Part 4

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, 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.

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.

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.

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 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.

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.