Chapter 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.

364. _Secular Displacement of the Stars._--Though the proper motion of the stars is apparently slight, it will, in the course of many ages, produce a marked change in the configuration of the stars. Thus, in Fig. 420, the left-hand portion shows the present configuration of the stars of the Great Dipper. The small arrows attached to the stars show the direction and comparative magnitudes of their motion. The right-hand portion of the figure shows these stars as they will appear thirty-six thousand years from the present time.

Fig. 421 shows in a similar way the present configuration and proper motion of the stars of _Cassiopeia_, and also these stars as they will appear thirty-six thousand years hence.

Fig. 422 shows the same for the constellation _Orion_.

365. _The Secular Motion of the Sun._--The stars in all parts of the heavens are found to move in all directions and with all sorts of velocities. When, however, the motions of the stars are averaged, there is found to be an apparent proper motion common to all the stars. The stars in the neighborhood of _Hercules_ appear to be approaching us, and those in the opposite part of the heavens appear to be receding from us. In other words, all the stars appear to be moving away from Hercules, and towards the opposite part of the heavens.

This apparent motion common to all the stars is held by astronomers to be due to the real motion of the sun through space. The point in the heavens towards which our sun is moving at the present time is indicated by the small circle in the constellation Hercules in Fig. 423. As the sun moves, he carries the earth and all the planets along with him. Fig. 424 shows the direction of the sun's motion with reference to the ecliptic and to the axis of the earth. Fig. 425 shows the earth's path in space; and Fig. 426 shows the paths of the earth, the moon, Mercury, Venus, and Mars in space.

Whether the sun is actually moving in a straight line, or around some distant centre, it is impossible to determine at the present time. It is estimated that the sun is moving along his path at the rate of about a hundred and fifty million miles a year. This is about five-sixths of the diameter of the earth's orbit.

366. _Star-Drift._--In several instances, groups of stars have a common proper motion entirely different from that of the stars around and among them. Such groups probably form connected systems, in the motion of which all the stars are carried along together without any great change in their relative positions. The most remarkable case of this kind occurs in the constellation _Taurus_. A large majority of the brighter stars in the region between _Aldebaran_ and the _Pleiades_ have a common proper motion of about ten seconds per century towards the east. Proctor has shown that five out of the seven stars which form the Great Dipper have a common proper motion, as shown in Fig. 427 (see also Fig. 420). He proposes for this phenomenon the name of _Star-Drift_.

367. _Motion of Stars along the Line of Sight._--A motion of a star in the direction of the line of sight would produce no displacement of the star that could be detected with the telescope; but it would cause a change in the brightness of the star, which would become gradually fainter if moving from us, and brighter if approaching us. Motion along the line of sight has, however, been detected by the use of the tele-spectroscope (152), owing to the fact that it causes a displacement of the spectral lines. As has already been explained (169), a displacement of a spectral line towards the red end of the spectrum indicates a motion away from us, and a displacement towards the violet end, a motion towards us.

* * * * *

By means of these displacements of the spectral lines, Huggins has detected motion in the case of a large number of stars, and calculated its rate:--

STARS RECEDING FROM US.

Sirius 20 miles per second.
Betelgeuse 22 miles per second.
Rigel 15 miles per second.
Castor 25 miles per second.
Regulus 15 miles per second.

STARS APPROACHING US.

Arcturus 55 miles per second.
Vega 50 miles per second.
Deneb 39 miles per second.
Pollux 49 miles per second.
Alpha Ursæ Majoris 46 miles per second.

These results are confirmed by the fact that the amount of motion indicated is about what we should expect the stars to have, from their observed proper motions, combined with their probable distances. Again: the stars in the neighborhood of Hercules are mostly found to be approaching the earth, and those which lie in the opposite direction to be receding from it; which is exactly the effect which would result from the sun's motion through space. The five stars in the Dipper, which have a common proper motion, are also found to have a common motion in the line of sight. But the displacement of the spectral lines is so slight, and its measurement so difficult, that the velocities in the above table are to be accepted as only an approximation to the true values.

Chemical and Physical Constitution of the Stars.

368. _The Constitution of the Stars Similar to that of the Sun._--The stellar spectra bear a general resemblance to that of the sun, with characteristic differences. These spectra all show Fraunhofer's lines, which indicate that their luminous surfaces are surrounded by atmospheres containing absorbent vapors, as in the case of the sun. The positions of these lines indicate that the stellar atmospheres contain elements which are also found in the sun's, and on the earth.

369. _Four Types of Stellar Spectra._--The spectra of the stars have been carefully observed by Secchi and Huggins. They have found that stellar spectra may be reduced to four types, which are shown in Fig. 428. In the spectrum of _Sirius_, a representative of _Type I._, very few lines are represented; but the lines are very thick.

Next we have the solar spectrum, which is a representative of _Type II._, one in which more lines are represented. In _Type III._ fluted spaces begin to appear, and in _Type IV._, which is that of the red stars, nothing but fluted spaces is visible; and this spectrum shows that something is at work in the atmosphere of those red stars different from what there is in the simpler atmosphere of _Type I._

Lockyer holds that these differences of spectra are due simply to differences of temperature. According to him, the red stars, which give the fluted spectra, are of the lowest temperature; and the temperature of the stars of the different types gradually rises till we reach the first type, in which the temperature is so high that the dissociation (161) of the elements is nearly if not quite complete.