Fig. 69´.Fig. 69´.
Such would be the relation of the body that affords the meteoric shower of November, provided its revolution is accomplished in six months; but it is still somewhat uncertain whether the period be half a year or a year; it must be one or the other.
If we inquire, now, why the meteors always appear to radiate from a point in the constellation Leo, recollecting that this is the point to which the body is projectedamong the stars, the answer is, that this is the very point towards which the earth is moving in her orbit at that time; so that if, as we have proved, the earth passed through or near a nebulous body on the thirteenth of November, that body must necessarily have been projected into the constellation Leo, else it could not have lain directly in her path. I consider it therefore as established by satisfactory proof, that the meteors of November thirteenth emanate from a nebulous or cometary body, revolving around the sun, and coming so near the earth at that time that the earth passes through itsskirts, or extreme portions, and thus attracts to itself some portions of its matter, giving to the meteors a greater velocity than could be imparted by gravity alone, in consequence of passing rapidly by them.
All these conclusions were made out by a process of reasoning strictly inductive, without supposing that the meteoric body itself had ever been seen. But there are some reasons for believing that we do actually see it, and that it is no other than that mysterious appearance long known under the name of thezodiacal light. This is a faint light, which at certain seasons of the year appears in the west after evening twilight, and at certain other seasons appears in the east before the dawn, following or preceding the track of the sun in a triangular figure, with its broad base next to the sun, and its vertex reaching to a greater or less distance, sometimes more than ninety degrees from that luminary. You may obtain a good view of it in February or March, in the west, or in October, in the morning sky. The various changes which this light undergoes at different seasons of the year are such as to render it probable, to my mind, that this is the very body which affords the meteoric showers; its extremity coming, in November, within the sphere of the earth's attraction. But, as the arguments for the existence of a body in the planetary regions, which affords these showers, were drawn without the least reference to the zodiacal light,and are good, should it finally be proved that this light has no connexion with them, I will not occupy your attention with the discussion of this point, to the exclusion of topics which will probably interest you more.
It is perhaps most probable, that the meteoric showers of August and December emanate from the same body. I know of nothing repugnant to this conclusion, although it has not yet been distinctly made out. Had the periods of the earth and of the meteoric body been so adjusted to each other that the latter was contained an exact even number of times in the former; that is, had it beenexactlyeither a year or half a year; then we might expect a similar recurrence of the meteoric shower every year; but only a slight variation in such a proportion between the two periods would occasion the repetition of the shower for a few years in succession, and then an intermission of them, for an unknown length of time, until the two bodies were brought into the same relative situation as before. Disturbances, also, occasioned by the action of Venus and Mercury, might wholly subvert this numerical relation, and increase or diminish the probability of a repetition of the phenomenon. Accordingly, from the year 1830, when the meteoric shower of November was first observed, until 1833, there was a regular increase of the exhibition; in 1833, it came to its maximum; and after that time it was repeated upon a constantly diminishing scale, until 1838, since which time it has not been observed. Perhaps ages may roll away before the world will be again surprised and delighted with a display of celestial fire-works equal to that of the morning of November 13, 1833.
——"O, majestic Night!Nature's great ancestor! Day's elder born,And fated to survive the transient sun!By mortals and immortals seen with awe!A starry crown thy raven brow adorns,An azure zone thy waist; clouds, in heaven's loomWrought, through varieties of shape and shade,In ample folds of drapery divine,Thy flowing mantle form; and heaven throughoutVoluminously pour thy pompous train."—Young.
——"O, majestic Night!Nature's great ancestor! Day's elder born,And fated to survive the transient sun!By mortals and immortals seen with awe!A starry crown thy raven brow adorns,An azure zone thy waist; clouds, in heaven's loomWrought, through varieties of shape and shade,In ample folds of drapery divine,Thy flowing mantle form; and heaven throughoutVoluminously pour thy pompous train."—Young.
Sincethe solar system is but one among a myriad of worlds which astronomy unfolds, it may appear to you that I have dwelt too long on so diminutive a part of creation, and reserved too little space for the other systems of the universe. But however humble a province our sun and planets compose, in the vast empire of Jehovah, yet it is that which most concerns us; and it is by the study of the laws by which this part of creation is governed, that we learn the secrets of the skies.
Until recently, the observation and study of the phenomena of the solar system almost exclusively occupied the labors of astronomers. But Sir William Herschel gave his chief attention to thesidereal heavens, and opened new and wonderful fields of discovery, as well as of speculation. The same subject, has been prosecuted with similar zeal and success by his son, Sir John Herschel, and Sir James South, in England, and by Professor Struve, of Dorpat, until more has been actually achieved than preceding astronomers had ventured to conjecture. A limited sketch of these wonderful discoveries is all that I propose to offer you.
The fixed stars are so called, because, to common observation, they always maintain the same situations with respect to one another. The stars are classed by their apparentmagnitudes. The whole number of magnitudes recorded aresixteen, of which the first six only are visible to the naked eye; the rest aretelescopicstars. These magnitudes are not determined by any very definite scale, but are merely ranked according to their relative degrees of brightness, and this is left in a great measure to the decision of the eye alone. The brightest stars, to the number of fifteen or twenty, are considered as stars of the first magnitude; the fifty or sixty next brightest, of the second magnitude; the next two hundred, of the third magnitude; and thus the number of each class increases rapidly, as we descend the scale, so that no less than fifteen or twenty thousand are included within the first seven magnitudes.
