Thirty-two years later another stranger appeared and was seen by Kepler, who wrote a paper about it entitled “The New Star in the Foot of the Serpent.” It shone out in the same sudden manner and faded in the same leisurely way.
Since 1860 there have been several such apparitions, and since 1876 it has been possible to study them with the spectroscope, which has immensely increased our knowledge of their constitution. Indeed, this instrument of research has really opened our eyes to what they are. Nova Cygni, in 1876, Nova Aurigæ, in 1892, and Nova Persei, in 1901, besides several others found by Mrs. Fleming on the Arequipa plates, were excellent examples, and all agreed in their main features, showing that novæ constitute a type of stars by themselves, whose appearing in the first place and whose behavior afterwards prove them to have started from like cause and to have pursued parallel lines of development.
As a typical case we may review the history of Nova Aurigæ. On February 1, 1892, an anonymous post-card was received by Dr. Copeland of the Royal Observatory, Edinburgh, that read as follows: “Nova in Aurigæ. In Milky Way, about 2° south of χ Aurigæ, preceding 26 Aurigæ. Fifth magnitude slightly brighter than χ.” The observatory staff at once looked for the nova and easily found it with an opera glass. They then examined it through a prism placed before their 24-inch reflector and found its spectrum. It proved to be that of a “blaze star.”
Dr. Thomas D. Anderson turned out to be the writer of the anonymous post-card—his name modestly self-obliterated by the nova’s light. He had detected the star on January 24, but had only verified it as a new one on the 31st. Harvard College Observatory then looked up its archived plates. The plates showed that it had appeared sometime between December 1 and 10. Its maximum had been attained on December 20, after which it declined, to record apparently another maximum on February 3 of the 3.5 magnitude. From this time its light steadily waned till on April 1 it was only of the 16th magnitude or ¹/₁₀₀₀₀₀ of what it had been. In August it brightened again and then waned once more.
Meanwhile its spectrum underwent equally strange fluctuations. At first it exhibited the bright lines characteristic of the flaming red solar prominences, the calcium, hydrogen, and helium lines flanked by their dark correlatives upon a continuous background, showing that both glowing and cooler gases were here concerned. The sodium lines, too, appeared, like those that come out in comets as they approach the furnace of the Sun. An outburst such as occurs in miniature in the solar chromosphere or outermost gaseous layer of the Sun was here going on upon a gigantic scale. A veritable spectral chaos next supervened, staying until the star had practically faded away. Then, on its reappearance, in August, Holden, Schaeberle, and Campbell discovered to their surprise not what had been at all, but something utterly new: the soberly bright lines only of a nebula. Finally, ten years later, January, 1902, Campbell found its spectrum had become continuous, the body having reverted to the condition of a star.
Now how are we to interpret these grandiose vicissitudes, visually and spectrally revealed? That we witnessed some great catastrophe is clear. The sudden increase of light of many thousand fold from invisibility to prominence shows that a tremendous cataclysm occurred. The bright lines in the spectrum confirm it and imply that vast upheavals like those that shake the Sun were there in progress, but on so stupendous a scale that, if for no other reason, we must dismiss the idea that explosions alone can possibly be concerned. The dark correlatives of the bright lines have been interpreted as indicating that two bodies were concerned, each travelling at velocities of hundreds of miles a second. But in Nova Aurigæ shiftings of the spectral lines implying six bodies at least were recorded, if such be attributed to motion in the line of sight, and Vogel was minded to throw in a few planets as well—as Miss Clerke pithily puts it. There is not room for so many on the stage of the cosmic drama. Other causes, as we now know, may also displace the spectral lines. Great pressure has been shown to do it, thanks to the labors of Humphreys and Mohler at Baltimore. “Anomalous refraction” may do it, as Professor Julius of Utrecht has found out. Finally, changes of density may produce it, as Michelson has discovered. To these causes we may confidently ascribe most of the shiftings in the stellar spectrum, for just such forces must be there at work.
Mr. Monck suggested the idea that new stars are the result of old dark stars rushing through gaseous fields in space and rendered luminous by the encounter. Seeliger revived and developed this idea, which in certain cases is undoubtedly the truth. Probably this occurred to the new star of 1885 which suddenly blazed out almost in the centre of the great nebula in Andromeda. It behaved like a typical nova and in due course faded to indistinguishability. Something like it happened, too, in the nova of 1860, which suddenly flared up in the star cluster 80 Messier, outdoing in lustre the cluster itself, and then, too, faded away.
But just as psychology teaches us that not only do we cry because we are sorrowful, but that we are sorrowful because we cry, so while a nova may be made by a nebula, no less may a nebula be made by a star.
Let us see how this might be brought about and what sign manuals it would present. Suppose that the two bodies actually grazed. Then the disruption would affect the star’s cuticle, first raising the outer parts, consisting rather of carbon than of the metals, since that substance is the lighter, to intense heat and the gases about it at the same time. The glowing carbon would be intensely bright, and at first its light would overpower that from the gases, and not till its great glow had partially subsided would theirs be seen. Then the gases, hydrogen, helium, and so forth, would make themselves evident. Finally only the most tenuous ones, those peculiar to a nebula, would remain visible. After which the more solid particles due to the disruption would fall together and light up again by their individual collisions. Much the same would result if without striking the stars passed close.