It remains to be asked what leads to the sudden brightening of a faint star, which results in the production of the aforementioned phenomena. There are two main lines of explanation available. Newton was one of many enquirers who have looked outside the star for the source of the necessary energy and have postulated collision or the inflow of matter as the exciting cause. Laplace held the view that a great surface conflagration of a star was all that was involved and this view has been held in varying form by many others. The breakdown of equilibrium in a cooling star through chemical combination at low temperature, through radio active forces, or through unstable pulsations, have all been sug gested, as have also tidal eruptions through the close passage of a second star or companion. The latter theory is the bridge to the view that we have a collision between a star and a planet or comet, or even a collision between two stars. The frequency of the phe nomenon makes an explanation in terms of the collision of two stars difficult to accept, and Seeliger's view, that what we behold in a nova's outburst is the result of a star (or a binary star) enter ing into a nebula, seems on the whole the best solution of the problem so far proposed. E. W. Brown has worked out the effect
to be observed when a star enters a nebula, and his picture closely resembles what the camera shows at the present time for the region round N Persei, 1901—a spherical expanding shell and a fan-like appendage on one side of the star. In the case of this star there was evidence of nebulosity round the star at the time of outburst. It seems then not unlikely that some of the novae are to be explained in terms of Seeliger's hypothesis of an encounter of a star with dark nebulosity. Differences between the behaviour of various novae may well argue against a common cause for the initial brightening of all novae. It is, perhaps, fairest to return an open verdict on the cause for the nova's original brightening, though we can now follow with reasonable certainty the main effects which follow the great outburst of radiation.