This is especially the case in the class known as Orion stars, because many of them are found in the constella- tion Orion. These stars are mostly very white or even blue in colour, and show a number of fine dark lines which are to a greater or less extent the same in all Orion stars, but are not those produced by any known chemical element. We therefore have reason to believe that there are in the stars other chemical elements than those with which we are acquainted. There is a very curious case in which an element first 296 THE FIXED STARS excited interest through its being found in the sun and stars. For some time after the study of the sun's spec- trum had been commenced, it was known that certain well- marked lines in it were not produced by any substance then known. But continued research led to the discovery that this substance existed in a Norwegian mineral, cleveite, and perhaps elsewhere on the earth. From its existence on the sun it was called helium. Its spectrum was no sooner made known than it was found that helium existed in many stars which are, for that reason, called "helium stars." Density and Heat of the Stars In many cases some idea can be obtained of the density of a star, or, in ordinary language, of its specific gravity. It is very remarkable that, in near- ly all such cases the density is found to be far less than that of our ordinary solid or liquid substances ; frequently no greater than that of air, sometimes even less. In this respect our sun, although its den- sity is so small, seems to be an exception, and it is likely that only a very small proportion of the stars are as dense as the sun. This affords one proof of the high temperature of these bodies, which must be such that all liquid or solid substances exposed to it would boil away as water boils when put on a fire, thus changing its substance into a vapour. We have reason to be- lieve that the stars are for the most part masses of this intensely hot vapour, surrounded perhaps by a somewhat colder surface. Possibly many of the stars DENSITY AND HEAT OF THE STARS 297 may be of the nature of bubbles, but this is far from being estabhshed. A star, Hke the sun, must be hotter in the interior than at its surface. From the latter alone can heat be radiated ; hence the surface is continually cooling off, and if the matter composing the body were at rest, the cooling would soon go so far that a crust would form, as it does on a mass of molten iron. The only way in which this can be prevented is that, as the superficial portions cool, the greater density which they thus acquire causes them to sink down into the seething mass below, portions of w^hich arise to take their place, cool off, and sink in their turn. Thus there is a continual interchange of matter between the inside and the surface, much as in a boiling pot the water at the bottom is continually being forced up to the top, while that on the top continually sinks down. It follows from this that there must be a limit to the smallness of a star. If such a body were no larger than the moon, it would, in a few thousand years, so far cool off that a crust would form over its surface. This would cut off the currents by which the hot matter is brought to the surface and the star would soon cease to shine. As there can be little doubt that the age of most of the stars is to be reckoned by millions of years, it follows that they must be so large that they can lose heat for millions of years and yet a cool crust not form on their surface. We have said that our sun is among the colder of the stars and also that it is among the smaller. These two facts fit well together. The smaller a star is the more 298 THE FIXED STARS rapidly it cools off, just as a cup of water cools off faster than a pot full. The revelations of the spectroscope makes it very prob- able that every star has a life history. It began as a nebula, which, in the course of ages, slowly condensed into an intensely hot, blue-coloured star. The condensa- tion going on, the star becomes yet hotter, until it reaches its highest temperature. Then, cooling off, its colour changes to white, yellow, and red, and the lines in its spectrum become darker and more numerous. Finally, its light dies away, as a fire flickers out when the supply of fuel is exhausted, and the star becomes a dark opaque body, — its life has ended. The greater the mass of the star the longer its life. Thus it is that the stars we observe seem to be of all ages, from the infantile nebula to the star dying of old age. II Aspect of the Sky Not only to the ordinary beholder, but to the learned student of the heavens, the most wonderful feature of the sky is the Milky Way. This is a girdle apparently spanning the sky and perhaps, in reality, spanning the entire universe of stars, uniting them, as it were, into a single system — one "stupendous whole." It may be seen at some time of the night every day of the year, and at some convenient hour