And indeed there can be but little doubt that the process we have just been considering is that of the birth of stars. Even a casual glance at photographs of nebulae suffices to shew that the matter which has been ejected into the equatorial plane of a nebula does not lie uniformly spread out in that plane; it is seen to have fallen into bunches, knots or condensations. These are apparent enough in many of the nebular photographs already shewn, but they can be seen still more clearly in nebulae which are viewed nearly full-on, such as for instance the two striking nebulae shewn in Plates XVII and XVIII. These bunches are invariably too large to be interpreted as single stars; they are more probably groups of stars. In the largest telescopes they break up into great numbers of points of light in the way already exhibited in Plate XI (p. 70). We have already mentioned the reasons which compel us to regard these points of light as actual stars, the principal being that some of them shew the characteristic light-fluctuations of the Cepheid variables. It is not altogether clear whether the stars are formed directly as condensations in the equatorial plane of the nebula, or whether larger condensations form first, namely the bunches observable in nebular photographs, which subsequently form smaller condensations, the stars. On the whole it seems likely that there are two processes involved—first the break-up of the nebular matter into big condensations, and then the break-up of these big condensations into stars. Such a succession of processes might well accompany a gradual cooling of the matter, and it is of course possible that there are even more than two processes involved. There is no need to form a final opinion on this at present, as it is in no way essential to the progress of the main argument. A collection of nebular photographs enables us to follow nebular evolution from the earliest stages shewn in Plate XVI (p. 207), through the first appearance of granular bunches, such as are shewn in Plate XVII, and the first distinct appearance of stars shewn in Plate XVIII, down to the later stages, such as are shewn in Plates XIX and XX, in which the nebula appears to be but little more than a cloud of stars. Hubble has found it possible to follow the sequence still further, and can trace a continuous transition from the nebulae of this last type to pure star-clouds such as the Greater and Lesser Magellanic Clouds shewn in Plate XXI. [Illustration: PLATE XVII _Mt Wilson Observatory_ The “Whirlpool” Nebula _M_ 51 in Canes Venatici] [Illustration: PLATE XVIII _Mt Wilson Observatory_ The Nebula _M_ 81 in Ursa Major]