I. GENERAL ASPECT OF THE HEAVENS.

322. The Magnitude of the Stars.—The stars that are visible to the naked eye are divided into six classes, according to their brightness. The brightest stars are called stars of the first magnitude; the next brightest, those of the second magnitude; and so on to the sixth magnitude. The last magnitude includes the faintest stars that are visible to the naked eye on the most favorable night. Stars which are fainter than those of the sixth magnitude can be seen only with the telescope, and are called telescopic stars. Telescopic stars are also divided into magnitudes; the division extending to the sixteenth magnitude, or the faintest stars that can be seen with the most powerful telescopes.

The classification of stars according to magnitudes has reference only to their brightness, and not at all to their actual size. A sixth magnitude star may actually be larger than a first magnitude star; its want of brilliancy being due to its greater distance, or to its inferior luminosity, or to both of these causes.

None of the stars present any sensible disk, even in the most powerful telescope: they all appear as mere points of light. The larger the telescope, the greater is its power of revealing faint stars; not because it makes these stars appear larger, but because of its greater light-gathering power. This power increases with the size of the object-glass of the telescope, which plays the part of a gigantic pupil of the eye.

The classification of the stars into magnitudes is not made in accordance with any very accurate estimate of their brightness. The stars which are classed together in the same magnitude are far from being equally bright.

The stars of each lower magnitude are about two-fifths as bright as those of the magnitude above. The ratio of diminution is about a third from the higher magnitude down to the fifth. Were the ratio two-fifths exact, it would take about

323. The Number of the Stars.—The total number of stars in the celestial sphere visible to the average naked eye is estimated, in round numbers, at five thousand; but the number varies much with the perfection and the training of the eye and with the atmospheric conditions. For every star visible to the naked eye, there are thousands too minute to be seen without telescopic aid. Fig. 364 shows a portion of the constellation of the Twins as seen with the naked eye; and Fig. 365 shows the same region as seen in a powerful telescope.

Struve has estimated that the total number of stars visible with Herschel's twenty-foot telescope was about twenty million. The number that can be seen with the great telescopes of modern times has not been carefully estimated, but is probably somewhere between thirty million and fifty million.

The number of stars between the north pole and the circle thirty-five degrees south of the equator is about as follows:—

The number of stars of the several magnitudes is approximately in inverse proportion to that of their brightness, the ratio being a little greater in the higher magnitudes, and probably a little less in the lower ones.

324. The Division of the Stars into Constellations.—A glance at the heavens is sufficient to show that the stars are not distributed uniformly over the sky. The larger ones especially are collected into more or less irregular groups. The larger groups are called constellations. At a very early period a mythological figure was allotted to each constellation; and these figures were drawn in such a way as to include the principal stars of each constellation. The heavens thus became covered, as it were, with immense hieroglyphics.

There is no historic record of the time when these figures were formed, or of the principle in accordance with which they were constructed. It is probable that the imagination of the earlier peoples may, in many instances, have discovered some fanciful resemblance in the configuration of the stars to the forms depicted. The names are still retained, although the figures no longer serve any astronomical purpose. The constellation Hercules, for instance, no longer represents the figure of a man among the stars, but a certain portion of the heavens within which the ancients placed that figure. In star-maps intended for school and popular use it is still customary to give these figures; but they are not generally found on maps designed for astronomers.

325. The Naming of the Stars.—The brighter stars have all proper names, as Sirius, Procyon, Arcturus, Capella, Aldebaran, etc. This method of designating the stars was adopted by the Arabs. Most of these names have dropped entirely out of astronomical use, though many are popularly retained. The brighter stars are now generally designated by the letters of the Greek alphabet,—alpha, beta, gamma, etc.,—to which is appended the genitive of the name of the constellation, the first letter of the alphabet being used for the brightest star, the second for the next brightest, and so on. Thus Aldebaran would be designated as Alpha Tauri. In speaking of the stars of any one constellation, we simply designate them by the letters of the Greek alphabet, without the addition of the name of the constellation, which answers to a person's surname, while the Greek letter answers to his Christian name. The names of the seven stars of the "Dipper" are given in Fig. 366. When the letters of the Greek alphabet are exhausted, those of the Roman alphabet are employed. The fainter stars in a constellation are usually designated by some system of numbers.

