Part 2

Spica, the Ear of Corn in the hand of the Virgin, is immeasurably distant from us, and its companion is invisible even in the most powerful telescopes. It was discovered by means of the spectroscope, for, when the light of the star is drawn out into a long rainbow-coloured ribbon crossed by dark lines, it is found that there are two ribbons, one bright, the other very faint, and that the lines of these two spectra draw apart and then come together again once in every four days, showing that there are two stars close together and revolving round one another in this short time. The joint mass of the pair is two and a half times that of the sun.

Spica is one of the first stars whose invisible companion was discovered in this way, and it is also connected with another discovery, made nearly twenty centuries earlier by Hipparchus in the island of Rhodes. He was patiently plotting the positions of all the visible stars, when he found a slight discrepancy between the places given to Spica by himself and another Greek astronomer, who had observed about a hundred and fifty years earlier. Examining into this led him to discover that all stars change their apparent positions very slowly, completing a cycle of change in nearly 26,000 years, so that 240 centuries more must elapse before any astronomer sees Spica in exactly the same place as where Hipparchus saw her. It remained for modern astronomers to discover that the apparent change is due to a slow nodding motion of the Earth’s axis.

Brightest of all stars in the whole sky is Sirius, the Dog-Star. It was worshipped by the ancient Egyptians, and the day on which it rose just before the sun was counted as the first day of their year. The Arabs, when they learned the astronomy of more ancient nations, were forbidden to adopt their star-worship, hence the saying in the Koran, often quoted by Arab writers: “The Highest saith, He is lord of Sirius.”

Sirius is moving rapidly through space, not uniformly but with an oscillating movement, and Bessel in 1844 “founded the astronomy of the invisible” by showing that these irregularities might be caused by a dark disturbing companion. Eight years later, Mr. Alvan Clark, wishing to test a large lens just made by his firm, turned it on Sirius, and lo! there was the satellite in the position required to explain the vagaries of Sirius. It is not therefore wholly dark, but it shines with so feeble a light that, if it were brought as near to us as our own sun, it would appear only one-hundredth as bright as he is, even though it is a somewhat more massive body. Sirius itself is only about two and a half times as massive as our sun, but immensely more brilliant. It is the typical star of the “Sirian” class, to which belong many of the brightest stars in the heavens, white stars in whose spectra broad hydrogen lines form the most striking feature, indicating a very extensive atmosphere of glowing hydrogen. Bright Sirius and his dim companion revolve round their common centre of gravity in fifty years.

Fomalhaut, the mouth of the Fish, belongs to the Sirian type of stars, and is also very brilliant, giving out fourteen and a half times as much light as our sun. Its distance is 25 light-years.

Canopus, the rudder of the Ship Argo, must be a giant sun, for its distance is altogether beyond reach of measurement and it is steadily receding from us, yet it shines as the brightest star in the sky except Sirius.

In Southern India it is called Agastya, after a Brahmin rishi who led an early Aryan colony to the south, and before whom the Vindhya Mountains prostrated themselves as he passed.

Greek astronomers noticed that this star rose only just above the horizon of Rhodes, but 7½ degrees above it at Alexandria, from which Poseidonius calculated that the circumference of the whole earth, _i.e._ 360°, must be 240,000 stadia. This is equal to nearly 23,500 miles, a value surprisingly near the correct figure, considering how difficult it must have been to measure the distance over the sea between Rhodes and Alexandria.

Antares, the brightest star in the Scorpion, was so named by the Greeks because it rivals the red planet Mars (Greek Ares) in colour. It is red because a dense atmosphere shuts out most of its blue rays. Like most red stars it is very distant, and its light takes 155 years to reach us. Nevertheless it shines 2000 times as brightly as our sun would do at the same distance, hence it must be of an enormous size. This immense red star is accompanied by a little green satellite, and there is also a very close companion which can never be seen, but is known to exist through the shifting of lines in the spectrum, like that of Spica.

What an amazing variety among these ten stars! Though the eye can only decide that all are much brighter than the average, and that they differ somewhat in colour among themselves, science tells us that they vary enormously in many ways. Half belong to the class of blue Orion stars, others to the white Sirian, yellow solar, red Antarian classes; more than half are known to be double or multiple, and among these we find twins, while others have faint companions differing from themselves in colour. In one case the pair is widely separated, and revolves in a period of half a century, while another pair is so close that the circuit is performed in four days. Again, while one star is the same size as the sun, others are much greater, and their distances from us vary all the way from four light-years to spaces we are powerless to plumb.

