What is a Nova?
When watching such an outburst as that of Nova Persei or Nova Aquilae, or even when reading about it, one is impelled to ask “What can have caused such a stupendous explosion?” We have to account for hundreds of thousands of times the energy of our Sun being liberated in a few hours, for temperatures of hundreds of millions of degrees being suddenly produced, and for velocities, often exceeding a thousand miles per second, being found in the outrushing gases. No theory which fails to explain such things, need be considered for a moment. But this is by no means all. The sudden fading of the star-like point of light, implying the dissipation of astounding quantities of energy, provides a still more searching test. If any theory survives this, it still has to face the evidence afforded by the succession of changing spectra, and the later development of the planetary nebular stage.