326. The Milky-Way, or Galaxy.—The Milky-Way is a faint luminous band, of irregular outline, which surrounds the heavens with a great circle, as shown in Fig. 367. Through a considerable portion of its course it is divided into two branches, and there are various vacant spaces at different points in this band; but at only one point in the southern hemisphere is it entirely interrupted.

The telescope shows that the Galaxy arises from the light of countless stars too minute to be separately visible with the naked eye. The telescopic stars, instead of being uniformly distributed over the celestial sphere, are mostly condensed in the region of the Galaxy. They are fewest in the regions most distant from this belt, and become thicker as we approach it. The greater the telescopic power, the more marked is the condensation. With the naked eye the condensation is hardly noticeable; but with the aid of a very small telescope, we see a decided thickening of the stars in and near the Galaxy, while the most powerful telescopes show that a large majority of the stars lie actually in the Galaxy. If all the stars visible with a twelve-inch telescope were blotted out, we should find that the greater part of those remaining were in the Galaxy.

The increase in the number of the stars of all magnitudes as we approach the plane of the Milky-Way is shown in Fig. 368. The curve acb shows by its height the distribution of the stars above the ninth magnitude, and the curve ACB those of all magnitudes.

327. Star-Clusters.—Besides this gradual and regular condensation towards the Galaxy, occasional aggregations of stars into clusters may be seen. Some of these are visible to the naked eye, sometimes as separate stars, like the "Seven Stars," or Pleiades, but more commonly as patches of diffused light, the stars being too small to be seen separately. The number visible in powerful telescopes is, however, much greater. Sometimes hundreds or even thousands of stars are visible in the field of view at once, and sometimes the number is so great that they cannot be counted.

328. NebulÆ.—Another class of objects which are found in the celestial spaces are irregular masses of soft, cloudy light, known as nebulÆ. Many objects which look like nebulÆ in small telescopes are shown by more powerful instruments to be really star-clusters. But many of these objects are not composed of stars at all, being immense masses of gaseous matter.

The general distribution of nebulÆ is the reverse of that of the stars. NebulÆ are thickest where stars are thinnest. While stars are most numerous in the region of the Milky-Way, nebulÆ are most abundant about the poles of the Milky-Way. This condensation of nebulÆ about the poles of the Milky-Way is shown in Figs. 367 and 369, in which the points represent, not stars, but nebulÆ.

II. THE STARS.

The Constellations.

329. The Great Bear.—The Great Bear, or Ursa Major, is one of the circumpolar constellations (4), and contains one of the most familiar asterisms, or groups of stars, in our sky; namely, the Great Dipper, or Charles's Wain. The positions and names of the seven prominent stars in it are shown in Fig. 370. The two stars Alpha and Beta are called the Pointers. This asterism is sometimes called the Butcher's Cleaver. The whole constellation is shown in Fig. 371. A rather faint star marks the nose of the bear, and three equidistant pairs of faint stars mark his feet.

330. The Little Bear, Draco, and Cassiopeia.—These are all circumpolar constellations. The most important star of the Little Bear, or Ursa Minor, is Polaris, or the Pole Star. This star may be found by drawing a line from Beta to Alpha of the Dipper, and prolonging it as shown in Fig. 372. This explains why these stars are called the Pointers. The Pole Star, with the six other chief stars of the Little Bear, form an asterism called the Little Dipper. These six stars are joined with Polaris by a dotted line in Fig. 372.

The stars in a serpentine line between the two Dippers are the chief stars of Draco, or the Dragon; the trapezium marking its head. Fig. 373 shows the constellations of Ursa Minor and Draco as usually figured.

To find Cassiopeia, draw a line from Delta of the Dipper to Polaris, and prolong it about an equal distance beyond, as shown in Fig. 372. This line will pass near Alpha of Cassiopeia. The five principal stars of this constellation form an irregular W, opening towards the pole. Between Cassiopeia and Draco are five rather faint stars, which form an irregular K. These are the principal stars of the constellation Cepheus. These two constellations are shown in Fig. 374.

