THE NEBULÆ AND SWARMS OF SUNS.—J. E. Gore
We will now consider the nebulæ, properly so called, that is to say, objects which the spectroscope shows to consist of glowing gas. These are sometimes large and irregular in form, like the great nebula in the “Sword” of Orion, sometimes with spiral convolutions, and sometimes of a definite shape, like the planetary and annular nebulæ.
Of the large and irregular nebulæ, one of the most remarkable is that known as “the great nebula in Orion.” It surrounds the multiple star, Theta Orionis. It is a curious fact that it escaped the searching eye of Galileo, although he gave special attention to the constellation of Orion, for even with a good opera-glass a nebulous gleam is distinctly visible round the central star of the “Sword.” The nebula seems to have been discovered by Cysat, a Swiss astronomer, in the year 1618, and it was sketched by Huygens in 1656. It has been called the “fish-mouth” nebula, from the fancied resemblance of the centre portion to the mouth of a fish. A number of small stars are visible over the surface of the nebula, and at one time Lord Rosse thought it showed indications of resolution into stars when examined with his giant telescope; but this is now known to have been a mistake, for Dr. Huggins finds, with the spectroscope, that it consists of nothing but glowing gas.
The brightest line in the nebular spectrum—the “chief nebular line,” as it is called—has not yet been identified with that of any terrestrial substance.
Mr. W. H. Pickering and Dr. Max Wolf have photographed another nebula surrounding the star Zeta Orionis—the southern star of the “Belt,” which seems to be connected with the nebula in the “Sword”; and Professor Barnard, using the “lens of a cheap oil lantern” of 1½ inches aperture and 3½ inches focal length, has photographed “an enormous curved nebulosity” stretching over nearly the whole of the constellation of Orion, and involving the “great nebula.”
Professor Keeler found, with the spectroscope, that the Orion nebula is apparently receding from the earth at the rate of nearly eleven miles a second, but this motion may be, in part at least, due to the sun’s motion in space in the opposite direction. Professor Pickering considers that the parallax of the nebula is probably not more than 0.″003, which corresponds to a thousand years’ journey for light!
In the southern constellation, Argo is a magnificent nebula, somewhat similar in appearance to the great nebula in Orion. It surrounds the famous variable star Eta Argûs. It is sometimes spoken of as the “keyhole” nebula, owing to a curious opening of that shape near its centre. It was carefully drawn by Sir John Herschel at the Cape of Good Hope in the years 1834-38. It lies in a very brilliant portion of the Milky Way, and Sir John Herschel thus describes it: “It is not easy for language to convey a full impression of the beauty and sublimity of the spectacle which the nebula offers as it enters the field of view of a telescope, fixed in right ascension, by the diurnal motion, ushered in as it is by so glorious and innumerable a procession of stars, to which it forms a sort of climax, and in a part of the heavens otherwise full of interest,” and he adds: “In no part of its extent does this nebula show any appearance of resolvability into stars, being, in this respect, analogous to the nebula of Orion. It has, therefore, nothing in common with the Milky Way, on the ground of which we see it projected, and may therefore be, and not improbably is, placed at an immeasurable distance behind that stratum.” Sir John Herschel’s conclusion as to its physical constitution has been fully confirmed by the spectroscope, which shows it to consist of luminous gas. As in the Orion nebula, there are numerous stars scattered over it. Some of these may possibly have a physical connection with the nebula, while others may belong to the Milky Way. The nebula is of great extent, covering an apparent space about five times the area of the full moon, and its real dimensions must be enormous. It was photographed by Mr. Russell, director of the Sydney Observatory, in July, 1890, and the photograph shows that “one of the brightest and most conspicuous parts of the nebula”—the swan-shaped form near the centre of Herschel’s drawing—has “wholly disappeared,” and its place is now occupied by “a great, dark oval.” Mr. Russell first missed the vanished portion of the nebula in the year 1871, while examining it with a telescope of 11½ inches aperture, and the photograph now confirms the disappearance, which is very remarkable, and shows that changes are actually in progress in these wonderful nebulæ, changes which may be detected after a comparatively short interval of time.
Smaller than the nebula in Argo, but somewhat similar in general appearance, is that known as 30 Doradus, which forms one of the numerous and diverse objects which together constitute the greater Magellanic Cloud. Sir John Herschel drew it carefully at the Cape of Good Hope, and describes it as “one of the most singular and extraordinary objects which the heavens present,” and he says “it is unique even in the system to which it belongs, there being no other object in either nubecula to which it bears the least resemblance.” It is sometimes called the “looped nebula,” from the curious openings it contains. One of these is somewhat similar to the “key-hole” opening in the Argo nebula. Near its centre is a small cluster of stars, and scattered over the nebula are many faint stars, of which Sir John Herschel gives a catalogue of 105, ranging from the ninth to the seventeenth magnitude. I do not know whether this nebula has been examined with the spectroscope, but its appearance would suggest that it is gaseous. It is remarkable as being the only object of its class which is found outside the zone of the Milky Way.
Among the nebulæ of irregular shape, although its spectrum is said to be not gaseous, may be mentioned that known as the “trifid nebula,” or 20 Messier. It lies closely north of the star 4 Sagittarii in a magnificent region of the heavens. In the drawing made by Sir John Herschel at the Cape of Good Hope, the principal portion consists of three masses of nebulous matter separated by dark “lanes” or “rifts.” Near the junction of the three “rifts” is a triple star. A beautiful drawing of this nebula has also been made by Trouvelot. It agrees fairly well with that of Sir John Herschel, but shows more detail.