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They are properly called Variable Stars. One in the Whale has a period of 344 days, and is remarkable for the magnitude of its variations. From being a star of the second magnitude, it becomes so dim as to be seen with difficulty through powerful telescopes. Some are remarkable for the shortness of the period of their variation. Algol has a period of between two and three days; Delta Cephei, of 5 days; Beta Lyræ, of 6 2-5 days; and Mu Antinoi, of 7 days.

The regular succession of these variations precludes the supposition of an actual destruction of the stars; neither can the variations be supposed to arise from a change of distance; for, as the stars invariably retain their apparent places, it would be necessary to suppose that they approach to, and recede from the earth in straight lines, which is very improbable. The most probable supposition is, that the stars revolve, like the sun and planets, about an axis. "Such a motion," says the elder Herschel, "may be as evidently proved, as the diurnal motion of the earth. Dark spots, or large portions of the surface, less luminous than the rest, turned alternately in certain directions, either toward or from us, will account for all the phenomena of periodical changes in the luster of the stars, so satisfactorily, that we certainly need not look for any other cause.'


244. On examining the stars with telescopes of considerable power, many of them are found to be composed of two or more stars, placed contiguous to each other, or of which the distance subtends a very minute angle. This appearance is, probably, in many cases, owing solely to the optical effect of their position relative to the spectator; for it is evident that two stars will appear contiguous if they are placed nearly in the same line of vision, although their real distance may be immeasurably great.

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Here the observer on the left sees a large and small star at A, apparently near together-the lowest star being much the smallest. But instead of their being situated as they appear to be, with respect to each other, the true position of the smaller star may be at B instead of A; and the difference in their apparent magnitudes may be wholly owing to the greater distance of the lower star.

Upon this subject Dr. Herschel remarks, that this nearness of the stars to each other, in certain cases, might be attributed to some accidental cause, did it occur only in a few instances; but the frequency of this companionship, the extreme closeness, and, in many cases, the near equality of the stars so conjoined, would alone lead to a strong suspicion of a more near and intimate relation than mere casual juxtaposition.

245. There are, however, many instances in which the angle of position of the two stars varies in such a manner as to indi

are these unsteady stars called? What specimens referred to, and their periods? What does this regular succession, &c., prove? What theory did Dr. Herschel adopt respecting the variable stars? 244. What said of double stars? Are they always really near each other? Illustrate on blackboard. Remark of Dr. Herschel? 245. Are they

cate a revolution about each other and about a common center. In this case they are said to form a Binary system performing to each other the office of sun and planet, and are connected together by laws of gravitation like those which prevail in the solar system.

The recent observations of Sir John Herschel and Sir James South, have established the truth of this singular fact beyond a doubt. Motions have been detected, so rapid as to become measurable within very short periods of time; and at certain epochs, the satellite or feebler star has been observed to disappear, either passing behind or before the primary, or approaching so near to it that its light has been absorbed by that of the other.

246. The most remarkable instance of a regular revolution of this sort, is that of Mizar, in the tail of the Great Bear; in which the angular motion is 6 degrees and 24 minutes of a great circle, annually; so that the two stars complete a revolution about one another in the space of 58 years. About eleventwelfths of a complete circuit have been already described since its discovery in 1781, the same year in which the planet Herschel was discovered.

A double star in Ophiuchus presents a similar phenomenon, and the satellite has a motion in its orbit still more rapid. Castor in the Twins, Gamma Virginis, Zeta in the Crab, Zi Bootis, Delta Serpentis, and that remarkable double star 61 Cygni, together with several others, amounting to 40 in number, exhibit the same evidence of a revolution about each other and about

a common center. (For a more particular description of these stars, see Telescopic Objects and the Map.)

But it is to be remembered that these are not the revolutions of bodies of a planetary nature around a solar center, but of sun around sun-each, perhaps, accompanied by 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.

247. The examination of double stars was first undertaken by the late Sir William Herschel, with a view to the question of parallax. His attention was, however, soon arrested by the new and unexpected phenomena which these bodies presented.