331. The Lion, Berenice's Hair, and the Hunting-Dogs.—A line drawn from Alpha to Beta of the Dipper, and prolonged as shown in Fig. 375, will pass between the two stars Denebola and Regulus of Leo, or the Lion. Regulus forms a sickle with several other faint stars, and marks the heart of the lion. Denebola is at the apex of a right-angled triangle, which it forms with two other stars, and marks the end of the lion's tail. This constellation is visible in the evening from February to July, and is figured in Fig. 376.

In a straight line between Denebola and Eta, at the end of the Great Bear's tail, are, at about equal distances, the two small constellations of Coma Berenices, or Berenice's Hair, and Canes Venatici, or the Hunting-Dogs. These are shown in Fig. 377. The dogs are represented as pursuing the bear, urged on by the huntsman BoÖtes.

332. BoÖtes, Hercules, and the Northern Crown.—Arcturus, the principal star of BoÖtes, may be found by drawing a line from Zeta to Eta of the Dipper, and then prolonging it with a slight bend, as shown in Fig. 378. Arcturus and Polaris form a large isosceles triangle with a first-magnitude star called Vega. This triangle encloses at one corner the principal stars of BoÖtes, and the head of the Dragon near the opposite side. The side running from Arcturus to Vega passes through Corona Borealis, or the Northern Crown, and the body of Hercules, which is marked by a quadrilateral of four stars.

BoÖtes, who is often represented as a husbandman, Corona Borealis, and Hercules, are delineated in Fig. 379. These constellations are visible in the evening from May to September.

333. The Lyre, the Swan, the Eagle, and the Dolphin.—Altair, the principal star of Aquila, or the Eagle, lies on the opposite side of the Milky-Way from Vega. Altair is a first-magnitude star, and has a faint star on each side of it, as shown in Fig. 380. Vega, also of the first magnitude, is the principal star of Lyra, or the Lyre. Between these two stars, and a little farther to the north, are several stars arranged in the form of an immense cross. The bright star at the head of this cross is called Deneb. The cross lies in the Milky-Way, and contains the chief stars of the constellation Cygnus, or the Swan. A little to the north of Altair are four stars in the form of a diamond. This asterism is popularly known as Job's Coffin. These four stars are the chief stars of Delphinus, or the Dolphin. These four constellations are shown together in Fig. 381. The Swan is visible from June to December, in the evening.

334. Virgo.—A line drawn from Alpha to Gamma of the Dipper, and prolonged with a slight bend at Gamma, will reach to a first-magnitude star called Spica (Fig. 382). This is the chief star of the constellation Virgo, or the Virgin, and forms a large isosceles triangle with Arcturus and Denebola.

Virgo is represented in Fig. 383. To the right of this constellation, as shown in the figure, there are four stars which form a trapezium, and mark the constellation Corvus, or the Crow. This bird is represented as standing on the body of Hydra, or the Water-Snake. Virgo is visible in the evening, from April to August.

335. The Twins.—A line drawn from Delta to Beta of the Dipper, and prolonged as shown in Fig. 384, passes between two bright stars called Castor and Pollux. The latter of these is usually reckoned as a first-magnitude star. These are the principal stars of the constellation Gemini, or the Twins, which is shown in Fig. 385. The constellation Canis Minor, or the Little Dog, is shown in the lower part of the figure. There are two conspicuous stars in this constellation, the brightest of which is of the first magnitude, and called Procyon.

The region to which we have now been brought is the richest of the northern sky, containing no less than seven first-magnitude stars. These are Sirius, Procyon, Pollux, Capella, Aldebaran, Betelgeuse, and Rigel. They are shown in Fig. 386.

Betelgeuse and Rigel are in the constellation Orion, being about equally distant to the north and south from the three stars forming the belt of Orion. Betelgeuse is a red star. Sirius is the brightest star in the heavens, and belongs to the constellation Canis Major, or the Great Dog. It lies to the east of the belt of Orion. Aldebaran lies at about the same distance to the west of the belt. It is a red star, and belongs to the constellation Taurus, or the Bull. Capella is in the constellation Auriga, or the Wagoner. These stars are visible in the evening, from about December to April.

336. Orion and his Dogs, and Taurus.—Orion and his Dogs are shown in Fig. 387, and Orion and Taurus in Fig. 388. Aldebaran marks one of the eyes of the bull, and is often called the Bull's Eye. The irregular V in the face of the bull is called the Hyades, and the cluster on the shoulder the Pleiades.

