STRANGE NEWS FROM THE AUSTRALIAN SKIES. TORE than a year
ago a discovery was announced by an LYE astronomer in the Southern hemisphere which seemed so strange and so perplexing, that Sir John Herschel, commenting on it, remarked "that no phenomenon in astronomy had yet turned up presenting anything like the same interest, or calculated to raise so many and such momentous points for inquiry and speculation." One of those mysterious nebulous masses which astronomers had been in the habit of regarding as galaxies, resembling in extent and magnificence the sidereal scheme to which the sun belongs, seemed to be undergoing a most astounding series of changes. During these winter nights, when Orion shines with full glory, the famous nebula which clings around his pendant sword presents to our northern observers an object similar to the nebula in question. Every one has heard of the strange interest which attaches to this Orion nebula, of the mysterious far-reaching arms which extend from it, the dark central vacancy, and the brilliant array of stars which the six- feet mirror of Lord Rosse has brought into view in the very heart of the nebula. But in the Southern skies there is an object of the same class even more glorious and more mysterious. In the richest part of the southern heavens, a part so rich indeed that, according to the argument of a well-known astronomer, the splen- dour of the constellations comprised in it illumines the heavens as a new moon would, there lies the great nebula known among astronomers as "the Nebula in Argo." The Orion nebula can only be seen on the darkest nights, but the great Argo nebula shines as brilliantly as a third-magnitude star, and is scarcely obliterated even by the effulgence of the full moon. It is, in fact, the most splendid nebula in the whole heavens. Yet this glorious object, whose contemplation has led our most thoughtful astronomers to form new ideas of the grandeur of the universe, whose dimensions seemed immeasurable by any unit of length men could devise, the whole of this magnificent nebula, is drifting about like a cloud before a shifting wind.
For the news which seemed so surprising to Sir John Herschel bas just been confirmed by the revelations of a new telescope of enormous power. The news had come, first of all, from a small telescope,—only five inches, indeed, in aperture ; and it seemed quite possible that the weakness of this instrument (compared with the 19-inch reflector used by Sir John Herschel during his survey -of the southern heavens) might have led to an erroneous impres- sion of change. But now the new four-feet mirror is at work among the southern stars. Surpassed only by the Rosse reflector and matched only by the fine reflector with which Lassell is sur- veying the heavens at Malta, the great Melbourne reflector is about to place our knowledge of the Southern heavens nearly on the same footing as that we possess respecting the Northern .stars. And if the work to be done by this great reflector in after ,years is shadowed forth by its first great exploit, we may well look eagerly forward for the discoveries it will effect.
Sir John Herschel had said, a year and more ago, that the strange inquiries suggested by the news then lately received about the Argo nebula "must be settled." We cannot do better than use the ipsissima rerba of the great astronomer :—" The question," he said, "is not one of minute variations of subordinate features, which may or may not be attributable to differences of optical power in the instruments used by different observers, as in the case of the Orion nebula, but of a total change of form and character— a complete subversion of all the greater and most striking features —accompanied with an amount of relative movement between the star and the nebula, and of the brighter portions of the latter inter se, which reminds us more of the capricious changes of form and place in a cloud drifted by the wind, than of anything hereto- lore witnessed in the sidereal heavens."
Urged on, doubtless, by the importance thus attached to the question by the greatest astronomer of the day, Mr. Le Sueur turned the newly-mounted reflector to the great nebula. The result is now before us. There seems no longer the least room to doubt that the nebula has changed in the most marvellous manner since Sir John Herschel, a third of a century ago, mapped its most striking features. The stars which are strewn over the nebula, and which had been spoken of by Sir John Herschel as probably much nearer to us, have remained unchanged in position, and with one exception have not changed much in relative brilliancy. So that Mr. Le Sueur has been led to form the opinion that the nebula is much nearer to us than the stars,—a view clearly tending to diminish our ideas of the real dimensions of the nebula, and so rendering the observed changes somewhat leas astounding than they otherwise would be. Forbearing to specu- late, as, indeed, we have no means of forming an opinion, about the physical causes to which these marvellous changes may be due, let us consider a little the conclusion to which Mr. Le Sueur has been drawn.
Because the stars seen with the nebula have remained un- changed while the nebula itself has shifted about so strangely, the opinion is suggested, says Mr. Le Sueur, that the nebula and the stars are in no way associated. And certainly one would expect to find the changes of the nebula accompanied by very remarkable changes in the star-group, if there were any bond of association between one and the other. Changes more remarkable perhaps than have been noticed in any other part of the sideral heavens might be looked for.
What, however, if this were actually the case, despite the fixity observed among the stars examined by Le Sueur ? We have spoken of one exception to the constancy of these stars in bright- ness, what if that exception should be more than sufficient of itself to compensate for the fixity of the other stars?
The star that has changed is the famous Eta Argils, the most wonderful star in the whole heavens, and only surpassed in interest by one object,—the Nebula in the midst of which it is situated.
It was marked in Halley's catalogue as a fourth-magnitude star ; in Lacaille's as of the second magnitude ; in 1843 it surpassed every star in the heavens in brilliancy except the Dog Star ;—at present, it cannot be seen with the naked eye. When Sir John Herschel was at the Cape the star was nearly at its brightest, and then the nebula could not be seen even on the darkest night. Now, when the star is invisible, the nebula shines with a lustre which renders it visible before the third-magnitude stars come out upon the evening skies ; and probably its brilliancy, if not diffused, would make it appear as soon each evening as though it were a first-magnitude star.
Is not the idea suggested that these interchanges of light are not merely apparent and fortuitous ? that in some way Eta Argus has given up its brilliancy to the surrounding nebula, to resume it again when the cycle of its changes is approaching completion? We have every reason to believe that the star's variations are really periodical, though the cycle of changes is exceedingly com- plex; indeed, astronomers have already indicated the close of the present century as the epoch when the star will have resumed its full splendour.
If this view is correct, the nebula must be looked upon as neither beyond the star-group, as Sir John Herschel surmised, nor nearer to us than that group, as Le Sueur thinks, but as mixed up with it. This view is not a new one. In March, 1868, long before the news had been received that the nebula is changing in figure, the author of this paper, in an article which appeared in the Student for that month, pointed out that there is an obvious connection between the nebula as depicted by Sir J. Herschel and the fixed stars seen in the same field. "There is not," be remarked, "a single remarkable condensation or projection in the nebula which is not marked by bright or clustering stars,—by stars which appear clearly to be leading stars, and there are not ten (out of some hundreds entered), whose influence on the nebula is not clearly discernible." If, now that the nebula has assumed a new figure, the same sort of connection should be observable, no doubt can any longer remain that this nebula (and therefore presumably every other nebula of the same type) is associated with the stars which are seen in the same field of view with it.
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