DOUBLE AND MULTIPLE STARS—COLOURED AND VARIABLE STARS—CLUSTERS, GROUPS, AND NEBULÆ—THE GALAXY, OR MILKY WAY—HOW TO FIND OUT THE PRINCIPAL STARS.

Although not very clearly visible to the naked eye, there are in the sky some pairs of stars very close together apparently; but when these double stars are examined with a good telescope we find that though we fancy they are two stars very close, in reality an immense distance separates them. By Vega, which we have already mentioned, there is apparently a star, which on examination will be found really to be two stars. It is also in the constellations of the Lyre, but of much lower magnitude than Vega. But in some instances there are three or four stars thus placed together, and the frequency of the occurrence of this fact establishes the farther fact that these combinations are not accidental—that the stars are interdependent and physically connected.

There are now at least six thousand double stars known,27 and this is a very small proportion of the forty millions or so of suns which are believed to exist in space. But of these six thousand a larger proportion have been ascertained to be physically connected. More than six hundred of these pairs are double suns, while again there are other combinations of three and perhaps more. When two are thus connected we have what are termed binary systems, and when more are associated they are called triple and multiple stars. An example of the last-mentioned class is the small star above mentioned near Vega. It is e LyrÆ, and is a double of a double. In ordinary telescopes this will not be perceived, but with a high power the combination will be noticed. The same phenomenon is observable in one of the stars of Hercules and in Andromeda.

The revolution of these double suns, or binary systems, has been closely observed, and Professor Newcomb has given us a list of the binary systems of short period which are well determined. These are as follows:—

It must be borne in mind that although these double stars appear close together from our standpoint, they may be far apart—one behind the other in a straight line. When such “pairs” exist they are known as optical pairs, or optically double stars, as distinguished from the actually physical “pairs” which revolve round the centre of a system. In Orion there has been discovered a regular system, and the ? in Orion, which appears in a common telescope a moderate star, and to the unaided eye only a speck of light, is really composed of seven stars—four are set in the form of a trapezium, as figured in the diagram in the margin by dots and asterisks. Two of these have been ascertained to possess attendants indicated by dots, and a seventh star was discovered by Lassell, and Humboldt remarks that in all probability this apparently tiny star in the constellation Orion constitutes a real system, for the five smaller stars have the same proper motion as the principal one.

Thus our imagination almost fails to grasp the infinity of the systems with our single sun, and with the distant double and even triple suns round which planets revolve perfectly independent of the other systems, as we are independent of them possessing heat and light from their own sun or suns as we receive it from ours, day and night seasons succeeding each other, and the wondrous varieties of the light produced by the appearance or withdrawal of a sun or two in the firmament of those most distant planets. These suns being double or triple would affect each other; the composition of the light given forth would produce—as we may assume—varying effects. We know something about the light of the stars by the spectroscope, and the colours of stars are due to the vapour which takes away a certain part of the light emitted, leaving the remainder to descend through the atmosphere to us.

Binary stars are most numerous of the doubles; for instance Castor, ? CoronÆ, Rigel, Polaris, Mivac, ? Leonis, ? Virginis, ? UrsÆ Majoris, a Hercules, 36 AndromedÆ, ? Ophiuchi, and p AquilÆ. The illustration in the margin is Castor (or a Geminorum), the most northerly of the Twins. The ? CoronÆ is also figured, as are Polaris (see fig. 610 ante.), BoÖtes, Rigel, and ? Leonis.

The cuts herewith illustrate the relative positions at the periods named of the “doubles,” and of the revolution of suns around other suns as mentioned. As a consequence of their proper motion the binary stars appear to vary in their distances from each other, as in the topmost of the three cuts on the (opposite) page representing ? Virginis. The stars have gradually approached each other, and so are the stars in Castor approximating, and when they have closed, and have appeared almost as a single star, as they will do, they will take open order again.

The shortest time occupied by a double star in its revolution is thirty-five years, and we have already given some of those which have been ascertained. We will close this section with a few other examples. For instance, ? Virginis revolves in one hundred and fifty years, Castor in two hundred and forty years, 4 Aquarii in three hundred years, 37 Pegasi in five hundred years. There are numerous other instances up to a period of three thousand years, and about eight hundred of these binary systems are known. We have mentioned that there are two or more suns in the multiple systems. These suns are the cause of the different colours of the stars.

Colours of the Stars.

The question of star-colour follows naturally the consideration of the multiple stars; for although single stars have been observed of a ruddy colour, there are no instances of a blue or green one unattended by a companion. This colouring has been attributed to the contrast between multiple stars, for the colours are frequently complementary; but investigation has shown that this cannot be the case. For instances have been known in which, when two are thus associated, and one is concealed from us, the other is just as bright, and retains its former colour.