The stars have been grouped inconstellationsfrom the most remote antiquity; a few, as Orion, Bootes, and Ursa Major, are mentioned in the most ancient writings, under the same names as they bear at present. The names of the constellations are sometimes founded on a supposed resemblance to the objects to which they belong; as the Swan and the Scorpion were evidently so denominated from their likeness to those animals; but in most cases, it is impossible for us to find any reason for designating a constellation by the figure of the animal or hero which is employed to represent it. These representations were probably once blended with the fables of pagan mythology. The same figures, absurd as they appear, are still retained for the convenience of reference; since it is easy to find any particular star, by specifying the part of the figure to which it belongs; as when we say, a star is in the neck of Taurus, in the knee of Hercules, or in the tail of the Great Bear. This method furnishes a general clue to its position; but the stars belonging to any constellation are distinguished according to their apparent magnitudes, as follows: First, by the Greek letters, Alpha, Beta, Gamma, &c. Thus,Alpha Orionisdenotes the largest star in Orion;Beta Andromedæthe second star in Andromeda; andGamma Leonis, the third brightest star in the Lion. When the number of the Greek letters is insufficient to include all the stars in a constellation, recourse is had to the letters of the Roman alphabet, a, b, c, &c.; and in all cases where these are exhausted the final resort is to numbers. This is evidently necessary, since the largest constellations contain many hundreds or even thousands of stars.Cataloguesof particular stars have also been published, by different astronomers, each author numbering the individual stars embraced in his list according to the places they respectively occupy in the catalogue. These references to particular catalogues are sometimes entered on large celestial globes. Thus we meet with a star marked 84 H., meaning that this is its number in Herschel's catalogue; or 140 M., denoting the place the star occupies in the catalogue of Mayer.
The earliest catalogue of the stars was made by Hipparchus, of the Alexandrian school, about one hundred and forty years before the Christian era. A new star appearing in the firmament, he was induced to count the stars, and to record their positions, in order that posterity might be able to judge of the permanency of the constellations. His catalogue contains all that were conspicuous to the naked eye in the latitude of Alexandria, being one thousand and twenty-two. Most persons, unacquainted with the actual number of the stars which compose the visible firmament, would suppose it to be much greater than this; but it is found that the catalogue of Hipparchus embraces nearly all that can now be seen in the same latitude; and that on the equator, where the spectator has both the northern and southern hemispheres in view, the number of stars that can be counted does not exceed three thousand. A careless view of the firmament in a clear night gives us the impression of an infinite number of stars; but when we begin to count them, they appear much more sparsely distributed than we supposed, and large portions of the sky appear almost destitute of stars.
By the aid of the telescope, new fields of stars present themselves, of boundless extent; the number continually augmenting, as the powers of the telescope are increased. Lalande, in his 'Histoire Celeste,' has registered the positions of no less than fifty thousand; and the whole number visible in the largest telescopes amounts to many millions.
When you look at the firmament on a clear Autumnal or Winter evening, it appears so thickly studded with stars, that you would perhaps imagine that the task of learning even the brightest of them would be almost hopeless. Let me assure you, this is all a mistake. On the contrary, it is a very easy task to become acquainted with the names and positions of the stars of the first magnitude, and of the leading constellations. If you will give a few evenings to the study, you will be surprised to find, both how rapidly you can form these new acquaintances, and how deeply you will become interested in them. I would advise you, at first, to obtain, for an evening or two, the assistance of some friend who is familiar with the stars, just to point out a few of the most conspicuous constellations. This will put you on the track, and you will afterwards experience no difficulty in finding all the constellations and stars that are particularly worth knowing; especially if you have before you a map of the stars, or, what is much better, a celestial globe. It is a pleasant evening recreation for a small company of young astronomers to go out together, and learn one or two constellations every favorable evening, until the whole are mastered. If you have a celestial globe,rectifyit for the evening; that is, place it in such a position, that the constellations shall be seen on it in the same position with respect to the horizon, that they have at that moment in the sky itself. To do this, I first elevate the north pole until the number of degrees on the brass meridian from the pole to the horizon corresponds to my latitude, (forty-one degrees and eighteen minutes.) I then find the sun's place in the ecliptic, by looking for the day of the month on the broad horizon, and against it noting the corresponding sign and degree. I now find the same sign and degree on the ecliptic itself, and bring that point to the brass meridian. As that willbe the position of the sun at noon, I set the hour-index at twelve, and then turn the globe westward, until the index points to the given hour of the evening. If I now inspect the figures of the constellations, and then look upward at the firmament, I shall see that the latter are spread over the sky in the same manner as the pictures of them are painted on the globe. I will point out a few marks by which the leading constellations may be recognised; this will aid you in finding them, and you can afterwards learn the individual stars of a constellation, to any extent you please, by means of the globes or maps. Let us begin with theConstellations of the Zodiac, which, succeeding each other, as they do, in a known order, are most easily found.
Aries(the Ram) is a small constellation, known by two bright stars which form his head,AlphaandBeta Arietis. These two stars are about four degrees apart; and directly south of Beta, at the distance of one degree, is a smaller star,Gamma Arietis. It has been already intimated that the Vernal equinox probably was near the head of Aries, when the signs of the zodiac received their present names.
Taurus(the Bull) will be readily found by the seven stars, orPleiades, which lie in his neck. The largest star in Taurus isAldebaran, in the Bull's eye, a star of the first magnitude, of a reddish color, somewhat resembling the planet Mars. Aldebaran and four other stars, close together in the face of Taurus, compose theHyades.
Gemini(the Twins) is known by two very bright stars,Castor and Pollux, five degrees asunder. Castor (the northern) is of the first, and Pollux of the second, magnitude.
Cancer(the Crab.) There are no large stars in this constellation, and it is regarded as less remarkable than any other in the zodiac. It contains, however, an interesting group of small stars, calledPræsepe, or the nebula of Cancer, which resembles a comet, and is often mistaken for one, by persons unacquainted with thestars. With a telescope of very moderate powers this nebula is converted into a beautiful assemblage of exceedingly bright stars.
Leo(the Lion) is a very large constellation, and has many interesting members.Regulus(Alpha Leonis) is a star of the first magnitude, which lies directly in the ecliptic, and is much used in astronomical observations. North of Regulus, lies a semicircle of bright stars, forming asickle, of which Regulus is the handle.Denebola, a star of the second magnitude, is in the Lion's tail, twenty-five degrees northeast of Regulus.