IV

STARS OF DIFFERENT AGES

What is the meaning of these different types of stars—the blue, the yellow, and the red? and can a star change its colour and type? Yes, we believe that all the blue stars, if they follow the normal course, will in countless ages become red, and though the life of the whole human race may perhaps not be long enough to witness a single instance, we feel confident that this process is going on. One walk through a forest is enough to teach us that a tree grows from seed to sapling, from maturity to decay, because we see trees around us in all these stages. So with the stars. Here the process of development is far more difficult to understand, and we are still ignorant concerning the birth and death of stars; but it is clear that we see a series of stages which pass gradually into one another, and that the cause of a star’s growing old is a gradual loss of heat by radiation. The blue Orion stars are the brightest and hottest of all those about which we have spoken, and are generally thought to have reached the climax of a star’s career. They are great globes of thin gas, much less dense than water, but glowing hot through and through. The outer layers naturally part with their heat first, and in cooling they absorb more and more light from the radiant centre. As the blue rays are always first and most powerfully affected, the blue tint is soon lost, and the colour slowly changes through white, yellow, and orange to deep-red. At the same time changes take place which cause different gases to become most conspicuous in the star’s spectrum at different stages.

But what are the early stages which precede the brilliance of an Orion star? Arguments have been brought forward to show that young stars also are comparatively cool, but even less dense than Orion stars, that as they grow denser and smaller through gravity, which causes all the particles of a star to condense towards its centre, they must at first grow gradually hotter and brighter until a certain point is reached, after which they will grow cooler and fainter again until they become quite cold and dark; and so there is a double progression, viz. from red towards blue, and back again to red, some stars changing in one direction and some in the other.

On the other hand, the stage which immediately precedes the Orion type somewhat resembles a peculiar class of nebulae, so the upward progression of the Orion stars may have been from nebulae through this stage.

Let us see what these stages are.

At one end of the series we find stars like Gamma in the Sails of the Ship Argo. When its light is spread out by prisms into a coloured band it is a wonderful sight, for not only are there dark lines and dark flutings, but wide bright bands mingle with these and stand out against the fainter background. We are fortunate to possess γ Velorum in the southern hemisphere, for it is the only bright star of its class. The type is known as WOLF-RAYET STARS, from the name of their discoverer. Only a few are known, and because of their faintness not very much has been discovered about them, but it is remarkable that all of them lie near the middle line of the Milky Way.

The bright lines in their spectra and some other features of Wolf-Rayet stars point (as we have said) to a connection with gaseous nebulae, and it is possible that they developed from nebulae not very long ago—as time is reckoned in astronomy. Though this past is doubtful, the future of these stars is clearly indicated: they are destined to become Orion stars, for in stars like τ Canis majoris we see an intermediate stage between the two types.

ORION STARS sometimes have bright lines in their spectra also, especially the very blue ones found near nebulae, but the most striking and characteristic feature is the series of dark helium lines, from which they are often called “helium stars.” The three in Orion’s belt are typical of this class; several first-magnitude stars belong to it, as we have seen, and it contains others which are very bright, such as β and δ Crucis, β Scorpii, α Lupi, α Pavonis, α Gruis, α Sagittarii. Spica is a distinctly blue Orion star; Rigel and γ Gruis have already lost the blue tint and are approaching the next stage.

For the Orion class passes by gradual transition into the SIRIAN STARS, which may be called “hydrogen stars,” from the wide hazy lines of hydrogen which are the most conspicuous feature in their spectrum. They are also very hot and bright, though less so than the Orion stars. Besides Sirius, a large number of bright southern stars belong to this class, among which we may mention Fomalhaut, γ Centauri, β Carinae, δ Velorum, and β Pavonis. Among stars visible to the naked eye this is by far the most numerous class, partly, no doubt, because they are intrinsically bright and therefore visible at distances where redder stars could not be seen. This cannot however be the whole explanation, or the Orion stars would be still more numerous instead of being comparatively rare.

The blue end of the spectrum begins to be darkened as a star reaches the stage of sun-like or SOLAR STARS, and although the hydrogen lines grow narrower and less intense, an immense number of fine lines cross the bright band and absorb much of the light. α Centauri, as we have seen, is a replica of the sun, and another star which almost exactly resembles it, in spectrum if not in mass, is β Hydri. ζ Gruis and one star of the naked-eye double in Grus, δ¹ Gruis, are also solar stars.

This class may be subdivided into three: the Sirian-solar, which is typified by Canopus, and also by η Crucis and α Hydri; the solar, closely resembling the sun; and the red-solar, which tend towards the red stars. The temperatures apparently are lower and lower in these three divisions in the order given, and all solar stars are cooler than those of the preceding classes. Among these red-solar stars is the beautiful ε Carinae, the foot of the False Cross,[5] which is a rich reddish orange even to the naked eye, and more brilliantly coloured in a binocular. Others are α Toucani, α Trianguli australi, ε Crucis (the little fifth star of the Southern Cross), the two brightest stars of Indus (α and β), almost all the bright stars of the Phoenix (α, β, δ, ε), and ε Scorpii which is beautifully coloured. This class is also extremely numerous among naked-eye stars, although they cannot be so bright intrinsically as the whiter solar stars: the significance of the fact is not easy to understand.