Of course in cases in which colour is apparent to the unaided vision, only the brightest stars betray colour. Antares, Betelgeuse and Aldebaran are red (orange) colour. Sirius and Canopus are white. Arcturus and Capella are yellowish, so is Pollux. Vega is bluish-white. These appearances are, of course, much more marked when the stars are examined through the telescope, and telescopic stars—which are stars unobservable without a glass—are very much coloured, and the multiple stars give us blue, green, violet, and other tints, besides those already mentioned.

Again, these coloured stars do not always remain the same colour. Sirius was once red; Mars was at times white. Spectrum analysis shows that the colours of many are due to absorption by the vapours of some of the rays; and the existence of certain vapours may cut off some, and at other times other vapours may exist and cut off other rays, and so the colours may be changed. Struve gives the following list of binary complements of “multiple” stars:—

We need scarcely pursue this question farther, though many ideas concerning the coloured stars will arise in every thoughtful reader’s mind. Supposing that every system has its sun or suns, can we fancy the effects of a green or blue or violet sunlight—a light unmixed? To employ the words of Sir John Herschel—“It may be more easily suggested in words than conceived in imagination what variety of illumination two suns, a red and a green, or a yellow and blue one, must afford to a planet circulating round either—or what charming contrasts and graceful vicissitudes a red and a green day, for instance, alternating with a white one and with darkness, might arise from the presence or absence of one or other or both above the horizon.”

Lost and New Stars.

We may have perhaps read the “Lost Pleiad,” and wondered what has become of the star supposed to have dropped out of the cluster so well known in the constellation Taurus—the Pleiades. There are seven stars, of which six are visible to the average eye, and the ancients used to declare that one of the seven sisters (the daughters of Atlas and Pleione) hid herself because she had married a mortal, while all her sisters wedded gods. It is not improbable that one of the seven, formerly distinguishable with the unassisted eye, may have disappeared or been lost; but it is certain that strong eyesight can see more than seven now, and in the telescope there are about one hundred.

And it is a fact that some stars whose places have been carefully marked in the catalogue have subsequently disappeared. Many errors may have been made, and stars put down where no star existed, so a succeeding observer has not been able to find the star indicated. But, on the other hand, we may admit that stars have been lost to sight, and to compensate us for any such disappearances new stars are frequently observed, and these are very remarkable phenomena. About 121 B.C. Hipparchus perceived a new star, which was visible even in the daytime, and on subsequent occasions others came into existence—viz., in the years 945, 1264, and 1572. In the last-mentioned year Tycho BrahÉ suddenly perceived the new star, which was at first very brilliant. It grew fainter and fainter, after first gaining in intensity, and disappeared entirely in 1574; and at other times stars have been seen which remained only for a short time, and then disappeared.

The star discovered by Tycho BrahÉ was seen by him when walking across the fields one night, and he encountered peasants who were gazing at the new luminary. It was so bright that it threw a shadow from BrahÉ’s stick. The new arrival appeared in “Cassiopeia,” under the lady’s chair, forming, as pictured in the diagram, an irregular square. The strange star is the largest.

Some stars exhibit extraordinary fluctuations, and one discovered by Mr. Birmingham in 1866, decreased rapidly and sank away to about the tenth magnitude, and then got brighter, and again diminished in splendour. The “Eta” ArgÛs has also been subjected to many fluctuations likewise, and such alterations have gained for these luminaries the name of “Variable Stars.”

In the accompanying little chart there will be perceived two particular stars, named Algol, “the demon,” and Mira “the wonderful.” The latter is the most celebrated for its variable qualities, and its cycle of change occupies nearly one of our years. For a few days it appears very bright, and then fades away for about three months, to disappear for five months, and then it reappears again, increasing in brilliancy up to the second magnitude for another three months or so. Some people account for these phenomena by stating that the sides of the star being less luminous present the dark and light portions in rotation; but we can give no satisfactory explanation of the reason, unless it be caused by an aggregation of spots upon its surface, like sun-spots on our sun, or perhaps by eclipse.

Star-Clusters and NebulÆ.

NebulÆ and Star-clusters are numerous in the heavens. The most important are the Great NebulÆ in Orion and in Andromeda. But there are other very beautiful “patches” of luminous matter or cloud appearances composed of minute stars invisible to the naked eye. We annex specimens of the NebulÆ, one or two having been already inserted. There must be thousands of these star-clouds, and they have been classified by Sir John Herschel from Sir William’s discoveries as follows:—

(1) Clusters of stars, in which the stars are clearly distinguishable, divided again into regular and irregular clusters.