Virgo(the Virgin) extends a considerable way from west to east, but contains only a few bright stars.Spica, however, is a star of the first magnitude, and lies a little east of the place of the Autumnal equinox. Eighteen degrees eastward of Denebola, and twenty degrees north of Spica, isVindemiatrix, in the arm of Virgo, a star of the third magnitude.
Libra(the Balance) is distinguished by three large stars, of which the two brightest constitute the beam of the balance, and the smallest forms the top or handle.
Scorpio(the Scorpion) is one of the finest of the constellations. His head is formed of five bright stars, arranged in the arc of a circle, which is crossed in the centre by the ecliptic nearly at right angles, near the brightest of the five,Beta Scorpionis. Nine degrees southeast of this is a remarkable star of the first magnitude, of a reddish color, calledCor Scorpionis, orAntares. South of this, a succession of bright stars sweep round towards the east, terminating in several small stars, forming the tail of the Scorpion.
Sagittarius(the Archer.) Northeast of the tail of the Scorpion are three stars in the arc of a circle, which constitute thebowof the Archer, the central star being the brightest, directly west of which is a bright star which forms thearrow.
Capricornus(the Goat) lies northeast of Sagittarius, and is known by two bright stars, three degrees apart, which form the head.
Aquarius(the Water-Bearer) is recognised by two stars in a line withAlpha Capricorni, forming the shoulders of the figure. These two stars are ten degrees apart; and three degrees southeast is a third star, which, together with the other two, make an acute triangle, of which the westernmost is the vertex.
Pisces(the Fishes) lie between Aquarius and Aries. They are not distinguished by any large stars, but are connected by a series of small stars, that form a crooked line between them.Piscis Australia, the Southern Fish, lies directly below Aquarius, and is known by a single bright star far in the south, having a declination of thirty degrees. The name of this star isFomalhaut, and it is much used in astronomical measurements.
The constellations of the zodiac, being first well learned, so as to be readily recognised, will facilitate the learning of others that lie north and south of them. Let us, therefore, next review the principalNorthern Constellations, beginning north of Aries, and proceeding from west to east.
Andromedais characterized by three stars of the second magnitude, situated in a straight line, extending from west to east. The middle star is about seventeen degrees north of Beta Arietis. It is in the girdle of Andromeda, and is namedMirach. The other two lie at about equal distances, fourteen degrees west and east of Mirach. The western star, in the head of Andromeda, lies in the equinoctial colure. The eastern star,Alamak, is situated in the foot.
Perseuslies directly north of the Pleiades, and contains several bright stars. About eighteen degrees from the Pleiades isAlgol, a star of the second magnitude, in the head of Medusa, which forms a part of the figure; and nine degrees northeast of Algol isAlgenib, of the same magnitude, in the back of Perseus. Between Algenib and the Pleiades are three bright stars, at nearly equal intervals, which compose the right leg of Perseus.
Auriga(the Wagoner) lies directly east of Perseus,and extends nearly parallel to that constellation, from north to south.Capella, a very white and beautiful star of the first magnitude, distinguishes this constellation. The feet of Auriga are near the Bull's horns.
TheLynxcomes next, but presents nothing particularly interesting, containing no stars above the fourth magnitude.
Leo Minorconsists of a collection of small stars north of the sickle in Leo, and south of the Great Bear. Its largest star is only of the third magnitude.
Coma Berenicesis a cluster of small stars, north of Denebola, in the tail of the Lion, and of the head of Virgo. About twelve degrees directly north of Berenice's hair, is a single bright star, calledCor Caroli, or Charles's Heart.
Bootes, which comes next, is easily found by means ofArcturus, a star of the first magnitude, of a reddish color, which is situated near the knee of the figure. Arcturus is accompanied by three small stars, forming a triangle a little to the southwest. Two bright stars,GammaandDelta Bootis, form the shoulders, andBeta, of the third magnitude, is in the head, of the figure.
Corona Borealis, (the Crown,) which is situated east of Bootes, is very easily recognised, composed as it is of a semicircle of bright stars. In the centre of the bright crown is a star of the second magnitude, calledGemma: the remaining stars are all much smaller.
Hercules, lying between the Crown on the west and the Lyre on the east, is very thickly set with stars, most of which are quite small. This constellation covers a great extent of the sky, especially from north to south, the head terminating within fifteen degrees of the equator, and marked by a star of the third magnitude, calledRas Algethi, which is the largest in the constellation.
Ophiucusis situated directly south of Hercules, extending some distance on both sides of the equator, the feet resting on the Scorpion. The head terminates near the head of Hercules, and, like that, is marked by a bright star within five degrees ofAlpha HerculisOphiucus is represented as holding in his hands theSerpent, the head of which, consisting of three bright stars, is situated a little south of the Crown. The folds of the serpent will be easily followed by a succession of bright stars, which extend a great way to the east.
Aquila(the Eagle) is conspicuous for three bright stars in its neck, of which the central one,Altair, is a very brilliant white star of the first magnitude.Antinouslies directly south of the Eagle, and north of the head of Capricornus.
Delphinus(the Dolphin) is a small but beautiful constellation, a few degrees east of the Eagle, and is characterized by four bright stars near to one another, forming a small rhombic square. Another star of the same magnitude, five degrees south, makes the tail.
Pegasuslies between Aquarius on the southwest and Andromeda on the northeast. It contains but few large stars. A very regular square of bright stars is composed ofAlpha Andromedæand the three largest stars in Pegasus; namely,Scheat,Markab, andAlgenib. The sides composing this square are each about fifteen degrees. Algenib is situated in the equinoctial colure.
We may now review theConstellations which surround the north pole, within the circle of perpetual apparition.