in the evening of every month ex- cept May. During this month it extends round the horizon in the early evening, and is invisible through the denser strata of the air. Of course it will even then become visible in the east and northeast later at night. The smallest telescope will show the Milky Way to be formed of immense congeries of stars, too faint in their light to be separately visible at their great distance from us. Careful observation, even with the naked eye, will show that these stars are not equally scattered along the whole extent of their course, but are frequently collected in great masses or clusters, with comparatively empty spaces around or between them. These are especially marked in the portions of the belt visible in the south in the evenings of summer and autumn. A remarkable fact connected with the universe is that 300 THE FIXED STARS the stars are not equally thick in all directions, there be- ing more in a given space around the belt of the Milky Way, and the number growing smaller as we pass away from that belt. This is true even of the brightest stars, and yet more true of the fainter ones. The poles of the Milky Way are those two points in the heavens which are ninety degrees from every point of the Milky Way. If we imagine one to hold a rod in his hand, so that the Milky Way shall be at right angles to it, the two ends of the rod w^ill point to the two poles in question. To give an idea of the thickness of the stars w^e may say that, near the poles of the Milky Way, a round circle of the sky one degree in diameter will commonly contain two or three stars visible in quite a small sized telescope. In the region of the Milky Way, such a circle may contain eight, ten, perhaps even fifteen or twenty such stars. Brightness of the Stars No one can look at the sky without seeing that the stars differ enormously in their brightness, or, in the language of astronomy, in their magnitude. They re- semble men in that a very few far outshine all their fel- lows, a greater number are less bright, and, as we come down to smaller and smaller stars, we find the number to continually increase. Those visible to the naked eye were classified by the ancient astronomers as of six orders of magnitude. About twenty of the brightest in the sky were designated as of the first magnitude. The forty next in order of brightness were called of the second mag- nitude ; a larger number were of the third, and so on to BRIGHTNESS OF THE STARS 301 the sixth magnitude, which included the faintest stars that the best eye could see under a clear sky. Modern astronomers carry this system down to the telescopic stars. Those which are one degree fainter than the smallest visible to the naked eye are called of the seventh magnitude; the next in brightness are of the eighth, and so on. The faintest that can be seen or photographed with the largest telescopes are probably of the fifteenth, sixteenth, or seventeenth magnitude. The reader will of course understand that the magni- tude of a star does not express its real brightness, because a shining body looks brighter the nearer it is to us. No matter how bright a star may be, if it were removed far enough away it would grow so faint as to be invisible. The smallest star in the heavens if brought near enough to us would shine as of the first magnitude. It was formerly believed that the actual brightness of the different stars was nearly the same, and that some looked brighter than others only because they were nearer to us. But the case is now known to be different. Estimates of the distance of the stars show that among the nearest to us are many quite invisible to the naked eye, while some of the first magnitude are so far away that their distance is immeasurable. The brightest ones probably emit hundreds of thousands of times as much light as the smallest ones. Number of Stars The whole number of stars in the heavens which can be seen by the ordinary eye is between five and six thou- 302 THE FIXED STARS sand. Possibly a very keen eye might see more than six thousand, but most eyes will see even less than five thou- sand. Of these only one half can be above the horizon at the same time, and of this half a great number will be so near the horizon as to be obscured by the great thick- ness of the atmosphere in that direction. The number which can readily be seen on a clear evening by an ordinarily good eye will probably range between fifteen hundred and two thousand. Stars visible to the naked eye are called lucid stars, to distinguish them from tele- scopic stars, which can be seen only by the aid of a telescope. It is impossible to make even an estimate of the total number of telescopic stars. It is commonly supposed that between fifty and one hundred million can be seen with large telescopes, and it is now possible, with spe- cially arranged telescopes, to photograph stars which are fainter than the smallest the eye can see in any tele- scope. There is