(2) Resolvable NebulÆ, which may be separated into distinct stars under powerful telescopes.

(3) NebulÆ, in which there is no appearance whatever of stars, divided into classes according to brightness, etc.

(4) Planetary NebulÆ.

(5) Stellar NebulÆ.

(6) Nebulous Stars.

We learn also from the foregoing authority that NebulÆ affect a certain district; that is, they have, as it were, a preference for it, and are not distributed in a random manner over the heavens, and are found in Leo, Leo Minor, Ursa Major, Canes Venatici, Coma, BÖotes, and Virgo, and more sparingly in Aries, Taurus, Orion, Perseus, Draco, Hercules, Lyra, etc. NebulÆ are found associated with stars, as is the case with ? Argus; these are called nebulous stars, and in the case of this particular star many very interesting investigations have been made. The NebulÆ are as equally variable as the stars they surround.

What is termed the Nebular Hypothesis was put forward by La Place, and by it he endeavoured to account for the regular development of the stellar system, which is supposed to have originated from an immense nebular cloud. This immense mass would rotate and contract, and the outer portions would separate and develop in rings like Saturn’s rings. Then the rings break into separate portions, and each portion condenses into a planet, or the small “bits” travel round the sun like asteroids, and in this manner various systems were formed. This theory was considered to be quite exploded when stars were discerned in nebulÆ by the more recent telescopes; but then the spectroscope came to our aid, and it was discovered that there were some nebulÆ which are simply masses of glowing gas or aggregations of stones which are dashing against each other in so forcible a manner as to produce heat and luminosity. Mr. Lockyer appears to favour the latter theory as to nebulÆ.

Mr. Proctor, however, has put forward a hypothesis that the star or meteor showers are the original cause of the sidereal system, and this rain of meteors has fallen for all time, gradually consolidating into orbs. The fact that the constituents of sun, earth and planets, comets and meteors being fundamentally the same lends probability to this hypothesis, which is fully explained by the author.

The Milky Way.

The Galaxy is familiar to all readers, and although visible all the year round, is perceived more plainly in August, September, and October, or at the beginning and ending of that period. This zone of stars was of course well known to the ancients, but it is to Galileo that we owe the first important information about the Galaxy; he decided that it was formed of stars. Sir John Herschel investigated the subject very closely, and to him much of the information concerning the Milky Way is due.

It is not very distinct in the north, but as it advances from Cepheus southwards by the Unicorn, it gets clearer, and opens out in Argo, and descends still south, becoming brighter near the Southern Cross. It then passes northward again, dividing into two branches, one of which dies out, and then over Sagittarius, and so on to Cygnus, then to Casseopeia and the starting-point. The number of stars in the Galaxy is about 18,000,000.

In this wonderful zone of stars the centre of our system, the sun, is placed. It was supposed to be divided as in the diagram above; the inner portion being the stars seen in their thickness, and the outer ring representing the stars viewed in the direction of the length and breadth. But afterwards, Herschel modified his opinions respecting the Milky Way, and since his death many astronomers—and Mr. Proctor more particularly—have devoted considerable time to an examination of this wonderful zone of stars; which, it must be remembered, is not a continuous stream; it is a series of luminous patches. On this point Professor Nichol says:—

“It is only to the most careless glance that the Milky Way appears a continuous zone. Let the naked eye rest thoughtfully on any part of it, and if circumstances are favourable, it will stand out rather as an accumulation of patches and streams of light in every conceivable variety of form and brightness; now side by side, now heaped on each other, again spanning across dark spaces ... and at other times darting off into the neighbouring skies in branches of capricious length and shape, which gradually thin away and disappear.”

The Milky Way has its greatest breadth in the “Swan,” and in the “Eagle” constellation it divides itself. In the “Southern Triangle” the zone is brightest, and in the “Southern Cross” the hole or space, termed by sailors the “Coal Sack,” is very distinct. It then contracts and expands, and there is in Argo another gap. Then it is lost for a space, then it branches out, and soon crosses the Equator, dilates, contracts, opens out again, and so returns to the “Swan” again.