Ursa Minor(the Little Bear) lies nearest the pole. The pole-star,Polaris, is in the extremity of the tail, and is of the third magnitude. Three stars in a straight line, four degrees or five degrees apart, commencing with the pole-star, lead to a trapezium of four stars, and the whole seven form together adipper,—the trapezium being the body and the three stars the handle.
Ursa Major(the Great Bear) is situated between the pole and the Lesser Lion, and is usually recognised by the figure of a larger and more perfect dipper which constitutes the hinder part of the animal. This has also seven stars, four in the body of the Dipper and three in the handle. All these are stars of much celebrity. The two in the western side of the Dipper, Alpha andBeta, are calledPointers, on account of their always being in a right line with the pole-star, and therefore affording an easy mode of finding that. The first star in the tail, next the body, is namedAlioth, and the second,Mizar. The head of the Great Bear lies far to the westward of the Pointers, and is composed of numerous small stars; and the feet are severally composed of two small stars very near to each other.
Draco(the Dragon) winds round between the Great and the Little Bear; and, commencing with the tail, between the Pointers and the pole-star, it is easily traced by a succession of bright stars extending from west to east. Passing under Ursa Minor, it returns westward, and terminates in a triangle which forms the head of Draco, near the feet of Hercules, northwest of Lyra.Cepheuslies eastward of the breast of the Dragon, but has no stars above the third magnitude.
Cassiopeiais known by the figure of achair, composed of four stars which form the legs, and two which form the back. This constellation lies between Perseus and Cepheus, in the Milky Way.
Cygnus(the Swan) is situated also in the Milky Way, some distance southwest of Cassiopeia, towards the Eagle. Three bright stars, which lie along the Milky Way, form the body and neck of the Swan, and two others, in a line with the middle one of the three, one above and one below, constitute the wings. This constellation is among the few that exhibit some resemblance to the animals whose names they bear.
Lyra(the Lyre) is directly west of the Swan, and is easily distinguished by a beautiful white star of the first magnitude,Alpha Lyræ.
TheSouthern Constellationsare comparatively few in number. I shall notice only the Whale, Orion, the Greater and Lesser Dog, Hydra, and the Crow.
Cetus(the Whale) is distinguished rather for its extent than its brilliancy, reaching as it does through forty degrees of longitude, while none of its stars, except one, are above the third magnitude.Menkar(Alpha Ceti)in the mouth, is a star of the second magnitude; and several other bright stars, directly south of Aries, make the head and neck of the Whale.Mira, (Omicron Ceti,) in the neck of the Whale, is a variable star.
Orionis one of the largest and most beautiful of the constellations, lying southeast of Taurus. A cluster of small stars forms the head; two large stars,Betalgeusof the first andBellatrixof the second magnitude, make the shoulders; three more bright stars compose the buckler, and three the sword; andRigel, another star of the first magnitude, makes one of the feet. In this constellation there are seventy stars plainly visible to the naked eye, including two of the first magnitude, four of the second, and three of the third.
Canis Majorlies southeast of Orion, and is distinguished chiefly by its containing the largest of the fixed stars,Sirius.
Canis Minor, a little north of the equator, between Canis Major and Gemini, is a small constellation, consisting chiefly of two stars, of which,Procyonis of the first magnitude.
Hydrahas its head near Procyon, consisting of a number of stars of ordinary brightness. About fifteen degrees southeast of the head is a star of the second magnitude, forming the heart, (Cor Hydræ;) and eastward of this is a long succession of stars of the fourth and fifth magnitudes, composing the body and tail, and reaching a few degrees south of Spica Virginis.
Corvus(the Crow) is represented as standing on the tail of Hydra. It consists of small stars, only three of which are as large as the third magnitude.
In assigning the places of individual stars, I have not aimed at great precision; but such a knowledge as you will acquire of the constellations and larger stars, by nothing more even than you can obtain from the foregoing sketch, will not only add greatly to the interest with which you will ever afterwards look at the starry heavens, but it will enable you to locate any phenomenon that may present itself in the nocturnal sky, andto understand the position of any object that may be described, by assigning its true place among the stars; although I hope you will go much further than this mere outline, in cultivating an actual acquaintance with the stars. Leaving, now, these great divisions of the bodies of the firmament, let us ascend to the next order of stars, composingClusters.
In various parts of the nocturnal heavens are seen large groups which, either by the naked eye, or by the aid of the smallest telescope, are perceived to consist of a great number of small stars. Such are the Pleiades, Coma Berenices, and Præsepe, or the Bee-hive, in Cancer. ThePleiades, or Seven Stars, as they are called, in the neck of Taurus, is the most conspicuous cluster. When we lookdirectlyat this group, we cannot distinguish more than six stars; but by turning the eyesidewaysupon it, we discover that there are many more; for it is a remarkable fact that indirect vision is far more delicate than direct. Thus we can see the zodiacal light or a comet's tail much more distinctly and better defined, if we fix one eye on a part of the heavens at some distance and turn the other eye obliquely upon the object, than we can by looking directly towards it. Telescopes show the Pleiades to contain fifty or sixty stars, crowded together, and apparently insulated from the other parts of the heavens.Coma Bereniceshas fewer stars, but they are of a larger class than those which compose the Pleiades. TheBee-hive, or Nebula of Cancer, as it is called, is one of the finest objects of this kind for a small telescope, being by its aid converted into a rich congeries of shining points. The head of Orion affords an example of another cluster, though less remarkable than those already mentioned. These clusters are pleasing objects to the telescope; and since a common spyglass will serve to give a distinct view of most of them, every one may have the power of taking the view. But we pass, now, to the third order of stars, which present themselves much more obscurely to the gaze of the astronomer, and require large instruments for the full developement of their wonderful organization. These are theNebulæ.
Figures 70, 71, 72, 73. CLUSTERS OF STARS AND NEBULÆ.Figures 70, 71, 72, 73. CLUSTERS OF STARS AND NEBULÆ.
