Part 5

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

Midway between the Southern Cross and the False Cross there is a nebula visible to the naked eye, and in it once shone a bright star. When Halley was observing in the southern hemisphere towards the end of the seventeenth century, he catalogued it as of fourth magnitude, but Lacaille and later astronomers marked it as second. Sir John Herschel first saw it in 1834, when he was at the Cape, and he says that it remained steady for three years, from 1834 to 1837. On the 16th of December 1837 he began his observations as usual by noting the brightest stars in the heavens and arranging them in order on a list, when to his astonishment he saw “a new candidate for distinction among the very brightest stars of first magnitude” in a part of the sky where he was quite sure no such brilliant object had been seen before. He consulted a map and satisfied himself that it was his “old friend Eta Argūs,” but nearly three times as bright as usual. He made careful comparisons with other bright stars then visible, and says that “Fomalhaut and Alpha Gruis were at the time not quite so high, and Alpha Crucis much lower, but all were fine and clear, and Eta Argūs would not bear to be lowered to their standard.” It was a little brighter than Rigel, and the only stars which outshone it were Sirius and Canopus.

Still it grew brighter, for twelve days later it greatly surpassed Rigel and could only be compared with α Centauri. After this the light began to fade, and by April 1838 it was not much brighter than Aldebaran.

Herschel now returned to England, and therefore he did not see the still more startling changes of this wonderful star, but he has recorded what he heard from others. In March 1843 it became much brighter than Rigel or α Centauri, but its light wavered, and he says: “We have here an epoch of great interest, a temporary minimum, with a kind of trepidation or fluttering of light, followed, however, by another step in advance even yet more extraordinary.” This was in the following month, April 1843, when Eta became almost equal to Sirius, the brightest of all stars. It was the highest point reached by this extraordinary star, and two years later Maclear at the Cape wrote to the Astronomer-Royal in England: “When you see Sir John Herschel again, tell him that Eta Argūs has been for some time rather larger than Canopus, and seems again on the decline.”

Sir John’s concluding remarks seem to indicate something of pained surprise that a star could behave in so erratic and unaccountable a fashion: “A strange field of speculation is opened by this phenomenon. Here we have a star fitfully variable to an astonishing extent, and whose fluctuations extend over centuries.... What origin are we to ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so uncertain a source?”

Eta Argūs continued to fade, and for many years it has not been visible to the naked eye. When the present writer looked for it in April 1908 it was beyond the power of the binocular, although seventh-magnitude stars in the neighbourhood were clearly distinguished and identified. In a telescope it in no way stands out from the crowd of small stars scattered over the nebula whose light it once almost blotted out by its brilliance. Reports now and then arise that Eta is brightening again, but it always turns out that some neighbour in the throng, a little brighter than the faded star, has been mistaken for it.

A few other cases are known in which a bright star has appeared where none had been seen before. It is said that it was the appearance of such a “new star” in Scorpio in the year 136 B.C. which led Hipparchus of Rhodes to draw up his famous star-catalogue. In A.D. 1572 “Tycho’s star” blazed out in Cassiopeia, and in 1604 “Kepler’s star” in Ophiuchus astonished everyone. The old chronicle says: “It was exactly like one of the stars, except that in the vividness of its lustre and the quickness of its sparkling it exceeded anything Kepler had ever seen before. It was every moment changing into some of the colours of the rainbow, as yellow, orange, purple, and red, though it was generally white when it was at some distance from the vapours of the horizon.” This “new star” must have been even brighter than Eta Argūs, for it outshone Jupiter, and was only surpassed by Venus. It remained visible for about a year and then vanished.

Now often called Eta Carinae, since Argo has been subdivided (see p. 7).

Since the skies have been more carefully watched by astronomers all over the world, and especially since they have been frequently photographed, quite a considerable number of new stars have been recorded, and about half a dozen have become visible to the naked eye. In the twenty-seven years between 1885 and 1912, twenty were recorded, and half of these were discovered by Mrs. Fleming of Harvard Observatory from the examination of photographs.

Astronomers are still asking, like Herschel, what is the origin of these mysterious objects? Are they literally new stars, or is it the last flare-up of a dying system, or are we witnessing some catastrophe which only overtakes a few suns among the universe of stars? A collision between two dark stars, or between a star and a nebula, is a supposition which naturally suggests itself, and some probability is lent to this supposition by the fact that nearly every new star has appeared in or near the Milky Way, where stars throng most thickly; but there are difficulties in the way of accepting this hypothesis. There is a strong likeness between all that have been examined spectroscopically, and in the declining stage they become so distinctly nebular in type that we seem justified in saying that new stars change into small gaseous nebulae.

Does this mean that they are dying, or is it the first stage in the life-history of a star, immediately preceding the not altogether dissimilar Wolf-Rayet stage? We do not know enough yet about nebulae to answer this question.