337. The Wagoner.—The constellation Auriga, or the Wagoner (sometimes called the Charioteer), is shown in Fig. 389. Capella marks the Goat, which he is represented as carrying on his back, and the little right-angled triangle of stars near it the Kids. The five chief stars of this constellation form a large, irregular pentagon. Gamma of Auriga is also Beta of Taurus, and marks one of the horns of the Bull.

338. Pegasus, Andromeda, and Perseus.—A line drawn from Polaris near to Beta of Cassiopeia will lead to a bright second-magnitude star at one corner of a large square (Fig. 390). Alpha belongs both to the Square of Pegasus and to Andromeda. Beta and Gamma, which are connected with Alpha in the figure by a dotted line, also belong to Andromeda. Algol, which forms, with the last-named stars and with the Square of Pegasus, an asterism similar in configuration to the Great Dipper, belongs to Perseus. Algenib, which is reached by bending the line at Gamma in the opposite direction, is the principal star of Perseus.

Pegasus is shown in Fig. 391, and Andromeda in Fig. 392. Cetus, the Whale, or the Sea Monster, shown in Fig. 393, belongs to the same mythological group of constellations.

339. Scorpio, Sagittarius, and Ophiuchus.—During the summer months a brilliant constellation is visible, called Scorpio, or the Scorpion. The configuration of the chief stars of this constellation is shown in Fig. 394. They bear some resemblance to a boy's kite. The brightest star is of the first magnitude, and called Antares (from anti, instead of, and Ares, the Greek name of Mars), because it rivals Mars in redness. The stars in the tail of the Scorpion are visible in our latitude only under very favorable circumstances. This constellation is shown in Fig. 395, together with Sagittarius and Ophiuchus. Sagittarius, or the Archer, is to the east of Scorpio. It contains no bright stars, but is easily recognized from the fact that five of its principal stars form the outline of an inverted dipper, which, from the fact of its being partly in the Milky-Way, is often called the Milk Dipper.

Ophiuchus, or the Serpent-Bearer, is a large constellation, filling all the space between the head of Hercules and Scorpio. It is difficult to trace, since it contains no very brilliant stars. This constellation and Libra, or the Balances, which is the zodiacal constellation to the west of Scorpio, are shown in Fig. 396.

340. Capricornus, Aquarius, and the Southern Fish.—The two zodiacal constellations to the east of Sagittarius are Capricornus and Aquarius. Capricornus contains three pairs of small stars, which mark the head, the tail, and the knees of the animal.

Aquarius is marked by no conspicuous stars. An irregular line of minute stars marks the course of the stream of water which flows from the Water-Bearer's Urn into the mouth of the Southern Fish. This mouth is marked by the first-magnitude star Fomalhaut. These constellations are shown in Fig. 397.

341. Pisces and Aries.—The remaining zodiacal constellations are Pisces, or the Fishes, Aries, or the Ram (Fig. 398), and Cancer, or the Crab.

The Fishes lie under Pegasus and Andromeda, but contain no bright stars. Aries (between Pisces and Taurus) is marked by a pair of stars on the head,—one of the second, and one of the third magnitude. Cancer (between Leo and Gemini) has no bright stars, but contains a remarkable cluster of small stars called PrÆsepe, or the Beehive.

Clusters.

342. The Hyades.—The Hyades are a very open cluster in the face of Taurus (334). The three brightest stars of this cluster form a letter V, the point of the V being on the nose, and the open ends at the eyes. This cluster is shown in Fig. 399. The name, according to the most probable etymology, means rainy; and they are said to have been so called because their rising was associated with wet weather. They were usually considered the daughters of Atlas, and sisters of the Pleiades, though sometimes referred to as the nurses of Bacchus.

343. The Pleiades.—The Pleiades constitute a celebrated group of stars, or a miniature constellation, on the shoulder of Taurus. Hesiod mentions them as "the seven virgins of Atlas born," and Milton calls them "the seven Atlantic sisters." They are referred to in the Book of Job. The Spaniards term them "the little nanny-goats;" and they are sometimes called "the hen and chickens."