[5] The stars ε and ι Carinae, κ and δ Velorum, form a cross much like the Southern Cross, but less bright, and this is called the False Cross.

The deep-red stage is reached in suns like Antares, whose spectrum shows not only lines like those in the sun but also a series of broad bands or flutings which absorb much of its light, and in photographs a great part of the violet end is wholly cut off. Were it not such an enormous size, this would be a very dim star. ANTARIAN STARS are almost all very remote. They are often very faint, and many of them are unstable in their light, as if fluctuating towards extinction. β and δ² Gruis and γ Hydri are among them, and Mira, the wonderful variable star. Compare the ruddy γ in the Cross with pale δ to see the contrast between stars of this class and the Orion-type. Because of the want of blue rays in the former it makes so little impression on a photographic plate compared with α, β, and δ, which are all Orion stars, that one can scarcely recognise the form of the Cross in a photograph.

The brightest of these red stars are in the northern hemisphere, viz. Betelgueux in the shoulder of Orion, and Aldebaran, the eye of the Bull: both are slightly brighter than Antares.

Nearly all the stars we know have a place in this series, though there are individual peculiarities, but there is one class which seems to lie outside it. Stars of this class are red and have spectra crossed by dark bands, but they are unlike the Antarian bands and resemble instead those seen in the spectra of comets and of candle-flames. They are due to carbon compounds, so these stars may be called CARBON STARS. Most of them are extremely remote, and all are so faint that among the very brightest is U Hydrae, just visible to the naked eye a little east of Alphard. They are probably aged stars, but no links between them and the other types have yet been discovered to enable us to place them in the series.

Strange and interesting discoveries have been made by grouping large numbers of stars into their classes and comparing the average motions, distances, &c., of the groups. It is found that the redder stars are on an average moving more rapidly and in a more random fashion than blue and white stars. Thus, Wolf-Rayet and Orion stars have a low average speed, and both are very much more numerous in and near the Milky Way than elsewhere; Sirian stars are travelling a good deal faster, show a marked tendency to congregate in two streams, and move chiefly parallel with the plane of the Milky Way; solar stars, including our own sun, move more rapidly still and show less preference for the Galaxy and the two streams: Antarians have the most rapid motions of all, but these appear to be haphazard, and the stars are scattered all over the sky in every direction.

These facts are very unexpected and very difficult to explain. It looks as if the Milky Way were the birthplace of the stars, and that as they develop they gradually scatter through space; but how are we to explain the fact that speed and direction of movement differ for different types? There seems to be no reason why a cooler star should move more quickly than a hotter one, and none of the theories yet advanced can be considered final.

V

SOME NEAR NEIGHBOURS

If it is remarked that Sirius is fifty millions of millions of miles away from us, it is not at once obvious that he is one of our very near neighbours; but this is equal to 8 light-years, not twice the distance of α Centauri, our next-door neighbour among the stars. Some faint stars in the south must also be counted as very close to us: such are a little star in Cetus, τ Ceti, only 10 light-years away; ε Indi, 11½; and two in the River Eridanus, ε and δ Eridani, 10½ and 18 respectively.

But the most interesting among these near neighbours of the south is a little yellow star in Pictor, too faint to be seen without a good binocular or a telescope, and bearing the very modern name of CZ 5ʰ.243. This stands for Cordoba Zones 5.243 hours, and means that it was catalogued at Cordoba Observatory in South America and its position fixed in the fifth hour of Right Ascension. After this it was observed by Mr. Innes at the Cape, and he was startled to find (like Hipparchus comparing his work with that of Timocharis) that its position no longer agreed with that found at Cordoba. “Can this be motion?” he asked, and found that the star had indeed a larger visible motion across the sky than any other, not even excepting the famous “runaway star” in Ursa Major. Apparent motion, however, depends upon distance as well as real speed, and when the distance of CZ 5ʰ.243 had been calculated it was found that its real speed, amazing as it is, yet falls slightly short of that of the northern star. The rates are 163 and 174 miles per second.

A few other stars are known to move at speeds approaching 100 miles a second, and one was announced in December 1913 to have a velocity of 200 miles a second, but runaway stars are rare. They do not seem to show any preference for special parts of the sky or special stellar types, and it is impossible to say what causes them to rush with such headlong haste through space, or what is their goal. The average rate for a star is about 13 miles a second, but, as we have seen, it differs with different types, the average speed increasing progressively from the blue to the red classes.