Nebulæ are faint misty appearances which are dimly seen among the stars, resembling comets, or a speck of fog. They are usually resolved by the telescope into myriads of small stars; though in some instances, no powers of the telescope have been found sufficient thus to resolve them. TheGalaxyor Milky Way, presents a continued succession of large nebulas. The telescope reveals to us innumerable objects of this kind. Sir William Herschel has given catalogues of two thousand nebulæ, and has shown that the nebulous matter is distributed through the immensity of space in quantities inconceivably great, and in separate parcels, of all shapes and sizes, and of all degrees of brightness between a mere milky appearance and the condensed light of a fixed star. In fact, more distinct nebulæ have been hunted out by the aid of telescopes than the whole number of stars visible to the naked eye in a clear Winter's night. Their appearances are extremely diversified. In many of them we can easily distinguish the individual stars; in those apparently more remote, the interval between the stars diminishes, until it becomes quite imperceptible; and in their faintest aspect they dwindle to points so minute, as to be appropriately denominatedstar-dust. Beyond this, no stars are distinctly visible, but only streaks or patches of milky light. The diagram facing page 379 represents a magnificent nebula in the Galaxy. In objects so distant as the fixed stars, any apparent interval must denote an immense space; and just imagine yourself situated any where within the grand assemblage of stars, and a firmament would expand itself over your head like that of our evening sky, only a thousand times more rich and splendid.
Many of the nebulæ exhibit a tendency towards a globular form, and indicate a rapid condensation towards the centre. This characteristic is exhibited inthe forms represented in Figs. 70 and 71. We have here two specimens of nebulæ of the nearer class, where the stars are easily discriminated. In Figs. 72 and 73 we have examples of two others of the remoter kind, one of which is of the variety calledstar-dust. These wonderful objects, however, are not confined to the spherical form, but exhibit great varieties of figure. Sometimes they appear as ovals; sometimes they are shaped like a fan; and the unresolvable kind often affect the most fantastic forms. The opposite diagram, Fig. 74, as well as the preceding, affords a specimen of these varieties, as given in Professor Nichols's 'Architecture of the Heavens,' where they are faithfully copied from the papers of Herschel, in the 'Philosophical Transactions.'
Sir John Herschel has recently returned from a residence of five years at the Cape of Good Hope, with the express view of exploring the hidden treasures of the southern hemisphere. The kinds of nebulæ are in general similar to those of the northern hemisphere, and the forms are equally various and singular. TheMagellan Clouds, two remarkable objects seen among the stars of that hemisphere, and celebrated among navigators, appeared to the great telescope of Herschel (as we are informed by Professor Nichols) no longer as simple milky spots, or permanent light flocculi of cloud, as they appear to the unassisted eye, but shone with inconceivable splendor. TheNubecula Major, as the larger object is called, is a congeries of clusters of stars, of irregular form, globular clusters and nebulæ of various magnitudes and degrees of condensation, among which is interspersed a large portion of irresolvable nebulous matter, which may be, and probably is, star-dust, but which the power of the twenty-feet telescope shows only as a general illumination of the field of view, forming a bright ground on which the other objects are scattered. TheNubecula Minor(the lesser cloud) exhibited appearances similar, though inferior in degree.
Figure 74. VARIOUS FORMS OF NEBULÆ.Figure 74. VARIOUS FORMS OF NEBULÆ.
Figure 75. A NEBULA IN THE MILKY WAY.Figure 75. A NEBULA IN THE MILKY WAY.
It is a grand idea, first conceived by Sir WilliamHerschel, and generally adopted by astronomers, that the whole Galaxy, or Milky Way, is nothing else than a nebula, and appears so extended, merely because it happens to be that particular nebula to which we belong. According to this view, our sun, with his attendant planets and comets, constitutes but a single star of the Galaxy, and our firmament of stars, or visible heavens, is composed of the stars ofournebula alone. An inhabitant of any of the other nebulæ would see spreading over him a firmament equally spacious, and in some cases inconceivably more brilliant.
It is an exalted spectacle to travel over the Galaxy in a clear night, with a powerful telescope, with the heart full of the idea that every star is a world. Sir William Herschel, by counting the stars in a single field of his telescope, estimated that fifty thousand had passed under his review in a zone two degrees in breadth, during a single hour's observation. Notwithstanding the apparent contiguity of the stars which crowd the Galaxy, it is certain that their mutual distances must be inconceivably great.
It is with some reluctance that I leave, for the present, this fairy land of astronomy; but I must not omit, before bringing these Letters to a conclusion, to tell you something respecting other curious and interesting objects to be found among the stars.
Variable Starsare those which undergo a periodical change of brightness. One of the most remarkable is the starMira, in the Whale, (Omicron Ceti.) It appears once in eleven months, remains at its greatest brightness about a fortnight, being then, on some occasions, equal to a star of the second magnitude. It then decreases about three months, until it becomes completely invisible, and remains so about five months, when it again becomes visible, and continues increasing during the remaining three months of its period.
Another very remarkable variable star isAlgol, (Beta Persei.) It is usually visible as a star of the second magnitude, and continues such for two days and fourteen hours, when it suddenly begins to diminish in splendor, and in about three and a half hours is reduced to the fourth magnitude. It then begins again to increase, and in three and a half hours more is restored to its usual brightness, going through all its changes in less than three days. This remarkable law of variation appears strongly to suggest the revolution round it of some opaque body, which, when interposed between us and Algol, cuts off a large portion of its light. "It is," says Sir J. Herschel, "an indication of a high degree of activity in regions where, but for such evidences, we might conclude all lifeless. Our sun requires almost nine times this period to perform a revolution on its axis. On the other hand, the periodic time of an opaque revolving body, sufficiently large, which would produce a similar temporary obscuration of the sun, seen from a fixed star, would be less than fourteen hours." The duration of these periods is extremely various. While that of Beta Persei, above mentioned, is less than three days, others are more than a year; and others, many years.