Usually only six stars in this cluster can be seen with the naked eye, and this fact has given rise to the legend of the "lost Pleiad." On a clear, moonless night, however, a good eye can discern seven or eight stars, and some observers have distinguished as many as eleven. Fig. 400 shows the Pleiades as they appear to the naked eye under the most favorable circumstances. Fig. 401 shows this cluster as it appears in a powerful telescope. With such an instrument more than five hundred stars are visible.

344. Cluster in the Sword-handle of Perseus.—This is a somewhat dense double cluster. It is visible to the naked eye, appearing as a hazy star. A line drawn from Algenib, or Alpha of Perseus (338), to Delta of Cassiopeia (330), will pass through this cluster at about two-thirds the distance from the former. This double cluster is one of the most brilliant objects in the heavens, with a telescope of moderate power.

345. Cluster of Hercules.—The celebrated globular cluster of Hercules can be seen only with a telescope of considerable power, and to resolve it into distinct stars (as shown in Fig. 402) requires an instrument of the very highest class.

346. Other Clusters.—Fig. 403 shows a magnificent globular cluster in the constellation Aquarius. Herschel describes it as appearing like a heap of sand, being composed of thousands of stars of the fifteenth magnitude.

Fig. 404 shows a cluster in the constellation Toucan, which Sir John Herschel describes as a most glorious globular cluster, the stars of the fourteenth magnitude being immensely numerous. There is a marked condensation of light at the centre.

Fig. 405 shows a cluster in the Centaur, which, according to the same astronomer, is beyond comparison the richest and largest object of the kind in the heavens, the stars in it being literally innumerable. Fig. 406 shows a cluster in Scorpio, remarkable for the peculiar arrangement of its component stars.

Star clusters are especially abundant in the region of the Milky-Way, the law of their distribution being the reverse of that of the nebulÆ.

Double and Multiple Stars.

347. Double Stars.—The telescope shows that many stars which appear single to the naked eye are really double, or composed of a pair of stars lying side by side. There are several pairs of stars in the heavens which lie so near together that they almost seem to touch when seen with the naked eye.

Pairs of stars are not considered double unless the components are so near together that they both appear in the field of view when examined with a telescope. In the majority of the pairs classed as double stars the distance between the components ranges from half a second to fifteen seconds.

Epsilon LyrÆ is a good example of a pair of stars that can barely be separated with a good eye. Figs. 407 and 408 show this pair as it appears in telescopes magnifying respectively four and fifteen times; and Fig. 409 shows it as seen in a more powerful telescope, in which each of the two components of the pair is seen to be a truly double star.

348. Multiple Stars.—When a star is resolved into more than two components by a telescope, it is called a multiple star. Fig. 410 shows a triple star in Pegasus. Fig. 411 shows a quadruple star in Taurus. Fig. 412 shows a sextuple star, and Fig. 413 a septuple star. Fig. 414 shows the celebrated septuple star in Orion, called Theta Orionis, or the trapezium of Orion.

349. Optically Double and Multiple Stars.—Two or more stars which are really very distant from each other, and which have no physical connection whatever, may appear to be near together, because they happen to lie in the same direction, one behind the other. Such accidental combinations are called optically double or multiple stars.

350. Physically Double and Multiple Stars.—In the majority of cases the components of double and multiple stars are in reality comparatively near together, and are bound together by gravity into a physical system. Such combinations are called physically double and multiple stars. The components of these systems all revolve around their common centre of gravity. In many instances their orbits and periods of revolution have been ascertained by observation and calculation. Fig. 415 shows the orbit of one of the components of a double star in the constellation Hercules.

351. Colors of Double and Multiple Stars.—The components of double and multiple stars are often highly colored, and frequently the components of the same system are of different colors. Sometimes one star of a binary system is white, and the other red; and sometimes a white star is combined with a blue one. Other colors found in combination in these systems are red and blue, orange and green, blue and green, yellow and blue, yellow and red, etc.

If these double and multiple stars are accompanied by planets, these planets will sometimes have two or more suns in the sky at once. On alternate days they may have suns of different colors, and perhaps on the same day two suns of different colors. The effect of these changing colored lights on the landscape must be very remarkable.

New and Variable Stars.

352. Variable Stars.—There are many stars which undergo changes of brilliancy, sometimes slight, but occasionally very marked. These changes are in some cases apparently irregular, and in others periodic. All such stars are said to be variable, though the term is applied especially to those stars whose variability is periodic.