no sign of any limit to the number. As we pass to fainter and fainter degrees of brightness the stars are found to be more and more numerous. All that we can say of the total number is that it must be counted by hundreds of millions. We have, in fact, some reason for inferring that the great majority of the stars are invisible in the most powerful telescope we can make, owing to their distance. The distance of the great majority is such that only the brightest of them can become known to us. Minute stars are here and there collected into clusters in various parts of the sky. Some of these clusters are NUMBER OF STARS 303 visible to the naked eye. Those in and near the Milky Way frequently contain hundreds or even thousands of stars too small to be seen separately without a telescope. The stars differ from each other in colour, although not in so marked a degree as terrestrial objects. The most casual observer cannot fail to note the difference between the bluish white of Alpha Lyrae and the reddish light of Arcturus. There seems to be a regular grada- tion in the colour of the stars from blue, through yellow, to red. These differences of colour are connected with differences in the spectra of the stars. As a general rule, the redder a star is, the greater the number and intensity of the dark lines that can be seen in the green and blue parts of its spectrum. Constellations A slight examination of the heavens shows that the stars are not scattered equally over the sky, but that there is more or less of a tendency to collect into constellations. This is especially the case with the brighter stars. But no well-marked dividing line between the constellations is possible; that is, we cannot draw a line showing exactly where one constellation ends and another begins. Never- theless a division into constellations was made in ancient times and has been followed by astronomers down to the present time. How and by whom the constellations were first mapped out and named no one knows. The Chinese had their asterisms — collections of stars smaller than what we call constellations — in the earliest years of their history. 304 THE FIXED STARS What we know of the constellations dates from Ptolemy,, who lived in the second century after Christ. His names are still in use. As many of them are those of the gods, goddesses, and heroes of Grecian mythology — Perseus, Andromeda, Cepheus, Hercules, etc. — it seems likely that they were assigned during or after the heroic age. In modern times quite a number of new constellations have been carved out of or drawn between the older ones. This is especially the case in the southern hemisphere, which was imperfectly known to the ancient Greeks. Ill Description of the Constellations The present chapter is intended for those who wish to be able to recognise the principal constellations, and to know where to look for the several planets. The problem of pointing out the constellations is complicated by the effect of the twofold motion of the earth ; on its axis and around the sun. In consequence of the former the con- stellations change their apparent position in the course of the night, and the result of the latter is that different constellations are seen at different seasons. We explained in a former chapter how, in consequence of the motion of the earth in its orbit round the sun, the latter seems to us to perform an annual circuit among the constellations. Hence, if a star is east of the sun, we shall see it approach nearer to the sun every day. If we look out night after night at the same hour we shall find it farther and farther advanced toward the west. In consequence of this change it must rise and set earlier every day than it did the day before. More exactly, the time between two risings and settings of the same star is twenty-three hours fifty-six minutes four and a half sec- onds. While in the course of a year the sun rises three hundred and sixty-five times, a star rises three hundred and sixty-six times. The latter will therefore during the year have risen at every hour of the day and night. 306 THE FIXED STARS Astronomers avoid all confusion from this cause by the use of sidereal time, that is star-time, or time meas- ured by the stars. As already explained, a sidereal day is the interval between two successive pasages of a star over the meridian, and is three minutes fifty-six seconds less than our ordinary day. It is divided into twenty- four sidereal hours, and each hour into sidereal minutes and seconds. A sidereal clock gains three minutes fifty- six seconds daily on an ordinary clock and thus shows the same time at the same position of the stars the year around. One who wishes to keep the run of the stars will find it very convenient to have some idea of sidereal time. This may be had by the following rule : Double the number of the month; the product will be the sidereal time at six o'clock in the evening. At seven o'clock it will be one hour