Temporary Starsare new stars, which have appeared suddenly in the firmament, and, after a certain interval, as suddenly disappeared, and returned no more. It was the appearance of a new star of this kind, one hundred and twenty-five years before the Christian era, that prompted Hipparchus to draw up a catalogue of the stars, the first on record. Such, also, was the star which suddenly shone out, A.D. 389, in the Eagle, as bright as Venus, and, after remaining three weeks, disappeared entirely. At other periods, at distant intervals, similar phenomena have presented themselves. Thus the appearance of a star in 1572 was so sudden, that Tycho Brahe, returning home one day, was surprised to find a collection of country people gazing at a star which he was sure did not exist half an hour before. It was then as bright as Sirius, and continued to increase until it surpassed Jupiter when brightest, and was visible at mid-day. In a month it began to diminish; and, in threemonths afterwards, it had entirely disappeared. It has been supposed by some that, in a few instances, the same star has returned, constituting one of the periodical or variable stars of a long period. Moreover, on a careful reexamination of the heavens, and a comparison of catalogues, many stars are now discovered to be missing.
Double Starsare those which appear single to the naked eye, but are resolved into two by the telescope; or, if not visible to the naked eye, are seen in the telescope so close together as to be recognised as objects of this class. Sometimes, three or more stars are found in this near connexion, constituting triple, or multiple stars. Castor, for example, when seen by the naked eye, appears as a single star, but in a telescope even of moderate powers, it is resolved into two stars, of between the third and fourth magnitudes, within five seconds of each other. These two stars are nearly of equal size; but more commonly, one is exceedingly small in comparison with the other, resembling a satellite near its primary, although in distance, in light, and in other characteristics, each has all the attributes of a star, and the combination, therefore, cannot be that of a planet with a satellite. In most instances, also, the distance between these objects is much less than five seconds; and, in many cases, it is less than one second. The extreme closeness, together with the exceeding minuteness, of most of the double stars, requires the best telescopes united with the most acute powers of observation. Indeed, certain of these objects are regarded as the severesttestsboth of the excellence of the instruments and of the skill of the observer. The diagram on page 382, Fig. 76, represents four double stars, as seen with appropriate magnifiers. No. 1, exhibits Epsilon Bootis with a power of three hundred and fifty; No. 2, Rigel, with a power of one hundred and thirty; No. 3, the Pole-star, with a power of one hundred; and No. 4, Castor, with a power of three hundred.
Our knowledge of the double stars almost commenced with Sir William Herschel, about the year 1780. At the time he began his search for them, he was acquainted with onlyfour. Within five years he discovered nearlyseven hundreddouble stars, and during his life, he observed no less than twenty-four hundred. In his Memoirs, published in the Philosophical Transactions, he gave most accurate measurements of the distances between the two stars, and of the angle which a line joining the two formed with a circle parallel to the equator. These data would enable him, or at least posterity, to judge whether these minute bodies ever change their position with respect to each other. Since 1821, these researches have been prosecuted, with great zeal and industry, by Sir James South and Sir John Herschel, in England; while Professor Struve, of Dorpat, with the celebrated telescope of Fraunhofer, has published, from his own observations, a catalogue of three thousand double stars, the determination of which involved the distinct and most minute inspection of at least one hundred and twenty thousand stars. Sir John Herschel, in his recent survey of the southern hemisphere, is said to have added to the catalogue of double stars nearly three thousand more.
Fig. 76.Fig. 76.
Two circumstances add a high degree of interest to the phenomena of double stars: the first is, that a few of them, at least, are found to have a revolution around each other; the second, that they are supposed to afford the means of ascertaining the parallax of the fixed stars. But I must defer these topics till my next Letter.
"O how canst thou renounce the boundless storeOf charms that Nature to her votary yields?The warbling woodland, the resounding shore,The pomp of groves, and garniture of fields;All that the genial ray of morning yields,And all that echoes to the song of even,All that the mountain's sheltering bosom shields,And all the dread magnificence of heaven,—O how canst thou renounce, and hope to be forgiven!"—Beattie.
"O how canst thou renounce the boundless storeOf charms that Nature to her votary yields?The warbling woodland, the resounding shore,The pomp of groves, and garniture of fields;All that the genial ray of morning yields,And all that echoes to the song of even,All that the mountain's sheltering bosom shields,And all the dread magnificence of heaven,—O how canst thou renounce, and hope to be forgiven!"—Beattie.
In1803, Sir William Herschel first determined and announced to the world, that there exist among the stars separate systems, composed of two stars revolving about each other in regular orbits. These he denominatedbinary stars, to distinguish them from other double stars where no such motion is detected, and whose proximity to each other may possibly arise from casual juxtaposition, or from one being in the range of the other. Between fifty and sixty instances of changes, to a greater or less amount, of the relative positions of double stars, are mentioned by Sir William Herschel; and a few of them had changed their places so much, within twenty-five years, and in such order, as to lead him to the conclusion that they performed revolutions, one around the other, in regular orbits. These conclusions have been fully confirmed by later observers; so that it is now considered as fully established, that there exist among the fixed stars binary systems, in which two stars perform to each other the office of sun and planet, and that the periods of revolution of more than one such pair have been ascertained with some degree of exactness. Immersions and emersions of stars behind each other have been observed, and real motions among them detected, rapid enough to become sensible and measurable in very short intervals of time. The periods of the double stars are very various, ranging, in the case of those already ascertained, from forty-three yearsto one thousand. Their orbits are very small ellipses, only a few seconds in the longest direction, and more eccentric than those of the planets. A double star in the Northern Crown (Eta Coronæ) has made a complete revolution since its first discovery, and is now far advanced in its second period; while a star in the Lion (Gamma Leonis) requires twelve hundred years to complete its circuit.