VIII

MIRA, THE WONDERFUL STAR

The head of Cetus, the Sea-Monster, is formed of three stars in a crooked line (α, γ, δ); and a little beyond them, as far from δ as that is from α, you may sometimes see another star, marked on the map by the name “Mira,” which means “Wonderful.” Watch it carefully, and if it is on the upward grade you will see it slowly brighten until it equals δ, then γ, and if you are lucky it may even approach α in brightness; and meanwhile it will pass from red to a clear orange-yellow; then it will wane once more and gradually be lost to view, though you can follow it much longer in even a small opera-glass, and you will notice that as it grows fainter the colour becomes deeper and deeper crimson.

Unlike η Argūs with its one brilliant phase in two centuries, Mira waxes and wanes once in every eleven months, although there is a capricious uncertainty in both the period and the brightness which makes her a most fascinating object to observe. Sometimes the maximum brightness is several days earlier, sometimes later, than the average; sometimes she only equals δ, she has been seen to excel α; and no one can foretell exactly what and when her maximum will be.

Quite a number of other stars have been discovered which behave like Mira, and anyone who wishes to contribute something to astronomical research without having to buy large and expensive instruments, or to study difficult problems, cannot do better than observe some of these stars, carefully comparing them night after night with stars in the neighbourhood. Here is a list of a few southern “variable stars of long period,” as they are called, all of which are easily visible at maximum brightness with a binocular, and some even without. A map should be made of the surrounding stars, and a list drawn up of those which are of the different magnitudes through which the variable passes. Every fine night the star should be compared with these, and recorded in a note-book as brighter than one, fainter than others, perhaps equal to another, and so on, several comparisons being made to check each other. When the variable passes out of the range of the binocular, this should be noted. The British Astronomical Association, which has a branch in Australia, has a Variable-star Section, and anyone who becomes a member will receive ready help and advice, and may have the pleasure of feeling that he is doing useful work in astronomy.

Most long-period variables are red stars of the Antarian or of the carbon star classes with banded spectra, but they differ from ordinary red stars by occasionally showing bright lines, which indicate an uprush of intensely hot hydrogen gas in their atmospheres. These bright lines always appear at times of maximum, and prove that the star periodically undergoes some physical change: but what is the nature and the cause of this change? It can scarcely be due to the near approach of a satellite, because of the irregularity in the time of maximum. There is a certain resemblance between the way in which the light waxes and wanes and the waxing and waning in the number of sunspots on the sun, and spectra of these Mira stars also somewhat resemble sunspot spectra: can it be that our sun is an incipient variable star with a period of about eleven years? It is true that sunspots seem to be cooler rather than hotter regions on the sun, but a time of maximum spots is also a time of maximum activity; slightly more heat is actually radiating, the corona is brighter and larger, and the bright scarlet flames of hydrogen and calcium which we call prominences are larger and more abundant. Possibly it is a tremendous display of these on Mira which makes the hydrogen lines bright at maximum.

SOME SOUTHERN VARIABLE STARS OF LONG PERIOD

+------------+----------+----------+---------+----------------------+ | Name. | Maximum | Minimum | Period. | Remarks. | | |Magnitude.|Magnitude.| | | +------------+----------+----------+---------+----------------------+ | | | | Days. | | |Mira | 1.7 | 9.6 | 331½ | Discovered in | | | | | |1596. Large irregular-| | | | | |ities in period | | | | | |and max. brightness. | | | | | | | |R Sculptoris| 6.2 | 8.8 | 376½ | A red “carbon | | | | | |star.” | | | | | | | |R Leporis | 6.1 | 9.7 | 436 | Crimson. A | | | | | |“carbon star.” | | | | | | Observed from 1852 | | | | | |to 1883, but very | | | | | |few observations | | | | | |published since, so | | | | | |new work would | | | | | |be valuable. | | | | | | | |T Centauri | 6.5 | 8.0 | 90¼ | Period unusually | | | | | |short for this | | | | | |class of variables. | | | | | | | |R Centauri | 5.3 | 13.0 | 568 | An Antarian | | | | | |star. Double max. | | | | | |and min. The | | | | | |secondary min. | | | | | |about mag. 8½. | | | | | | | |R Scuti | 4.8 | 7.8 | ? | Bright and faint | | | | | |minima, usually | | | | | |alternative. Period | | | | | |perhaps irregular. | | | | | | | |S Sculptoris| 5.8 | 11.8? | 366 | | |R Horologii | 5.9 | 12.0 | 405 | | |L² Puppis | 3.4 | 6.2 | 140 | | | | | |or longer| | |R Carinae | 4.5 | 10.0 | 309 |{ Antarian stars. | | | | | |{ Spectra very similar| |S Carinae | 5.8 | 9.0 | 148 |{ to that of Mira. | |R Hydrae | 4.0 | 9.8 | 425 | | |S Virginis | 5.6 | 12.3 | 379½ | | |R Aquarii | 6.2 | 11.0 | 387 | | +------------+----------+----------+---------+----------------------+

Against this suggestion we must set the fact that no stars have been found to link together the slow and slight change represented by the sunspot period with the rapid and violent change suffered by Mira variables. Their periods, though never less than three months, are never more than two years, and the light radiated by Mira at maximum sometimes amounts to five thousand times as much as at a faint minimum.