later, and at eight it will be two hours later, and so on. Suppose, for example, that one looks at the sky in November at nine o'clock in the evening. This is the eleventh month; multiplying by two gives twenty-two, adding three gives twenty-five, from which we drop twenty-four, giving one hour as the sidereal time. The time thus obtained will not often be more than an hour in error, except during the first week or ten days of the month, when it may be an hour or more too great. It may then be diminished by one hour. Applying the same rule in January we have five hours as the sidereal time at nine in the evening. But early in the month the sidereal time at nine in the evening will be four hours instead of five. THE NORTHERN CONSTELLATIONS 307 At 0 hours sidereal time the equinoctial colure is on the meridian ; at six hours, the solstitial colure, and so on. The Northern Constellations With this preliminary explanation let us proceed to the study of the constellations. I assume the reader to be somewhere in the latitude of the United States. Then the principal northern constellations will never set, and will be visible in whole or in part every evening in the year. With them, therefore, we begin. A figure, showing these constellations, is found in the first part of the present book (Fig. 2). To see how they will appear hold the cut with the month at top ; we then have the position at eight o'clock in the evening. For a later hour turn it a little in the direction of the arrows. For example, in July, at ten o'clock, we hold it so as to have August at the top. The Roman numerals on top give the sidereal time without the trouble of calculating it. First find TJrsa Ma- jor, the Great Bear, generally called the Dip- per, an implement which the constellation resem- bles much more than it does a bear. This you can always do except perhaps in autumn when, if you are far south, it may be more or less below the northern horizon. Notice the pair of stars forming the outside ♦ * Fig. 50. — Ursa Major, or The Dipper. 308 THE FIXED STARS ^ STAR|^ Fig. 51. — Ursa Minor. of the bowl of the dipper. They are called the Pointers, because they point toward the pole star, as shown by the dotted line. This is the cen- tral star of the map. It is called Polaris. The pole star belongs to the constellation Ursa Minor, the Lesser Bear ; the rest of the constellation you will see by following a curved line of stars from the pole toward X\T hours. You will thus fall on another star as bright as Polaris but a little redder in colour. This is Beta Ursae Minoris. If you cannot see the pointers you will still easily find Polaris if you know the exact north, because it is nearly midway between the zenith and the northern horizon — nearer the latter, however, the farther south we are. It can be easily distinguished from its neighbour. Beta, by its whiter colour, Beta being slightly red or dingy in comparison. On the opposite side of the pole, at the same dis- tance as Ursa Major, is Cassiopeia, the Lady in the Chair. The chair has a very crooked back but could be made comfortable by a cushion in the hollow. There are several other constellations in the region Fig. 52. — Cassiopeia. THE AUTUMNAL CONSTELLATIONS 309 around the pole, but they have few bright stars and are of less interest than those we have mentioned. Among them is Draco, the Dragon, whose form coils itself up be- tween the Bears, and whose head is represented by a tri- angle of stars in XVIII hours, near the August zenith. The Autumnal Constellations The zenithal and southern constellations to be looked for will vary with the season. We begin with the posi- tion of the sphere at 0 hours sidereal time, which occurs at ten o'clock in October, eight in November, and six in December. The equinoctial colure is first to be imagined. It passes from the pole upward near the westernmost bright star of Cassiopeia and can be traced south through the eastern side of the square of Pegasus. The latter easily recognised landmark of the sky is formed by four stars of the second or third magnitude. The square is fifteen degrees on a side. Northeast from the northeast corner of the square is the Great Nebula of Andromeda. It is plainly visible to the naked eye as a whitish, ill-defined patch of light, and is a fine object when seen in a telescope. The Milky Way now spans the heavens like a slightly inclined arch, resting on the east and west regions of the horizon, and having its keystone a little north of the zenith, in Cassiopeia. Tracing it from this constellation toward the east, we first have Perseus, which stands in the Milky Way itself. The brightest star in this con- stellation is Alpha Persei, of the second magnitude. 