Stars of the southern skies

by M. A. Orr

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,[7] for it outshone Jupiter, and was only surpassed by Venus. It remained visible for about a year and then vanished.

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.