Winding among the constellations, and forming a gigantic belt round the whole star-sphere, lies that most wonderful feature of the heavens familiar to all under the name of the Milky Way. This great luminous girdle of the sky may be seen in some portion of its extent, and at some hour of the night, at all seasons of the year, though in May it is somewhat inconveniently placed for observation. Roughly speaking, it presents the appearance of a broad arch or pathway of misty light, 'whose groundwork is of stars'; but the slightest attention will reveal the fact that in reality its structure is of great complexity. It throws out streamers on either side and at all angles, condenses at various points into cloudy masses of much greater brilliancy than the average, strangely pierced sometimes by dark gaps through which we seem to look into infinite and almost tenantless space (Plate [XXVII.]), while in other quarters it spreads away in considerable width, and to such a degree of faintness that the eye can scarcely tell where it ends. At a point in the constellation Cygnus, well seen during autumn and the early months of winter, it splits up into two great branches which run separate to the Southern horizon with a well-marked dark gap dividing them.
When examined with any telescopic power, the Milky Way reveals itself as a wonderful collection of stars and star-clusters; and it will also be found that there is a very remarkable tendency among the stars to gather in the neighbourhood of this great starry belt. So much is this case that, in the words of Professor Newcomb, 'Were the cloud-forms which make up the Milky Way invisible to us, we should still be able to mark out its course by the crowding of the lucid stars towards it.' Not less remarkable is the fact that the distribution of the nebulæ with regard to the Galaxy is precisely the opposite of that of the stars. There are, of course, many nebulæ in the Galaxy; but, at the same time, they are comparatively less numerous along its course, and grow more and more numerous in proportion as we depart from it. It seems impossible to avoid the conclusion that these twin facts are intimately related to one another, though the explanation of them is not yet forthcoming.
In the year 1665 the famous astronomer Hooke wrote concerning the small star Gamma Arietis: 'I took notice that it consisted of two small stars very near together; a like instance of which I have not else met with in all the heavens.' This is the first English record of the observation of a double star, though Riccioli detected the duplicity of Zeta Ursæ Majoris (Mizar), in 1650, and Huygens saw three stars in Theta Orionis in 1656. These were the earliest beginnings of double-star observation, which has since grown to such proportions that double stars are now numbered in the heavens by thousands. Of course, certain stars appear to be double even when viewed with the unaided eye. Thus Mizar, a bright star in the handle of the Plough, referred to above, has not far from it a fainter companion known as Alcor, which the Arabs used to consider a test of vision. Either it has brightened in modern times, or else the Arabs have received too much credit for keenness of sight, for Mizar and Alcor now make a pair that is quite easy to very ordinary sight even in our turbid atmosphere. Alpha Capricorni, and Zeta Ceti, with Iota Orionis are also instances of naked-eye doubles, while exceptionally keen sight will detect that the star Epsilon Lyræ, which forms a little triangle with the brilliant Vega and Zeta Lyræ, is double, or at least that it is not single, but slightly elongated in form. Astronomers, however, would not call such objects as these 'double stars' at all; they reserve that title for stars which are very much closer together than the components of a naked-eye double can ever be. The last-mentioned star, Epsilon Lyræ, affords a very good example of the distinction. To the naked eye it is, generally speaking, not to be distinguished from a single star. Keen sight elongates it; exceptionally keen sight divides it into two stars extremely close to one another. But on using even a very moderate telescope, say a 2½-inch with a power of 100 or upwards, the two stars which the keenest sight could barely separate are seen widely apart in the field, while each of them has in its turn split up into two little dots of light. Thus, to the telescope, Epsilon Lyræ is really a quadruple star, while in addition there is a faint star forming a triangle with the two pairs, and a large instrument will reveal two very faint stars, the 'debilissima,' one on either side of the line joining the larger stars. These I have seen with 3⅞-inch.
What the telescope does with Epsilon Lyræ, it does with a great multitude of other stars. There are thousands of doubles of all degrees of easiness and difficulty—doubles wide apart, and doubles so close that only the finest telescopes in the world can separate them; doubles of every degree of likeness or of disparity in their components, from Alpha Geminorum (Castor), with its two beautiful stars of almost equal lustre, to Sirius, where the chief star is the brightest in all the heavens, and the companion so small, or rather so faint, that it takes a very fine glass to pick it out in the glare of its great primary. The student will find in these double stars an extremely good series of tests for the quality of his telescope. They are, further, generally objects of great beauty, being often characterized, as already mentioned, by diversity of colour in the two components. Thus, in addition to the examples given above, Eta Cassiopeiæ presents the beautiful picture of a yellow star in conjunction with a red one, while Epsilon Boötis has been described as 'most beautiful yellow and superb blue,' and Alpha Herculis consists of an orange star close to one which is emerald green. It has been suggested that the colours in such instances are merely complementary, the impression of orange or yellow in the one star producing a purely subjective impression of blue or green when the other is viewed; but it has been conclusively proved that the colours of very many of the smaller stars in such cases are actual and inherent.