310 THE FIXED STARS East of Alpha is a white mass Hke a httle cloud. With a small telescope, even with a good field glass, we see this mass to be a collection or cluster of small stars. It is the Great Cluster of Perseus and, in the figure of the con- stellation, forms the hilt of the hero's sword. In a sort of offshoot toward the south (or southeast as the constellation is now situated) lies a row of three stars. The middle and brightest of these is the wonder- ful variable star, Algol, whose changes will be described in a later chapter. It is also called Beta Persei. , Below Perseus, the first large constellation is Auriga, the Charioteer. It is marked by Capella, the Goat, a star of the first magnitude and one of the brightest now above the horizon — indeed, one of the four or five brightest in the sky. But it has no other striking stars. In the southeast are Aldebaran and the Pleiades, which will be described later. Meanwhile let us follow the course of the Milky Way from the zenith toward the west. The first collection of bright stars west of Cassiopeia is now Cygnus, the Swan, lying centrally in the Milky Way. Five stars are arranged somewhat in the form of a cross and mark the body, neck, and extended wings of the bird. The brightest of the group is Alpha Cygni, or Deneb, nearly, but not quite, of the first magnitude. Low and to the right of Cygnus, and a little outside of the Milky Way, is the constellation Lyra, the Harp, marked by the beautiful and very bright bluish star, Vega. It has no other star of greater magnitude than the third, but what it has will repay careful study. THE AUTUMNAL CONSTELLATIONS 311 In the figure given here, notice the star to the left of Vega ; Epsilon Lyrae it is called. A keen eye will, on care- ful examination, see that this star is really composed of two, lying so close together that it is not easy to dis- tinguish them. With an opera glass this will more easily be accomplished. But the most curious fact is that if a telescope be pointed at the pair, each of the stars will be found to be double, so that Epsilon Lyras is really composed of four stars. Another star, about as near to Vega as Epsilon is, lies at one corner of a par- allelogram or elongated diamond, which stretches south of Vega., At the farther blunt corner of the dia- mond lies Beta Lyras, marked B in the figure, a remark- able variable star. To the left of it is Gamma. The law of variation will be described in a later chapter. To the right of Lyra, and In the Milky Way, lies Aquila, the Eagle. It will be described later. The other constellations low in the west will be de- scribed later. At present we shall pass rapidly over the constellations of the Zodiac. If the ecliptic were painted on the sky we should now see it rising to the north of the east point of the horizon, passing in the south to mid-sky, where it would cross the Fig. 53. — Lyra, the Harp. 312 THE FIXED STARS equator at a small angle, and then, passing to the west, reach the western horizon twenty-three degrees south of west. At the time we suppose, Sagittarius, the Archer, is mostly below the western horizon. Capricornus, the Goat; Aquarius, the Water Bearer, and Pisces, the Fishes, fill up the space to the meridian. The stars of these constellations are mostly faint, few or none exceed- ing the third magnitude. Reaching the meridian, we see the square of Pegasus above the Zodiac, not far south of the zenith. East of it is the constellation Aries, the Ram. Three of its prin- cipal stars, of the second, third, and fourth magnitudes, form an obtuse triangle. The brightest is Alpha Arietis. Two thousand years ago this constellation marked the first sign of the zodiac, and the equinox was just below Alpha Arietis, as explained in speaking of the precession of the equinoxes. Southeast from the square of Pegasus is a widely extended constellation, Cetus, the Whale. Its two bright- est stars. Alpha and Beta, are of the second magnitude. The latter lies nearly below the southeast star of the square of Pegasus and is quite by itself. Alpha is some distance farther east. West of Alpha, and a little south, is a remarkable star, Mira Ceti, the wonderful star of Cetus, which is invisible to the naked eye except for a month or two in each year, when it attains the fourth, third, and often the second magnitude. A little west of south, quite low down, is Fomalhaut, nearly of the first magnitude, in the constellation Pisces Australis, the Southern Fish. THE WINTER CONSTELLATIONS 313 Fig. M.