Another remarkable fact lately discovered is that the different types of stars are not indiscriminately scattered through space. Our own sun seems to be surrounded, to a distance of about 100 light-years, by suns more or less like himself, while the greater number of the red Antarian stars lie at a much greater distance from us. Most distant of all, considered as a class, are the bluest of the Orion stars; for though the distances of individuals vary greatly, the average distance of these stars from us is more than 500 light-years. Yet they are often very bright, so this is another proof of their great intrinsic brilliancy.

VI

DOUBLE AND MULTIPLE STARS

Among the brighter stars at least one in four is double, and I shall only mention a few which for different reasons are of special interest.

The brightest double in the sky has already been mentioned, α Centauri, and we have also described another southern pair scarcely less brilliant, α Crucis, Sirius with his very dim companion, Rigel and Antares with theirs of contrasting colours. Other fine southern doubles are:

β Piscis australis, a white star with reddish companion, visible in a 3-inch telescope.

δ Corvi, an unequal distant pair, pale yellow and bluish, easily separated in a 4½-inch telescope.

σ Scorpii (near Antares), white and blue.

32 Eridani, yellow and blue-green—“magnifici, superbi,” according to Secchi.

β Capricorni (close to the splendid naked-eye double α Capricorni), orange-yellow and blue.

γ Crucis, orange-yellow, companion fifth magnitude, rather distant.

γ Leporis, companion crimson. There is also a third faint star, forming a triple group.

β Capricorni, like Antares, besides its visible companion, has a close invisible one, only known by the shifting of lines in the spectrum, and this is not an uncommon case. β Crucis consists similarly of two bright stars and a spectroscopic companion, and so also θ Eridani. κ Velorum and α Pavonis have spectroscopic companions only, one revolving in a period of 116½ days, the other of only 11¾ days, and the period of μ¹ Scorpii is counted in hours!—34 hours 42 minutes. The brief periods of these spectroscopic binaries[6] are in striking contrast with those of many visual binaries, such as ζ Sagittarii with a period of 19 years and γ Centauri with 150 years; and this is what one would expect, since the stars must be comparatively far apart and their orbits ample for them to be visible separately. Sometimes the stars of a pair or of a group are known to be moving together through space, though no movement of revolution round a common centre has yet been detected, probably because it is very slow.

[6] A “binary” is a system of two stars which are known to be comparatively close together and influencing one another’s movements. A “double star” may be a binary, or the two stars may really be very far apart and have no connection, merely happening to lie one nearly behind the other.

It is an interesting fact that most of the spectroscopic binaries, which are such close and rapid pairs, are found among the blue and white stars, the numbers steadily decreasing as we pass through the yellow to the red stars.

A significant fact about visual binaries is that companions which differ much in colour invariably differ much in brightness also. This is probably to be explained by supposing that one was from the beginning much larger than the other, and that there is consequently a difference in the rapidity with which each runs through its life-changes. Where the two are alike in colour and spectrum, like the two solar stars of α Centauri and the two Orion stars of α Crucis, it is found that they are also nearly equal in mass, so they keep the same pace and grow old together.

Among multiple stars there are some very remarkable instances in Scorpio. β Scorpii is a pair of bright stars (easily separated with a 3-inch telescope) with a third fainter companion, and besides these, one of the bright components is a spectroscopic pair, and the whole company is travelling together through space. The joint mass of the spectroscopic pair is twenty-one times as great as that of our sun, and they revolve about one another in seven days; but a very strange feature is that _some_ lines in their joint spectrum, due to calcium gas, behave differently from the rest, and it is thought that these two revolving stars may be enveloped in a calcium cloud which travels with them.

ξ Scorpii is a telescopic double which has been watched throughout a complete revolution of ninety-six years. It was discovered by Sir William Herschel in 1782. Here also there is a third star, much fainter and more distant than the brighter companion, and all are travelling together through space.

ν Scorpii is one of the “double-doubles,” of which a good many are known, where a star that looks single to the naked eye is seen as a pair with a telescope, and each of these becomes a pair with higher powers. It has been described as “perhaps the most beautiful quadruple in the heavens.” Both pairs journey together through the skies.

σ Orionis, the fourth-magnitude star just below Orion’s belt, separates very easily into two unequal components. Herschel found each of these to be triple, and called it a “double-treble.” Later it was found to be “double-quadruple,” with more stars between the two groups.

In the same way the beautiful naked-eye double star α Capricorni is seen in good telescopes to consist of two groups of stars, one (α¹) triple, the other (α²) quadruple.

If a group like this forms a connected system, the motions of the several stars must be highly complicated.

VII

THE ASTONISHING STAR, ETA ARGŪS