Nebula

The density of the diffuse nebulae must be extremely low. Dynamical considerations suggest that in the case of the Orion nebula it cannot be greater than i/r,000,000,000,000,000 of the density of ordinary air; it may he much less. It is certainly a much more perfect vacuum than anything we can yet produce in our laboratories, yet so great is its size that it may contain enough matter to form thousands of stars as large as the sun. Planetary Nebulae.—The planetary nebulae form quite a small class, there being less than iso of them known; and, owing to the ease with which they can be recognised from their gaseous spectra, it is probable that we know of most that exist, and that they are therefore a comparatively rare form. A few of them are several minutes of arc in diameter, but the majority are so small that when viewed in the telescope they are indistinguishable from stars, except under a high power. They have the appearance of discs, round or nearly so in shape, with sharply defined edges and a faint star as nucleus. In many cases the disc is more luminous at its edge than in the centre, and the nebula has the appearance of a ring (Pl. I., figs. 3 and 4) ; whence they are sometimes called "annular nebulae." If they were actually discs, it is extremely unlikely that we should see them all as circles. Their planes would naturally be inclined at various angles to our line of sight, and thus the discs would appear to us as ellipses of various degrees of elongation. Their real form is clearly spherical or nearly so; what appear to us as rings must be spherical shells, and the beautiful structure of ring within ring seen in some of these nebulae must be in reality a series of shell within shell.

The central stars of these nebulae are the bluest and hottest known (spectral class 0), and there can be but little doubt that they are responsible for the light emitted by their nebulous en velopes, in a way similar to the excitation of luminosity in the diffuse gaseous nebulae by the hot stars involved in them. The planetary nebulae have as a class high radial velocities, averaging over 3okilom. a second, higher than those of the Class 0 stars, which are not connected with nebulae, and consequently higher than stars of any spectral class; a few have radial velocities ex ceeding iookilom. a second. Exact determination of their motions across the line of sight is not easy, but it is clear that they are small, and that the average distance of these nebulae must be at least several hundred light-years. They are thus large bodies, several hundred times as large as the solar system.

The spectroscope has revealed that those which appear slightly flattened in form are rotating, and this is probably true in gen eral. The periods of rotation must be reckoned in thousands of years. In some of them internal motions more complicated than

a simple rotation have been detected by the spectroscope. Their densities seem to be very low, though perhaps not quite so low as those of the diffuse nebulae. Notwithstanding this their masses must be as large as those of the brightest stars.

The question as to what place, if any, the planetary nebulae take in the scheme of stellar evolution is a difficult one, to which at present there is no certain answer. Being so few in number, they cannot be regarded as forming a regular link in any evolu tionary chain. It is possible that they are a later stage in the life history of the new stars (novae), which as they decline in bright ness develop spectra of the same type as have the nuclei of planetary nebulae, and are also then usually surrounded by faint nebulous envelopes.

The Gaseous Spectrum of Nebulae.

The spectra of all gaseous nebulae are very similar, the differences consisting chiefly in the relative intensities of the bright lines. The lines in the spectrum are sharp, indicating that it is due to a gas of low den sity. The lines of hydrogen are prominent, and those of helium are usually present ; but there are several lines, and among them are the strongest in the spectrum, which are so far unknown to us in our laboratories. These strong lines form a pair in the green region of the spectrum ( XX 5007, 4959) and a close pair in the ultra-violet (XX 3726, 3729). The unidentified lines were origi nally ascribed to an element, unknown on earth, to which the name "Nebulium" was given, but it now seems clear that they are due to known elements in a state which has not yet been produced in the laboratory. This unfamiliar state may well be the ex tremely low density existing in these nebulae, far beyond anything we can produce in our vacuum tubes. Whereas in the latter the mean interval between atomic collisions is never more than one thousandth of a second, in the nebulae it may be as much as several minutes or even hours. I. S. Bowen has recently from theoretical considerations accounted for all the chief lines of "nebulium" in this way, the two strongest pairs being ascribed to ionised and doubly ionised oxygen in a "metastable" state.

All the lines of the nebular spectrum are characteristic of a form of radiation which requires a great amount of energy for its excitation. The source of this energy cannot be in the nebulae themselves, for that would postulate for them an impossibly high temperature. A possible source is the bombardment of the atoms of the nebula by electrons emitted by very hot bodies, and such bodies are to be found in all gaseous nebulae—the stars of spectral classes 0 and B, the hottest bodies known to us.