You may not at once see the reason why these revolutions of one member of a double star around the other, should be deemed facts of such extraordinary interest; to you they may appear rather in the light of astronomical curiosities. But remark, that the revolutions of the binary stars have assured us of this most interesting fact, thatthe law of gravitation extends to the fixed stars. Before these discoveries, we could not decide, except by a feeble analogy, that this law transcended the bounds of the solar system. Indeed, our belief of the fact rested more upon our idea of unity of design in the works of the Creator, than upon any certain proof; but the revolution of one star around another, in obedience to forces which are proved to be similar to those which govern the solar system, establishes the grand conclusion, that the law of gravitation is truly the law of the material universe. "We have the same evidence," says Sir John Herschel, "of the revolutions of the binary stars about each other, that we have of those of Saturn and Uranus about the sun; and the correspondence between their calculated and observed places, in such elongated ellipses, must be admitted to carry with it a proof of the prevalence of the Newtonian law of gravity in their systems, of the very same nature and cogency as that of the calculated and observed places of comets round the centre of our own system. But it is not with the revolution of bodies of a cometary or planetary nature round a solar centre, that we are now concerned; it is with that of sun around sun, each, perhaps, accompanied with its train of planets and their satellites, closely shrouded from our view by the splendor of their respective suns, and crowded into a space, bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself."
Many of the double stars are of different colors; and Sir John Herschel is of the opinion that there exist in nature suns of different colors. "It may," says he, "be easier suggested in words than conceived in imagination, what variety of illumination two suns, a red and a green, or a yellow and a blue one, must afford to a planet circulating about either; and what charming contrasts and 'grateful vicissitudes' a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon. Insulated stars of a red color, almost as deep as that of blood, occur in many parts of the heavens; but no green or blue star, of any decided hue, has ever been noticed unassociated with a companion brighter than itself."
Beside these revolutions of the binary stars,some of the fixed stars appear to have a real motion in space. There are severalapparentchanges of place among the stars, arising from real changes in the earth, which, as we are not conscious of them, we refer to the stars; but there are other motions among the stars which cannot result from any changes in the earth, but must arise from changes in the stars themselves. Such motions are called theproper motionsof the stars. Nearly two thousand years ago, Hipparchus and Ptolemy made the most accurate determinations in their power of the relative situations of the stars, and their observations have been transmitted to us in Ptolemy's 'Almagest;' from which it appears that the stars retain at leastvery nearlythe same places now as they did at that period. Still, the more accurate methods of modern astronomers have brought to light minute changes in the places of certain stars, which force upon us the conclusion,either that our solar system causes an apparent displacementof certain stars, by a motion of its own in space, or that they have themselves a proper motion. Possibly, indeed, both these causes may operate.
If the sun, and of course the earth which accompanies him, is actually in motion, the fact may become manifest from the apparent approach of the stars in the region which he is leaving, and the recession of those which lie in the part of the heavens towards which he is travelling. Were two groves of trees situated on a plain at some distance apart, and we should go from one to the other, the trees before us would gradually appear further and further asunder, while those we left behind would appear to approach each other. Some years since, Sir William Herschel supposed he had detected changes of this kind among two sets of stars in opposite points of the heavens, and announced that the solar system was in motion towards a point in the constellation Hercules; but other astronomers have not found the changes in question such as would correspond to this motion, or to any motion of the sun; and, while it is a matter of general belief that the sun has a motion in space, the fact is not considered as yet entirely proved.
In most cases, where a proper motion in certain stars has been suspected, its annual amount has been so small, that many years are required to assure us, that the effect is not owing to some other cause than a real progressive motion in the stars themselves; but in a few instances the fact is too obvious to admit of any doubt. Thus, the two stars, 61 Cygni, which are nearly equal, have remained constantly at the same or nearly at the same distance of fifteen seconds, for at least fifty years past. Mean-while, they have shifted their local situation in the heavens four minutes twenty-three seconds, the annual proper motion of each star being five seconds and three tenths, by which quantity this system is every year carried along in some unknown path, by a motion which for many centuries must be regarded as uniform and rectilinear. A greater proportion of the double stars than of any other indicate proper motions, especially the binary stars, or those which have a revolution around each other. Among stars not double, and no way differing from the rest in any other obvious particular, a star in the constellation Cassiopeia, (Mu Cassiopeiæ) has the greatest proper motion of any yet ascertained, amounting to nearly four seconds annually.
You have doubtless heard much respecting the "immeasurabledistances" of the fixed stars, and will desire to learn what is known to astronomers respecting this interesting subject.
We cannot ascertain the actual distance of any of the fixed stars, but we can certainly determine that the nearest star is more than twenty millions of millions of miles from the earth, (20,000,000,000,000.) For all measurements relating to the distances of thesun and planets, the radius of the earth furnishes the base line. The length of this line being known, and the horizontal parallax of the sun or any planet, we have the means of calculating the distance of the body from us, by methods explained in a previous Letter. But any star, viewed from the opposite sides of the earth, would appear from both stations to occupy precisely the same situation in the celestial sphere, and of course it would exhibit no horizontal parallax. But astronomers have endeavored to find a parallax in some of the fixed stars, by taking thediameter of the earth's orbitas a base line. Yet even a change of position amounting to one hundred and ninety millions of miles proved, until very recently, insufficient to alter the place of a single star, so far as to be capable of detection by very refined observations; from which it was concluded that the stars have not even anyannual parallax; that is, the angle subtended by the semidiameter of the earth's orbit, at the nearest fixed star, is insensible. The errors to which instrumental measurements are subject, arising from the defects of instruments themselves, from refraction, and from various other sources of inaccuracy, are such, that the angular determinations of arcs of the heavens cannot be relied on to less thanone second, and therefore cannot be appreciated by direct measurement. It follows, that, when viewed from the nearest star, the diameter of the earth's orbit would be insensible; the spider-line of the telescope would more than cover it. Taking, however, the annual parallax of a fixed star at one second, it can be demonstrated, that the distance of the nearest fixed starmust exceed95000000 × 200000 = 190000000 × 100000, or one hundred thousand times one hundred and ninety millions of miles. Of a distance so vast we can form no adequate conceptions, and even seek to measure it only by the time that light (which moves more than one hundred and ninety-two thousand miles per second, and passes from the sun to the earth in eight minutes and seven seconds) would take to traverse it, which is found to be more than three and a half years.