Sir William observed of them, in all, 2400. Sir James South and Herschel have given a catalogue of 880 in the Transactions of the Royal Society for 1824, and South added 458 in 1826. Sir John Herschel, in addition to the above, published an account of 1000, before he left England for the Cape of Good Hope, where he went to push his discoveries in the southern hemisphere. Professor Struve, with the great Dorpat telescope, has given a catalogue of 3,063 of the most remarkable of these stars.

The object of these catalogues is not merely to fix the place of the star within such limits as will enable us easily to discover it at any future time, but also to record a description

ever really near each other? What motion? What do these constitute? Is it certain that stars are ever thus in motion around a common center? 246. What remarkable instance cited? Its annual angular motion? Period? What other binary systems! Are these planetary systems like our own? 247. Who first undertook the examination of the double stars, and with what view? What number did he observe? What cata

of the appearance, position, and mutual distances of the individual stars composing the system, in order that subsequent observers may have the means of detecting their connected motions, or any changes which they may exhibit. Professor Struve has also taken notice of 52 triple stars, among which No. 11 of the Unicorn, Zeta of Cancer, and Zi of the Balance, appear to be ternary systems in motion. Quadruple and quintuple stars have likewise been observed, which also appear to revolve about a common center of gravity; in short, every region of the heavens furnishes examples of these curious phe



248. Many of the double stars exhibit the curious and beautiful phenomenon of contrasted colors, or complimentary tints. In such instances, the larger star is usually of a ruddy or orange hue, while the smaller one appears blue or green, probably in virtue of that general law of optics, which provides that when the retina is under the influence of excitement by any bright colored light, feebler lights, which, seen alone, would produce no sensation but that of whiteness, shall for the time appear colored with the tint complimentary to that of the brighter.

Thus, a yellow color predominating in the light of the brighter star, that of the less bright one, in the same field of view, will appear blue; while, if the tint of the brighter star verge to crimson, that of the other will exhibit a tendency to green-or even appear a vivid green. The former contrast is beautifully exhibited by Iota, in Cancer; the latter by Almaack, in Andromeda-both fine double stars. If, however, the colored star be much the less bright of the two, it will not materially affect the other. Thus, for instance, Eta Cassiopeiæ exhibits the beautiful combination of a large white star, and a small one of a rich ruddy purple.

249. It is not easy to conceive what variety of illumination two suns—a red and a green, or a yellow and a blue one-must afford to a planet revolving 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 the 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, we believe, ever been noticed, unassociated with a companion brighter than itself.


250. When we cast our eyes over the concave surface of the heavens in a clear night, we do not fail to observe that there are, here and there, groups of stars which seem to be compressed together more densely than those in the neighboring parts; forming bright patches or clusters.

logues? Their object? What triple stars? Ternary systems? Quadruple stars, &c.? 248. What said of the colors of the stars? What law of optics referred to? What illustrations? 249. What remarks respecting red and green suns, &c.? Of insulated stars of a red color? 250. What said of clusters? What specimen referred to? Pleiades?

The Pleiades are an instance of this kind, in which six or seven stars may be seen in near proximity, by the naked eye; and even more if the eye be turned carelessly upon it; for it is a remarkable fact that the center of the eye is far less sensible to feeble impressions of light, than the exterior portion of the retina. Rheita affirms that by the aid of a telescope he counted over 200 stars in this small cluster. See Map VIII., Fig. 28.

In the constellation called Coma Berenices there is another group more diffused, and consisting of much larger stars. In Cancer there is a nebulous cluster of very minute stars, called Præsepe, or the Beehive, which is sufficiently luminous to be seen by the naked eye, in the absence of the moon, and which any ordinary spyglass will resolve into separate stars. In the sword-handle of Perseus, also, is another such spot, crowded with stars. It requires, however, rather a better telescope to resolve it into individual stars. See p. 65, and Map VIII., Fig. 39.

