Nebula

When stars appear in these dark spaces, it seems likely that in general they lie between us and the obscuring cloud, though there are undoubtedly cases where the stars are actually involved in it. In fact the dark nebulae are commonly found associated with bright stars and luminous nebulosity (diffuse nebulae), in such a way as to suggest irresistibly that they are physically connected and therefore at the same distance from us. The distances of some of these bright stars are known and are of the order of 400 light-years (a light-year is the distance that light travels in a year, about six million million miles). The dark nebulae con nected with them must therefore be equally distant and their actual dimensions consequently very great, the dark lane in Ophiuchus being for example about 6o light-years long by 5 light years wide. The region just south of the star Orionis (Pl. I., fig. 2) is a typical example of the association of dark nebula, diffuse nebula, and stars. The left hand part of the field is covered by an obscuring cloud, and there are relatively few stars to be found in it. This cloud is edged with luminous nebulosity, of which there are also patches round the brighter stars. Beyond its boundary on the right the normal number of stars for a Milky Way region can be seen.

These clouds may be composed of particles of all sizes from mere molecules to bodies of almost stellar mass, but it seems clear from, their obscuring power that they must contain a large amount of fine dust. For a given mass of matter the maximum obscuration is obtained when it is in the form of particles the circumferences of which are of the order of a wave-length of light (about one two-thousandth of a millimetre), and the whole of the obscuration of the large cloud in Ophiuchus could be pro duced by matter equal in mass to about 12 stars like our sun if it were in the form of minute particles of this optimum size.

Diffuse Nebulae.

These are similar to the dark nebulae, except that they are luminous, which fact enables us to learn a good deal more about them. Almost without exception they are found to have bright stars involved in them, and in many cases they are fringed with dark nebulosity. This suggests that the bright and dark nebulosities are parts of the same cloud, which the bright stars involved in it render luminous in their immediate vicinity. That this is at any rate sometimes the case was first suggested by the discovery by V. M. Slipher that the stars of the Pleiades and the nebulosity surrounding them had similar spectra.

More recently a systematic study of the diffuse nebulae by E. Hubble has brought to light a well-marked connection between the size of the luminous patch and the brightness of the star in volved in it, the stars being found to illuminate the clouds around them to a distance proportional to their brightness. The method by which the clouds are made to shine cannot be always that of simple reflection of the star's light, because in many cases the nebulae shine with a different kind of light from that of the stars.

Diffuse nebulae exist with both the continuous spectrum char acteristic of the stars and with the bright-line spectrum which we have learned to associate with a rarified gas.

Only when the star involved in the nebula is an extremely hot one (spectral type Br or earlier), does the nebula have a bright line spectrum. When the star responsible for its light is of a cooler type the spectrum of the nebula is continuous and, in gen eral, of the same type as that of the star. In this case the nebula may shine by reflection of the star's light. But the light of those nebulae which have gaseous spectra cannot be explained by simple reflection; it is necessary to postulate excitation of the dark cloud to luminosity in some other way (see the Gaseous Spectrum of Nebulae below). It appears then that a diffuse nebula is in gen eral just a part of a dark cloud which is made luminous by the action of a star.

It is possible to determine with considerable accuracy the radial velocities (velocity in the line of sight) of those diffuse nebulae which show a gaseous spectrum, because their light though faint is concentrated in a few bright spectral lines ; and it is found to be low, the average being about i i kilom. a second. -Owing, however, to the general absence in them of sharply defined points suitable for accurate measurement, it is a far more difficult matter to determine the motions, either of the nebula as a whole or of its constituent parts, across the line of sight. In fact, with the possible exception of the "Crab" nebula in Taurus, no change has been detected in the structure of a nebula. On looking for the first time at a photograph of such a nebula as the great one in the sword of Orion (Pl. I., fig. I) one might well suppose that the internal motions of its parts would cause changes that would soon make a conspicuous alteration in its appearance. But when one considers its great distance (about 600 light-years), it is evident that sensible change of form could not be caused by motion at any reasonable velocities during the comparatively few years that the nebula has been studied by photography.

The same is true of all the diffuse nebulae, except the rare variable nebulae, of which the most remarkable are those in Monoceros and Corona Austrina (Nos. 2261 and 6729 in Dreyer's New General Catalogue). These two nebulae vary rapidly in both form and brightness, distinct differences being discernible in a few days. They are both fan-shaped, and each has a variable star at the apex of the fan. It is almost certain that the variations are caused, not by motion of the matter forming the nebulae, but by changes in the illumination of stationary nebulous matter emanat ing from. the variable stars with which they are connected.