— The Hyades. The Winter Constellations The next position of the stars we shall describe comes six hours after the preceding one ; that is at two o'clock A. M. in November and at eight o'clock P. M. in Feb- ruary. During this six- hour interval another sec- tion of the Milky Way has risen in the east and passed over toward the south. The Milky Way now passes nearly through the zenith, resting on the horizon near the north and south points. Near its course and east of the meridian we see the constellation Taurus, the Bull, of which the brightest star is Aldebaran, form- ing the eye of the bull in the mytho- logical figure. Alde- baran is easily rec- ognised by its red colour. It lies on the end of one branch of a V-shaped cluster called Hyades. No- tice the pretty pair of stars in the middle of one leg. Fig. 55. — The Pleiades, as seen with the 814 THE FIXED STARS Near bj is the best known cluster in the sky, the Pleiades, or "seven stars." Only six stars are made out by ordinary unaided vision, but to a good eye five others TAYCETA^* MAlAi^ PkElOME <^t ALCYONE. • eLECTRA||fi MIROPE Fig. 56. — Telescopic View of the Pleiades^ with Names of the Brighter Stars. are visible, making eleven in all. The term "seven stars" is therefore a misnomer ; as a reason for it, it was said in ancient times that the number was originally seven but that one faded away. This "lost Pleiad" is probably a myth, as we do not find stars fading away permanently. THE WINTER CONSTELLATIONS 315 With a telescope we find the cluster to contain quite a number of yet smaller stars, as can be seen by the tele- scopic view which we give. The central and brightest star of the group is called Alcyone, and was supposed by Maedler to be the central star of the universe. But this notion is quite baseless. East of Taurus and near the zenith is Gemini, the Twins, marked by two stars nearly of the first magnitude, Castor and Pollux. The latter is the northernmost and a little the brighter of the two. The next zodiacal constellation is Cancer, the Crab, but it contains no conspicuous stars. Its most noticeable feature is Prcesepe, a cluster of stars, which are singly invisible to the naked eye, and look collectively like a small patch of light. The smallest telescope will show a dozen stars in the patch. Leo, the Lion, is also well up in the east. It may be recognised by Regulus, a star nearly of the first magni- tude, and a curved row of stars in the form of a sickle, of which Regulus is the handle. In the south we now have the most brilliant constella- tion in the heavens, the beautiful Orion. The three stars of the second magnitude in a row forming the belt of the warrior are familiar from childhood to all who watch the sky. Below them hangs another row of three stars, the upper one quite faint. The middle one of these has a hazy aspect, and is really not a star at all, but one of the most splendid objects in the sky, the Great Nebula of Orion. A mere spy-glass will show its character, but a 316 THE FIXED STARS large telescope is required to bring out the magnificence of its form. The corners of the constellation are marked by four stars. The brighter of the two uppermost, Alpha Orionis, or Betel- guese, is reddish in colour. At the oppo- site corner is Rigel, blue in colour and also of the first mag- nitude. The two up- per stars are in the shoulders of the fig- ure. Midway and above them a triangle of small stars forms the head. East of Orion is Canis Minor, the Lit- tle Dog, containing Procyon, of the first magnitude. Below it and south- east of Orion is another collection of bright stars forming the constellation Canis Major, the Great Dog, containing Sirius, the Dog Star, the brightest fixed star in the heavens. The Spring Constellations The third position of the sphere, sidereal time twelve hours, occurs in February at two A. M. ; in May at eight P. M. Lyra has now risen in the northeast and Capella is going downward in the northwest. The Milky Fig. 61. — Ono7i. THE SPRING CONSTELLATIONS 317 Way may not be visible at all unless the air is very clear. It will then be seen skirting the northern and western horizon. Regulus has passed the meridian, and Orion and Canis Major have set, or are low down in the southwest. In mid-heaven, southeast of the zenith, is Arcturus, of a dingy yellow col- our, but one of the brightest first magni- tude stars. East of Arcturus (now below it) is Corona Borealis, the Northern Crown, a beautiful semicircle of stars, of which the brightest is of the second magnitude. Near the zenith is Coma Berenices, the Hair of Berenice, a collection of faint stars mostly of the fifth magnitude. East of south across the meridian from Leo is Virgo, the Virgin, con- spicuous only by Spica, a white star of nearly the first magnitude. Libra, the Balance, east and southeast of Virgo, has no conspicuous stars. The Summer Constellations The fourth position of the sphere, eighteen hours sidereal time, occurs in May at two A. M. ; in August at eight P. M. Capella has now set, Lyra is near the zenith. Fig. 58. — 77ie Northern Crown. 