If these conclusions are drawn with respect to the largest of the fixed stars, which we suppose to be vastly nearer to us than those of the smallest magnitude, the idea of distance swells upon us when we attempt to estimate the remoteness of the latter. As it is uncertain, however, whether the difference in the apparent magnitudes of the stars is owing to a real difference, or merely to their being at various distances from the eye, more or less uncertainty must attend all efforts to determine the relative distances of the stars; but astronomers generally believe, that the lower orders of stars are vastly more distant from us than the higher. Of some stars it is said, that thousands of years would be required for their light to travel down to us.
I have said that the stars have always been held, until recently, to have no annual parallax; yet it may be observed that astronomers were not exactly agreed on this point. Dr. Brinkley, a late eminent Irish astronomer, supposed that he had detected an annual parallax in Alpha Lyræ, amounting to one second and thirteen hundreths, and in Alpha Aquilæ, of one second and forty-two hundreths. These results were controverted by Mr. Pond, of the Royal Observatory of Greenwich; andMr. Struve, of Dorpat, has shown that, in a number of cases, the supposed parallax is in a direction opposite to that which would arise from the motion of the earth. Hence it is considered doubtful whether, in all cases of an apparent parallax, the effect is not wholly due to errors of observation.
But as if nothing was to be hidden from our times, the long sought for parallax among the fixed stars has at length been found, and consequently the distance of some of these bodies, at least, is no longer veiled in mystery. In the year 1838, Professor Bessel, of Köningsberg, announced the discovery of a parallax in one of the stars of the Swan, (61Cygni,) amounting to aboutone third of a second. This seems, indeed, so small an angle, that we might have reason to suspect the reality of the determination; but the most competent judges who have thoroughly examined the process by which the discovery was made, assent to its validity. What, then, do astronomers understand, when they say that a parallax has been discovered in one of the fixed stars, amounting to one third of a second? They mean that the star in question apparently shifts its place in the heavens, to that amount, when viewed at opposite extremities of the earth's orbit, namely, at points in space distant from each other one hundred and ninety millions of miles. On calculating the distance of the star from us from these data, it is found to be six hundred and fifty-seven thousand seven hundred times ninety-five millions of miles,—a distance which it would take light more than ten years to traverse.
Indirect methods have been proposed, for ascertaining the parallax of the fixed stars, by means of observations on thedouble stars. If the two stars composing a double star are at different distances from us, parallax would affect them unequally, and change their relative positions with respect to each other; and since the ordinary sources of error arising from the imperfection of instruments, from precession, and from refraction, would be avoided, (as they would affectboth objects alike, and therefore would not disturb their relative positions,) measurements taken with the micrometer of changes much less than one second may be relied on. Sir John Herschel proposed a method, by which changes may be determined that amount to only one fortieth of a second.
The immense distance of the fixed stars is inferred also from the fact, that the largest telescopes do not increase their apparent magnitude. They are still points, when viewed with glasses that magnify five thousand times.
With respect to theNATURE OF THE STARS, it would seem fruitless to inquire into the nature of bodies so distant, and which reveal themselves to us only as shining points in space. Still, there are a few very satisfactory inferences that can be made out respecting them. First,the fixed stars are bodies greater than our earth. If this were not the case, they would not be visible at such an immense distance. Dr. Wollaston, a distinguished English philosopher, attempted to estimate the magnitudes of certain of the fixed stars from the light which they afford. By means of an accurate photometer, (an instrument for measuring the relative intensities of light,) he compared the light of Sirius with that of the sun. He next inquired how far the sun must be removed from us, in order to appear no brighter than Sirius. He found the distance to be one hundred and forty-one thousand times its present distance. But Sirius is more than two hundred thousand times as far off as the sun; hence he inferred that, upon the lowest computation, it must actually give out twice as much light as the sun; or that, in point of splendor, Sirius must be at least equal to two suns. Indeed, he has rendered it probable, that its light is equal to that of fourteen suns. There is reason, however, to believe that the stars are actually of various magnitudes, and that their apparent difference is not owing merely to their different distances. Bessel estimates the quantity of matter in the two members of adouble star in the Swan, as less than half that of the sun.
Secondly,the fixed stars are suns. We have already seen that they are large bodies; that they are immensely further off than the furthest planet; that they shine by their own light; in short, that their appearance is, in all respects, the same as the sun would exhibit if removed to the region of the stars. Hence we infer that they are bodies of the same kind with the sun. We are justified, therefore, by a sound analogy, in concluding that the stars were made for the same end as the sun, namely, as the centres of attraction to other planetary worlds, to which they severally dispense light and heat. Although the starry heavens present, in a clear night, a spectacle of unrivalled grandeur and beauty, yet it must be admitted that the chief purpose of the stars could not have been to adorn the night, since by far the greater part of them are invisible to the naked eye; nor as landmarks to the navigator, for only a very small proportion of them are adapted to this purpose; nor, finally, to influence the earth by their attractions, since their distance renders such an effect entirely insensible. If they are suns, and if they exert no important agencies upon our world, but are bodies evidently adapted to the same purpose as our sun, then it is as rational to suppose that they were made to give light and heat, as that the eye was made for seeing and the ear for hearing. It is obvious to inquire, next, to what they dispense these gifts, if not to planetary worlds; and why to planetary worlds, if not for the use of percipient beings? We are thus led, almost inevitably, to the idea of aplurality of worlds; and the conclusion is forced upon us, that the spot which the Creator has assigned to us is but a humble province in his boundless empire.