353. Algol.—Algol, a star of Perseus, whose position is shown in Fig. 416, is a remarkable variable star of a short period. Usually it shines as a faint second-magnitude star; but at intervals of a little less than three days it fades to the fourth magnitude for a few hours, and then regains its former brightness. These changes were first noticed some two centuries ago, but it was not till 1782 that they were accurately observed. The period is now known to be two days, twenty hours, forty-nine minutes. It takes about four hours and a half to fade away, and four hours more to recover its brilliancy. Near the beginning and end of the variations, the change is very slow, so that there are not more than five or six hours during which an ordinary observer would see that the star was less bright than usual.

This variation of light was at first explained by supposing that a large dark planet was revolving round Algol, and passed over its face at every revolution, thus cutting off a portion of its light; but there are small irregularities in the variation, which this theory does not account for.

354. Mira.—Another remarkable variable star is Omicron Ceti, or Mira (that is, the wonderful star). It is generally invisible to the naked eye; but at intervals of about eleven months it shines forth as a star of the second or third magnitude. It is about forty days from the time it becomes visible until it attains its greatest brightness, and is then about two months in fading to invisibility; so that its increase of brilliancy is more rapid than its waning. Its period is quite irregular, ranging from ten to twelve months; so that the times of its appearance cannot be predicted with certainty. Its maximum brightness is also variable, being sometimes of the second magnitude, and at others only of the third or fourth.

355. Eta Argus.—Perhaps the most extraordinary variable star in the heavens is Eta Argus, in the constellation Argo, or the Ship, in the southern hemisphere (Fig. 417). The first careful observations of its variability were made by Sir John Herschel while at the Cape of Good Hope. He says, "It was on the 16th of December, 1837, that, resuming the photometrical comparisons, my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude in a part of the heavens where, being perfectly familiar with it, I was certain that no such brilliant object had before been seen. After a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected, and referring to a map for its configuration with other conspicuous stars in the neighborhood, I became satisfied of its identity with my old acquaintance, Eta Argus. Its light was, however, nearly tripled. While yet low, it equalled Rigel, and, when it attained some altitude, was decidedly greater." It continued to increase until Jan. 2, 1838, then faded a little till April following, though it was still as bright as Aldebaran. In 1842 and 1843 it blazed up brighter than ever, and in March of the latter year was second only to Sirius. During the twenty-five years following it slowly but steadily diminished. In 1867 it was barely visible to the naked eye; and the next year it vanished entirely from the unassisted view, and has not yet begun to recover its brightness. The curve in Fig. 418 shows the change in brightness of this remarkable star. The numbers at the bottom show the years of the century, and those at the side the brightness of the star.

356. New Stars.—In several cases stars have suddenly appeared, and even become very brilliant; then, after a longer or shorter time, they have faded away and disappeared. Such stars are called new or temporary stars. For a time it was supposed that such stars were actually new. They are now, however, classified by astronomers among the variable stars, their changes being of a very irregular and fitful character. There is scarcely a doubt that they were all in the heavens as very small stars before they blazed forth in so extraordinary a manner, and that they are in the same places still. There is a wide difference between these irregular variations, or the breaking-forth of light on a single occasion in the course of centuries, and the regular and periodic changes in the case of a star like Algol; but a long series of careful observation has resulted in the discovery of stars of nearly every degree of irregularity between these two extremes. Some of them change gradually from one magnitude to another, in the course of years, without seeming to follow any law whatever; while in others some slight tendency to regularity can be traced. Eta Argus may be regarded as a connecting link between new and variable stars.

357. Tycho Brahe's Star.—An apparently new star suddenly appeared in Cassiopeia in 1572. It was first seen by Tycho Brahe, and is therefore associated with his name. Its position in the constellation is shown in Fig. 419. It was first seen on Nov. 11, when it had already attained the first magnitude. It became rapidly brighter, soon rivalling Venus in splendor, so that good eyes could discern it in full daylight. In December it began to wane, and gradually faded until the following May, when it disappeared entirely.

A star showed itself in the same part of the heavens in 945 and in 1264. If these were three appearances of the same star, it must be reckoned as a periodic star with a period of a little more than three hundred years.