818 THE FIXED STARS Fig. 59. — Aquila. Cassiopeia is in the northeast, and the most splendid por- tion of the Milky Way is near the meridian. We have described all the constellations that lie near its course north of Lyra; let us now trace it to the south. One of the noticeable fea- tures of the Milky Way now to be seen is the great bifurcation, or separation into two branches. The split can be traced from Cygnus, where it begins, past Lyra and halfway to the south- ern horizon. Here we see Aquila, the Eagle, in the cleft, marked by Altair, of the first magnitude. It is in a line between two other stars of the third and fourth magnitudes. At this point the westernmost branch of the Milky Way diverges yet farther and seems to terminate, but if the air is clear we shall see that it recom- mences near the horizon. East of Aquila is a small but very pretty constellation of which the scientific name is Delphinus, the Dolphin, but which is popu- larly known as Job's Coffin. Between Lyra and the beautiful Corona, now some distance west of the zenith, lies the widely extended Fig. 60. — Delphimis, Dolphin. THE SUMMER CONSTELLATIONS 319 Fig. 6h—27ie Great Cluster of Eercuks, photographed at the Lick Observatory/. sw THE FIXED STARS constellation, Hercules. Alpha, its brightest star, is below the second magnitude and may be known by its reddish colour and by a white star. Alpha Ophiuchi, a little farther east. The most remarkable object in this con- stellation is the Great Cluster of Hercules which, to the naked eye, is a very faint patch, but which a great tele- scope resolves into a universe of stars. Near the horizon, west of south, is the zodiacal con- stellation ScorpiuSy the Scorpion. Its western boundary is a curved row of stars forming the claws of the animal; east of them is Antares, or Alpha Scor- pii, reddish in colour, and nearly of the first magnitude. In the Milky Way, due south, and therefore east of Scorpius, is Sag- ittarius, the Archer, wdth quite a collection of stars of the second and third magnitudes. The bow and arrow of the archer are easily imagined. Next toward the east are Capricornus and Aquarius, already mentioned. The brightest star in the former has a companion so close to it that it is a sign of not bad eyesight to be able to distinguish it. . • V • • * Fig. 62. — Scorpius, the Scorpion. IV The Distances of the Stars The principles on which distances in the heavens are determined was set forth in our chapter explaining how the heavens are measured. For distances of the moon and nearer planets, we use, as a base line for measurement, the radius of the earth, or the line joining two points of ob- servation on its surface. But this is entirely too short to serve for measuring a distance so great as that even of the nearest star. For this purpose we take as a base line the whole diameter of the earth's orbit. As the earth moves from one side of the orbit to the other, the stars must seem to have a slight motion in the opposite direction. But this motion is found to be almost immeasurably small. It can be made out with sufficient precision only by comparing the stars among themselves in the following way : Let the little circle on the left of the following figure represent the orbit of the earth. Let S be the star, sup- posed to be near us, of which we wish to measure the dis- tance. Let the dotted lines almost parallel to each other show the direction of a star T many times farther away. When the earth is at one side of its orbit, say at P, we measure the small angle SPT, which seems to us to sepa- rate these two stars. When the earth goes to the opposite side, it is readily seen that the corresponding angle SQT will be greater. We again measure it. The difference S22 THE FIXED STARS between these two angles will furnish a basis for com- puting, by trigonometric methods, the distance of the nearest star when that of the farthest is known. Practi- cally we have to assume that the star T is at an infinite dis- tance, so that the dotted lines are parallel. Then the measured difference between the angles will enable us to calculate the angle subtended by the radius of the earth's orbit, as seen from the star S. This angle is what astrono- FiG. 63. — Measurement of the Parallax of a Star. mers habitually use in their computations, not the dis- tance of the star. It is called the Parallax of the star. If we wish to obtain the distance of the star, we have to divide the number 206,265 by the parallax of the star expressed as a fraction of a second. This will give its distance in terms of the radius of the earth's orbit as a unit of measure. One second is the angle subtended by an object one inch in diameter at a distance of 206,265 inches, or more than three miles. * It is, of course, com- pletely invisible to the naked eye. It will be seen that this method of measurement implies tliat we know which of the two stars is the nearer ; in fact, that we know the farther star to be at an almost infinite distance. The question may be asked how this knowl- edge is obtained, and how a star is selected as being near to us. The most careful measures that can be made with the finest instruments show that the great mass of small THE DISTANCES OF THE STARS S2S telescopic stars do not have the slightest change in their relative positions, but remain as if fixed on the celestial sphere from year to year. Now and then, however, an exception is found. A very bright star is probably nearer to us than the fainter ones, and if a star shows any change in its position, the astronomer