admiral Smyth, and Adams are found to direct the powers of their minds to the task, we may rest assured that it is no vain undertaking. y Virginis is a star the approximate orbit of which has employed all the above eminent astronomers; taking the result of observations for seventy years, an orbit has been projected which answers as nearly as can be expected all the conditions of the case, and is considered by them to demonstrate the extension of the laws of gravity to the stellar universe. From a distance of 7" in 1720, the one star approached the other till, in 1836, the two overlapped and appeared as one; after a time they were again separated, and are now distant 3", but the one formerly to the right has advanced to the left. Mr. Adams calculates that an ellipse of great eccentricity (87965), inclined to the plane of projection 25° 27', and which will occupy the star 174 years to describe, will be found to be the orbit of the inferior member of this binary system, and this orbit satisfies all the observations as nearly as can be expected from the difficulties attending the measurement of such minute quantities. As doubts, and even sneers, have been thrown out against the attempt to reduce to system motions so slow of objects so remote-by those, it is true, who are ignorant of astronomy,-it is well that it should be known what class of men have been engaged in the pursuit of this especial branch of the science. When, aided by the telescope, we search the heavens, wonders multiply at every turn. Not only is it possible in some cases to separate a star into two components, but into three, four, or even a greater number, which give evidence of being connected by the same wonderful power. Some indication of a change of physical condition is given by the colour of the light which these multiple stars emit; and although, from the construction of the telescope, the colours are not consistent in all, still of some they are certain and easily determined. A few remarks on those represented below must conclude our observations on a subject which possesses an overwhelming attraction for the lovers of the sublime in creation. The pole star is the first in the drawing; it is the friend of the modern navigator, but it may be regarded as the sole guide of the ancient : Rudely their ships were navigated then No useful compass or meridian known; And knew no north but when the pole-star shone.* Little did they think that this star was accompanied by a minute companion, so small that its visibility is a good test of a telescope. The larger star is between the second and third magnitude, the smaller between the ninth and tenth; the former is yellow, the latter a pale white; the motion * Dryden. : of the smaller is so slow, that no possible attempt can as yet be made to fix any period of revolution, or even to decide whether it has any motion at all round its primary; the distance of the small star from the larger is 18". Lyræ is quadruple, or rather a double double star. In this system we have a remarkable exhibition of two pairs of stars, each pair revolving round its centre of gravity, and all four revolving round that of the entire system; the individuals of each pair are separated about 3"; the distance between the two pairs is 205". Observations extending from 1779 to the present time have distinctly indicated slow but certain revolutions that of one pair may be roundly stated to be completed in 2000 years; of the other in about half that time; and probably the orbit of both double systems about the common centre of gravity of the whole may be acccomplished in something less than one million of years.* E, or 51 Libræ, has three components. The largest star is of the fifth magnitude, the second of the fourth, the smallest of the seventh or eighth; the colours are respectively white, yellow, and grey, according to Admiral W. H. Smyth; but authorities differ on this last head. The second star describes, in a century, a circular orbit round the largest; hence its plane is perpendicular to a line joining the earth and star: the periodic time of the third star has not yet been computed. Attentive observers of the stars have remarked that some of them vary in degrees of brightness from time to time; and closer scrutiny, extended through a considerable period, has led them to the conclusion that this variation is periodic. From the analogy which well-known phenomena supply, we are led to associate periodicity with orbital motion; thus, as in a month, the time of her revolution, the moon passes through all her changes, from invisibility to her maximum brilliancy, so we can hardly avoid concluding that there must be some connection between à star's regular increase and diminution of light, and motion in a circular orbit. Now, if we suppose the star to be a sun, around which revolves a very large planet, opaque like those of our system, the failure of light might be accounted for by the interposition of the planet between us and the star. Ceti is a remarkable, variable star, which goes through its changes in about 332 days; when at its greatest brilliancy, it shines as a star of the second magnitude for about a fortnight; in three months it becomes less and less luminous, till it is invisible to the naked eye, and remains so for about five months, after which it continues to become more bright till its period has been accomplished. Algol in Perseus has a period of two days twenty hours forty-eight minutes, during which its light varies from that of a star of the second magnitude for two days thirteen hours of its period, when it suddenly begins to diminish in splendour, till it appears as a star of the fourth. From the first diminution till it recovers its original brightness seven hours intervene; so that, if the hypothesis of the interposition of an opaque planet be admitted, from the first appulse of its disk on that of the star till it passes quite across on the opposite side, that period of time must elapse. How strikingly does this phenomenon suggest a solar eclipse, which is caused by the dark body of the moon crossing the disk of the sun, and depriving us of more or less of his light! The resemblance indeed may be still more nearly traced; for, as the solar eclipses are not all of the same magnitude, the sun at times only losing a portion of his light, * Admiral W. H. Smyth, in the "Celestial Cycle." and at others the whole, so, in several variable stars that have been observed, the diminution of light is found to be not at all times of the same extent, which, on the supposition, may arise from the varying inclination of the orbit of the planet to the plane of projection; that is, that part of the ground of the heavens to which we refer the star in our observation. On any clear night a broad irregular band of faint light may be observed to span the heavens; this is the well-known "milky way"-the galaxy of the ancients-which the telescope has resolved into countless myriads of stars. It may be traced throughout the southern hemisphere, and it does, in fact, form a complete circle, which intersects the celestial equator 10° east of the equinoctial points, and is inclined to it at an angle of 63°; its whitish nebulous light arises from the blending of the light of numerous small stars, which defy any attempt at computation; for the more powerful the telescope through which they are viewed, so many more are brought to light. The milky way diverges into two branches at a certain point, which again unite and form a continuous band, only interrupted by a gap in the southern hemisphere of no very great width. In one part of this starry zone no less than 5000 stars have been distinctly seen in one square degree, a space about four times that occupied by the disk of the moon. When we gaze at the milky way then, we look through the densest and most thickly-strewn portion of the heavens, where world after world rises on our view in apparently infinite succession. Were it possible to count all the stars of this wondrous belt, enumeration would be at fault, and ages would pass before the whole could be surveyed, if indeed the powers of man could be equal to the task. Words would fail to produce an impression on that mind which could view this wonderful assemblage of distant suns and not be impressed with the grandeur and vastness of the universe, and the awful attributes of the Great Author and supporter of all, whose "glory" they show, and of whose "handy work" they tell. Though to an unpractised eye the stars may appear to be distributed at random over the celestial sphere, closer investigation has made it appear that the numbers decrease in proportion as we recede in either direction from the galactic circle. Sir William Herschel, whom no labours could affright, undertook, by means which we have not space to explain, to gauge the heavens, and estimate the proportional amount of stars in every direction. His plan was followed up by his son in the southern hemisphere, and the results of both sets of observations show distinctly that the points of the heavens, in which the stars are most sparingly scattered, are those 90° distant from the centre of the galactic circle; and from each of these points, as we approach that circle, the numbers of stars increase, until in the galaxy itself they become innumerable. From reasoning of which we can only give the deductions these eminent astronomers have concluded, that our sun is a star of the milky way, situated at a point removed considerably from the centre; and that the milky way itself, viewed from a station far beyond it, would appear like a nebulous cluster of stars, the sun being one, whose length would very much exceed its breadth, and that it would be found to divide itself into two branches, each about one-fourth the length of the whole. Amongst the wonders of the heavens are the nebula, which are, for the most part, clusters of stars viewed by us exactly under such circumstances as we have supposed the galaxy to be seen from a point without. Singularly enough, there does exist in the heavens a nebula of this bifurcated form; its place in the heavens 13 hrs. 23 min. of right ascension, declina |