Philosophers have frequently discoursed upon this phenomenon, but all statements must remain more or less speculative. From Kepler’s to the present time astronomers have been considering the Milky Way, and when the Nebular theory was given up, when the Galaxy was found to be composed of stars, there was, as we have noticed, the idea of the ring and the cloven disc. Mr. R. Proctor has likened the Galaxy to a coiled serpent, and considers the openings in the Milky Way as evidence that the stratum of stars is limited, and that here we can see beyond it. In fact, it would appear that it is a very complicated question; and as the zone itself is complicated “with outlying branches beyond the range of our most powerful telescopes,” so an actual knowledge of the Milky Way is beyond us at present. It is composed of most extraordinary aggregations of stars, which appear not only impossible to count, but each one to be independent of the other. Thus we must conclude our rapid survey of the Milky Way, and close with Mr. Proctor’s remark in his “Universe of Stars.” “The sidereal system,” he says, “is altogether more complicated, altogether more varied in structure than has hitherto been supposed. Within one and the same region co-exist stars of many orders of real magnitude, the greatest being thousands of times larger than the least. All the NebulÆ hitherto discovered, whether gaseous or stellar, irregular, planetary, ring-formed, or elliptic, exist within the limits of the sidereal system. They all form part and parcel of that wonderful system, whose nearer and brighter parts constitute the glories of our nocturnal heavens.”

And a little reflection will show how true this is. Not very long ago in the world’s life the solar system was supposed to consist of one sun with a few planets wandering around him. Then some more were found, and they were called “satellites.” For a long time man fancied he had reached the “ultima thule” of astronomy in these depths; but the whole idea was changed when it was discovered that beyond Mars there lie the asteroids and the host of bodies in this solar system which we cannot do more than allude to. Then when we consider that this “sun” of ours, which we think so enormous, and which keeps in subjection so many heavenly bodies, and illuminates them; when we reflect that there are in space, and visible, stars many times larger than our ruling star, each a sun, and that our sun would, if put where the great Sirius glows, be but a speck in the firmament, and his system invisible to our eyes, we may well wonder at the magnitude of the subject, and the Infinite Wisdom and Power “that telleth the number of the stars, and calleth them all by their names.”

How to read the Sky.

A few particulars, to enable a reader to identify the most prominent stars, may be given as starting-points from which some few excursions into the spangled heavens may be attempted. But the suggestions must be considered with reference to the ever-varying directions of the supposed lines in consequence of the daily revolution of the sphere. We have illustrated this in the cut in the margin, wherein the Lesser Bear is shown as swinging round the Polar Star in different positions. Sometimes the lines of direction will be vertical, sometimes inclined, but all retaining their relative positions.

We have already learnt that the “pointers” of the Great Bear indicate the Polar Star in the Lesser Bear, and we can (roughly) estimate the distance between the pointers as 5°. This will give us the distance between the pointers and the Polar Star as 29°. By following an imaginary line through the two northern stars of the “Waggon” (the Bear) away from the “horses,” we shall find Capella about 50° away.

If we pass from the first star next the waggon of “Charles’s Wain” to the Pole Star, and past it, we shall arrive at an irregular W. This is Cassiopeia, about as far beyond Polaris as the Bear is below it. When the latter is low, the former is at the zenith, and so on.

A line drawn from the Pole Star through the end star of the Great Bear leads to Arcturus. A line taken from Arcturus for about an equal distance will, with the Pole Star, make a triangle with Vega. The Polar Star may be called the Apex.

Regulus may be found southwards by drawing a line through the two first stars of the square in the Bear (opposite the pointers). From Vega, almost opposite the Pole Star, and through it about twice as far away on the other side, is Sirius, a brilliant “sun.” Procyon will be found to the westward of Regulus about 30°. From Procyon to the Pole Star a line will pass through Pollux and Castor.

Another line from the pole star through the middle of the three “horses” in the “Wain” will reach Spica Virginis about 70° beyond. So we can describe a large triangle with Spica, Regulus, and Arcturus, at the angles. Regulus is the apex, Spica and Arcturus a short base line.

From the pole star through Capella, passing between Betelgeux and Bellatrix, we shall describe a line leading to the three stars of Orion’s belt. Between it and the Pleiades is Aldebaran.

There are many other stars which could be indicated; but on a fine evening, if the observer will mark them upon a piece of paper, placing the pole star in the centre, he will be able to add to his star map very rapidly.

In the foregoing chapters of Astronomy we have seen how the earth and other planets move around the sun; we have glanced at the “fixed” stars and their groups, termed the constellations, and have noted the planets and their characteristics, with many other interesting facts. There is yet a great deal to be learnt, and much study will be required with daily (nightly) observations before the young reader will obtain success as a student of astronomy; but there is no study so interesting. We have seen what a very small portion of the universe is occupied by our solar system, and what a speck our earth is on the plain of creation. We find ourselves on the border-land of the incomprehensible, and we are lost in speculations upon the unseen.