may proceed to measure and determine its parallax. So far as has yet been determined, the nearest star to us is Alpha Centauri, a star of nearly the first magnitude, in the southern hemisphere. The parallax of this star is 0.75". By the rule we have given, its distance will be nearly 275,000 times that of the sun. Such a distance transcends all our power of conception over and over again. A crude idea of it may be obtained by reflecting that light itself, the speed of which we have already described, would require more than four years to reach us from this star. We see the latter, not as it is now, but as it was more than four years ago. At such a distance not only does the earth's orbit itself vanish away to a point, but a ball as large as the whole bedy- of Neptune would be barely visible to the naked eye as the minutest possible point. The next star in the order of distance is supposed to be about one half as far again as Alpha Centauri, and there are some half dozen others, within three or four times its distance. In all, the parallaxes of about one hundred stars have been determined with more or less exactness; but even in these cases the parallax is sometimes so small that we cannot be sure it is real. It seems likely that only about fifty stars are within seven times the distance of SM THE FIXED STARS Alpha Centauri. The distance of the stars whose paral- laxes are too small to be measured is a matter of judg- ment rather than calculation. The probability seems to be that at least the brighter stars are scattered through space with some approach to uniformity. If this is the case, many of the fainter telescopic stars, perhaps the large majority of the smallest ones found on photo- graphs of the heavens, must be more than one thousand times the distance of Alpha Centauri. The light by which their presence is made known to us must have been on its way to our system during the whole period of human history. V The Motions of the Stars If I were asked what is the greatest fact that the intel- lect of man has ever brought to light I should say it was this: Through all human history, nay, so far as we can dis- cover, from the infancy of time, our solar system — sun, planets, and moons — has been flying through space toward the constellation Lyra with a speed of which we have no example on earth. To form a conception of this fact the reader has only to look at the beautiful Lyra and reflect that for every second that the clock tells off^, we are ten miles nearer to that constellation. Every day that we live we are nearer to it by almost, perhaps quite, a million of miles. For every sentence that we utter, for every step that we take in the streets we are miles nearer to this star. We approached it by tens of thousands of miles while the writer has been penning these lines, and the reader has been carried nearer by a thousand miles while perusing them. This has been going on through all human history, and we have reason to believe that it will remain true for our remotest posterity. One of the greatest problems of astronomy is, when and how did this journey begin and when and how will it end.^ Before this question our science stands dumb. The astronomer can tell no more about the beginning or the end of the S26 THE FIXED STARS journey than can the untutored child. He can only im- press upon the mind of his followers the magnitude of the problem. Nothing can give us a better conception of the enor- mous distance of the stars than the reflection that not- withstanding the rapid motion, carrying us unceasingly forward through all the ages that the human race has existed on earth, ordinary observation would fail to show any change in the appearance of the constellation toward which we are travelling. From what we know of the dis- tance of Vega we have reason to suppose that our solar system will not reach the region in which that star is now situated until the end of a period ranging somewhere between half a million and a million of years from the present time. It does not follow, however, that our posterity, if any such shall then live on the earth, will find Vega when they arrive at its present j)lace. It also is going on its own journey and is passing away from its present location almost as rapidly as we are approaching it. What is true of our sun and of Vega is true, so far as we know, of ever}^ star in the heavens. Each of these bodies is flying straight ahead through space Hke a ball shot out from a cannon, with a speed which in most cases is almost inconceivable. It would be a very slow moving star of which the velocity did not exceed that of a cannon shot. In the great majority of cases it ranges from five to thirty miles per second — frequently more than fifty miles. Indeed there are two stars, of which Arcturus is one, whose speed we have reason to believe approaches THE MOTIONS OF THE STARS 327 two hundred miles a second. These motions of the stars are called their proper motions. 