Whatever be the nature of these clusters, it is certain that other laws of aggregation prevail in them, than those which have determined the scattering of stars over the general surface of the sky. Many of them, indeed, are of an exactly round figure, and convey the idea of a globular space filled full of stars, and constituting, in itself, a family or society apart, and subject only to its own internal laws.

"It would be a vain task," says the younger Herschel, "to attempt to count the stars in one of these globular clusters. They are not to be reckoned by hundreds; for it would appear that many clusters of this description must contain, at least, ten or twenty thou sand stars, compacted and wedged together in a round space, not more than a tenth part as large as that which is covered by the moon.


251. The Nebula, so called from their dim, cloudy appearance, form another class of objects which furnish matter for curious speculation and conjecture respecting the formation and structure of the sidereal heavens. When examined with a telescope of moderate powers, the greater part of the nebulæ are distinctly perceived to be composed of little stars, imperceptible to the naked eye, because, on account of their apparent proximity, the rays of light proceeding from each are blended together, in such a manner as to produce only a confused luminous appear


In other nebulæ, however, no individual stars can be perceived, even through the best telescopes; and the nebulæ exhibit only the appearance of a self-luminous phosphorescent patch of gaseous vapor, though it is possible that even in this case, the appearance may be owing to a congeries of stars so minute, or so distant, as not to afford, singly, sufficient light to make an impression on the eye.

Remarks upon their nature and the laws that govern them? Remarks of Herschel ? 251. What are nebulo, and why so called? How appear through telescopes? Are they all resolvable into stars?

252. One of the most remarkable nebulæ is in the swordhandle of Orion. It is formed of little flocky masses, like wisps of cloud, which seem to adhere to many small stars at its outskirts. It is not very unlike the mottling of the sun's disc, but of a coarser grain, and with darker intervals.. These wisps of light, however, present no appearance of being composed of small stars; but in the intervals between them, we fancy that we see stars, or that, could we strain our sight a little more, we should see them. These intervals may be compared to openings in the firmament, through which, as through a window, we seem to get a glimpse of other heavens, and brighter regions, beyond. See page 45, and Map VIII., Fig. 32.

253. Another very remarkable nebula is that in the girdle of Andromeda, which, on account of its being visible to the naked eye, has been known since the earliest ages of astronomy. It is often mistaken for a comet, by those unacquainted with the heavens. See page 20, and Map VIII., Fig. 22.

Marius, who noticed it in 1612, describes its appearance as that of a candle shining through horn; and the resemblance is certainly very striking. Its form is a long oval, increasing, by insensible gradations of brightness, from the circumference to a central point, which, though very much brighter than the rest, is not a star, but only a nebula in a high state of condensation. It occupies an area comparatively large-equal to that of the moon in quadrature. This nebula may be considered as a type, on a large scale, of a very numerous class of nebulæ, of a round or oval figure, increasing more or less in density toward the center.

254. Annular nebula are those in the form of a ring, but are among the rarest objects in the heavens. The most conspicuous of this class is to be found exactly half-way between the stars Beta and Gamma Lyræ, and may be seen with a telescope of moderate power. It is small, and particularly well defined; appearing like a flat oval ring. The central opening is not entirely dark, but is filled with a faint, hazy light, uniformly spread over it, like a fine gauze stretched over a hoop.

255. Planetary nebula are very extraordinary objects. They have, as their name imports, the appearance of planets, with round or slightly oval discs, somewhat mottled, but approaching, in some instances, to the vividness of actual planets. Some of them, upon the supposition that they are equally distant from us with the stars, must be of enormous magnitude. That one, for instance, which is situated in the left hand of Aquarius, must

252. What remarkable nebula mentioned? 253. What other? How long known, and why? Marius? Its form and extent? How considered?

Describe it? Point out on the map. Show on the map. How described by 254. What are Annular Nebula? are they common? What specimen referred to? 255. Planetary nebula? Their character and magnitude? Specimen? Stellar nebulæ ? General remarks respecting

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