THE EXTRA-GALACTIC NEBULAE General Characteristics.—The extra-galactic nebulae are found scattered all over the sky except in the Milky Way itself, which they rigidly avoid. This characteristic is, however, by no means the only thing that differentiates them from the galactic nebulae, and the most casual study soon shows them to be objects of quite a different nature. They vary in apparent size from the great spiral nebula in Andromeda, which is more than 2° long, to objects so small that only in the largest telescopes can they be distinguished from stars. Of these small nebulae there are sev eral hundred thousand within our reach. They are too minute to show any structure, but of the larger objects a few are irregu lar in shape and bear some resemblance to the Magellanic Clouds, but in general the extra-galactic nebulae are remarkable for their symmetry of form. They appear as circular or elliptical objects and form a progressive sequence, from the globular or elliptical, nebulae, which are quite structureless, to the spiral nebulae, which show spiral structure in various degrees of development.
The globular nebulae show no signs of resolution into separate stars even in the most powerful telescopes. They get gradually brighter towards the centre and have no well defined edge, as have the planetary nebulae. They are found with all degrees of elonga tion from circular to spindle shapes, but it seems from the fre quency of the circular form that many of them must be actually spherical or nearly so, though the majority may be lenticular in form. An example of a globular nebula can be seen in the lower left hand corner of the plate of the spiral nebula in Andromeda.
The chief characteristic of the typical spiral nebula is the emergence from opposite sides of its central nucleus of two spiral arms which wind themselves round it. Their forms are many and varied, some being very complicated and all very beautiful; but most of them can be graded into a sequence, with objects at one end differing but little from the structureless globular nebulae, and at the other end nebulae with highly developed spiral arms.
At the beginning of this sequence the nucleus has all the charac teristics of a globular nebula, but as progress is made along the sequence the nucleus becomes smaller and sharper in outline until at the other end it is almost stellar. The spiral arms, however, develop; unwinding themselves further and becoming more and more studded with condensations, which at the far end of the sequence can be resolved with the most powerful telescopes into separate stars.
It is tempting to suppose that this sequence gives us a picture of an evolutionary process, and that the spiral nebula actually develops from the structureless globular nebula to a final state of clusters of stars by the formation of spiral arms and the continued projection of the matter originally forming the globular nebula, or nucleus, along them. That such is a possible process in the case of a lenticular revolving mass, such as the globular nebula may be supposed to be, has been shown by J. H. Jeans, who by mathe matical reasoning has traced out the whole sequence ; how the nearly spherical and slowly rotating globular nebula spins faster as it contracts, and thus becomes more flattened; how the tides raised in it on opposite sides of its equator by some neighbouring nebula form protuberances, which are impelled forth as spiral arms; and how these arms as they emerge wind round the central mass and are disintegrated into groups of condensations which ultimately become stars. The process must be an extremely slow one as we reckon time, involving millions of millions of years, and we must not therefore expect to see in a lifetime much change in the form of a particular nebula. However, the spectroscope has shown that they are in rapid rotation, and, provided that they are not more distant from us than the galactic nebulae, it should be possible by comparing photographs taken only a few years apart to detect movements of the condensations in those nebulae which are turned so as to give us an open view of them.