358. Kepler's Star.—In 1604 a new star was seen in the constellation Ophiuchus. It was first noticed in October of that year, when it was of the first magnitude. In the following winter it began to fade, but remained visible during the whole year 1605. Early in 1606 it disappeared entirely. A very full history of this star was written by Kepler.

One of the most remarkable things about this star was its brilliant scintillation. According to Kepler, it displayed all the colors of the rainbow, or of a diamond cut with multiple facets, and exposed to the rays of the sun. It is thought that this star also appeared in 393, 798, and 1203; if so, it is a variable star with a period of a little over four hundred years.

359. New Star of 1866.—The most striking case of this kind in recent times was in May, 1866, when a star of the second magnitude suddenly appeared in Corona Borealis. On the 11th and 12th of that month it was observed independently by at least five observers in Europe and America. The fact that none of these new stars were noticed until they had nearly or quite attained their greatest brilliancy renders it probable that they all blazed up very suddenly.

360. Cause of the Variability of Stars.—The changes in the brightness of variable and temporary stars are probably due to operations similar to those which produce the spots and prominences in our sun. We have seen (188) that the frequency of solar spots shows a period of eleven years, during one portion of which there are few or no spots to be seen, while during another portion they are numerous. If an observer so far away as to see our sun like a star could from time to time measure its light exactly, he would find it to be a variable star with a period of eleven years, the light being least when we see most spots, and greatest when few are visible. The variation would be slight, but it would nevertheless exist. Now, we have reason to believe that the physical constitution of the sun and the stars is of the same general nature. It is therefore probable, that, if we could get a nearer view of the stars, we should see spots on their disks as we do on the sun. It is also likely that the varying physical constitution of the stars might give rise to great differences in the number and size of the spots; so that the light of some of these suns might vary to a far greater degree than that of our own sun does. If the variations had a regular period, as in the case of our sun, the appearances to a distant observer would be precisely what we see in the case of a periodic variable star.

The spectrum of the new star of 1866 was found to be a continuous one, crossed by bright lines, which were apparently due to glowing hydrogen. The continuous spectrum was also crossed by dark lines, indicating that the light had passed through an atmosphere of comparatively cool gas. Mr. Huggins infers from this that there was a sudden and extraordinary outburst of hydrogen gas from the star, which by its own light, as well as by heating up the whole surface of the star, caused the extraordinary increase of brilliancy. Now, the spectroscope shows that the red flames of the solar chromosphere (197) are largely composed of hydrogen; and it is not unlikely that the blazing-forth of this star arose from an action similar to that which produces these flames, only on an immensely larger scale.

Distance of the Stars.

361. Parallax of the Stars.—Such is the distance of the stars, that only in a comparatively few instances has any displacement of these bodies been detected when viewed from opposite parts of the earth's orbit, that is, from points a hundred and eighty-five million miles apart; and in no case can this displacement be detected except by the most careful and delicate measurement. Half of the above displacement, or the displacement of the star as seen from the earth instead of the sun, is called the parallax of the star. In no case has a parallax of one second as yet been detected.

362. The Distance of the Stars.—The distance of a star whose parallax is one second would be 206,265 times the distance of the earth from the sun, or about nineteen million million miles. It is quite certain that no star is nearer than this to the earth. Light has a velocity which would carry it seven times and a half around the earth in a second; but it would take it more than three years to reach us from that distance. Were all the stars blotted out of existence to-night, it would be at least three years before we should miss a single one.

Alpha Centauri, the brightest star in the constellation of the Centaur, is, so far as we know, the nearest of the fixed stars. It is estimated that it would take its light about three years and a half to reach us. It has also been estimated that it would take light over sixteen years to reach us from Sirius, about eighteen years to reach us from Vega, about twenty-five years from Arcturus, and over forty years from the Pole-Star. In many instances it is believed that it would take the light of stars hundreds of years to make the journey to our earth, and in some instances even thousands of years.

Proper Motion of the Stars.

363. Why the Stars appear Fixed.—The stars seem to retain their relative positions in the heavens from year to year, and from age to age; and hence they have come universally to be denominated as fixed. It is, however, now well known that the stars, instead of being really stationary, are moving at the rate of many miles a second; but their distance is so enormous, that, in the majority of cases, it would be thousands of years before this rate of motion would produce a sufficient displacement to be noticeable to the unaided eye.

Fig. 420.