Not only are there thousands of double stars in the heavens, but there are also many multiple stars, where the telescope splits an apparently single star up into three, four, or sometimes six or seven separate stars. Of these multiples, one of the best known is Theta Orionis. It is the middle star of the sword which hangs from the belt of Orion, and is, of course, notable from its connection with the Great Nebula; but it is also a very beautiful multiple star. A 2½-inch telescope will show that it consists of four stars in the form of a trapezium; large instruments show two excessively faint stars in addition. Again, in the same constellation lies Sigma Orionis, immediately below the lowermost star of the giant's belt. In a 3-inch telescope this star splits up into a beautiful multiple of six components, their differences in size and tint making the little group a charming object.
Looking at the multitude of double and multiple stars, the question can scarcely fail to suggest itself: Is there any real connection between the stars which thus appear so close to one another? It can be readily understood that the mere fact of their appearing close together in the field of the telescope does not necessarily imply real closeness. Two gas-lamps, for instance, may appear quite close together to an observer who is at some distance from them, when in reality they may be widely separated one from the other—the apparent closeness being due to the fact that they are almost in the same line of sight. No doubt many of the stars which appear double in the telescope are of this class—'optical doubles,' as they are called, and are in reality separated by vast distances from one another. But the great majority have not only an apparent, but also a real closeness; and in a number of cases this is proved by the fact that observation shows the stars in question to be physically connected, and to revolve around a common centre of gravity. Double stars which are thus physically connected are known as 'binaries.' The discovery of the existence of this real connection between some double stars is due, like so many of the most interesting astronomical discoveries, to Sir William Herschel. At present the number of stars known to be binary is somewhat under one thousand; but in the case of most of these, the revolution round a common centre which proves their physical connection is extremely slow, and consequently the majority of binary stars have as yet been followed only through a small portion of their orbits, and the change of position sufficient to enable a satisfactory orbit to be computed has occurred in only a small proportion of the total number. The first binary star to have its orbit computed was Xi Ursæ Majoris, whose revolution of about sixty years has been twice completed since, in 1780, Sir William Herschel discovered it to be double.
The star which has the shortest period at present known is the fourth magnitude Delta Equulei, which has a fifth magnitude companion. The pair complete their revolution, according to Hussey, in 5·7 years. Kappa Pegasi comes next in speed of revolution, with a period of eleven and a half years, while the star 85 of the same constellation takes rather more than twice as long to complete its orbit. From such swiftly circling pairs as these, the periods range up to hundreds of years. Thus, for example, the well-known double star Castor, probably the most beautiful double in the northern heavens, and certainly the best object of its class for a small telescope, is held to have a period of 347 years, which, though long enough, is a considerable reduction upon the 1,000 once attributed to it.
But the number of binary stars known is not confined to those which have been discovered and measured by means of the telescope and micrometer. One of the most wonderful results of modern astronomical research has been the discovery of the fact that many stars have revolving round them invisible companions, which are either dark bodies, or else are so close to their primaries as for ever to defy the separating powers of our telescopes. The discovery of these dark, or at least invisible, companions is one of the most remarkable triumphs of the spectroscope. It was in 1888 that Vogel first applied the spectroscopic method to the well-known variable star, Beta Persei—known as Algol, 'the Demon,' from its 'slowly-winking eye.' The variation in the light of Algol is very large, from second to fourth magnitude; Vogel therefore reasoned that if this variation were caused by a dark companion partially eclipsing the bright star, the companion must be sufficiently large to cause motion in Algol—that is, to cause both stars to revolve round a common centre of gravity. Should this be the case, then at one point of its orbit Algol must be approaching, and at the opposite point receding from the earth; and therefore the shift of the lines of its spectrum towards the violet in the one instance and towards the red in the other would settle the question of whether it had or had not an invisible companion. The spectroscopic evidence proved quite conclusive. It was found that before its eclipses, Algol was receding from the sun at the rate of 26⅓ miles per second, while after eclipse there was a similar motion of approach; and therefore the hypothesis of an invisible companion was proved to be fact. Vogel carried his researches further, his inquiry into the questions of the size and distance apart of the two bodies leading him to the conclusion that the bright star is rather more, and its companion rather less than 1,000,000 miles in diameter; while the distance which divides them is somewhat more than 3,000,000 miles. Though larger, both bodies prove to be less massive than our sun, Algol being estimated at four-ninths and its companion at two-ninths of the solar mass.
The class of double star disclosed in this manner is known as the 'spectroscopic binary,' and has various other types differing from the Algol type. Thus the type of which Xi Ursæ Majoris was the first detected instance has two component bodies not differing greatly in brightness from one another. In such a case the fact of the star being binary is revealed through the consideration that in any binary system the two components must necessarily always be moving in opposite directions. Hence the shift of the lines of their spectrum will be in opposite directions also, and when one of the stars (A) is moving towards us, and the other (B) away from us, all the lines of the spectrum which are common to the two will appear double, those of A being displaced towards the violet and those of B towards the red. After a quarter of a revolution, when the stars are momentarily in a straight line with us, the lines will all appear single; but after half a revolution they will again be displaced, those of A this time towards the red and those of B towards the violet.