Page:EB1911 - Volume 19.djvu/351

 Of planetary nebulae one of the best known is the “owl nebula” in the Great Bear about midway between “the pointers.” As seen with Lord Rosse’s reflector, it presented a startling appearance, resembling the face of a goblin; two faint stars shone in the centres of the two dark circles which represented the saucer-eyes of the creature. Some change has certainly taken place since then, for the two stars no longer could be supposed to represent the pupils of the eyes; the cause may, however, be merely the proper motion of the stars or of the nebula.

The discovery of great regions having a faint nebulous background is one of the most remarkable results of modern work. Particularly interesting is the fact that, whilst the large telescopes are unable to render them perceptible to the eye or to photograph them, they are revealed by what at first sight seems an absurdly simple apparatus. For the study of the ordinary nebulae large reflecting telescopes (preferably of short focal length) are used, the great light-gathering power being all important; but for photographing these diffused nebulosities portrait lenses of very small aperture and focal length are most successful. Thus the great extension of the Orion nebula was photographed by W. H. Pickering in 1890 with a lens 2·6 in. in aperture and of 8·6 in. focal length; the exposure was rather more than six hours. Other extensive nebulous regions of a similar character have been found by Barnard in the constellations Ophiuchus, Scorpio and Taurus.

Spectra of Nebulae.—Owing to the feebleness of their light the study of the spectra of nebulae is one of particular difficulty. Two varieties of spectra are recognized; the one consists of a few narrow bright lines with sometimes a faint continuous spectrum for a background; the other consists of a continuous spectrum crossed by dark lines and is indistinguishable from that of ordinary stars. The former variety unmistakably shows that the light proceeds from diffuse incandescent vapour; nebulae showing this spectrum are accordingly called “gaseous.” Irregular, annular and planetary nebulae are of this nature. The visual spectrum is marked by three bright lines in the blue and green of wave-lengths 5007, 4959 and 4861. Of these the last is the line H of the hydrogen series; the other two are of unknown origin, and as they are always found together and have always the same relative intensity, they have both been attributed to the same unknown element, which has been named “nebulium.” Usually there are no other conspicuous lines in the visual spectrum, but in the ultra-violet region numerous lines can be photographed, including most of the hydrogen series. The yellow line (D3) of helium can be detected in many nebulae. The great majority of the nebulae, however, show the second variety of spectrum, and are thus indistinguishable spectroscopically from irresolvable star-clusters. The great nebula of Andromeda and the spiral nebulae are of this kind. It is not necessary to conclude that they, therefore, are star-clusters whose components are, owing to their remoteness from us, too faint and close together to be separately distinguishable. A gaseous mass only gives a bright line spectrum when it is so rarefied as to be transparent through and through. If the density and thickness are such that a ray of light cannot pass through it the spectrum will, in general, be continuous like that of a solid body.

The inquiry into the physical state and constitution of the nebulae raises problems of great difficulty. In the case of “gaseous” nebulae it is very hard to understand how such extremely tenuous masses are maintained in a state of incandescence. Only one theory has been put forward which at all accounts for this fact, and unfortunately, it is not altogether satisfactory in other respects. This is Sir Norman Lockyer’s “Meteoritic Hypothesis,” which attributes the light to collisions between numbers of small discrete solid particles, these being vaporized and made luminous owing to the heat developed by their impacts. Formidable difficulties, however, prevent the entire acceptance of this suggestion. The spiral nebulae are not distributed at random over the sky, nor are they condensed along the galactic plane like the clusters which they spectroscopically resemble. There is a well-marked centre of aggregation of the northern nebulae near the north galactic pole. In the southern hemisphere they are more evenly distributed, but the avoidance of the galactic plane is marked. The remarkable Nubeculae or Magellanic Clouds in the southern hemisphere, which look like detached portions of the Milky Way, are found on telescopic examination to consist, not of stars alone, like the Milky Way, but of stars and nebulae clustering together. In the greater cloud Sir John Herschel counted 286 nebulae; in the lesser cloud they are rather less numerous.

NEBULAR THEORY, a theory advanced to account for the origin of the solar system. It is emphatically a speculation; it cannot be demonstrated by observation or established by mathematical calculation. Yet the boldness and the splendour of the nebular theory have always given it a dignity not usually attached to a doctrine which from the very nature of the case can have but little direct evidence in its favour.

There are very remarkable features in the solar system which point unmistakably to some common origin of many of the different bodies which it contains. We may at once put the comets out of view. It does not appear that they bear any testimony on either side of the question. We do not know whether the comets are really indigenous to the solar system or whether they may not be merely imported into the system from the depths of space. Even if the comets be indigenous to the system, they may, as many suppose, be merely ejections from the sun. In any case the orbits of comets are exposed to such tremendous perturbations from the planets that it is unsafe from the present orbit of a comet to conjecture what that orbit may have been in remote antiquity. On these grounds we discuss the nebular theory without much reference to comets. But even after the omission of all cometary objects we can still count in the solar system upwards of five hundred bodies, almost every one of which pronounces distinctly, though with varying emphasis, in favour of the nebular theory.

The first great fact to be noticed is that the planets revolve around the sun in the same direction. This is true not only of the major planets Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune; it is also true of the host of more than five hundred minor planets. It is also remarkable that all the great planets and many of the small ones have their orbits very nearly in the same plane, and nearly circular in form. Viewed as a question in probabilities, we calculate the chance that five hundred bodies revolving round the sun shall all be moving in the same direction. The improbability of such an arrangement is enormously great. It is represented by the ratio of a number containing about a hundred and sixty figures to unity, and so we are at once forced to the conclusion that this remarkable feature of the planetary motions must have some physical explanation. In a minor degree this conclusion is strengthened by observing the satellites. Discarding those of Uranus, in which the orbits of the satellites are highly inclined to the ecliptic, and in which manifestly some exceptional influences have been at work, we find that the satellites revolve around the primaries also in the same direction; while, to make the argument complete, the planets, so far as they can be observed, rotate on their axes in the same manner.

The nebular theory offers an explanation of this most remarkable uniformity. Laplace supposed the existence of a primeval