The one that naturally arises in our mind is that it was struck by another star. Two worlds, each moving at the rate of twenty miles a second, come into collision, and the result is the annihilation of both. The force of their impact, changed into heat, drives their elements into vapor. Such a catastrophe is quite possible in a universe like ours, where stars and worlds, millions and millions in number, sweep down the great avenues of space with a velocity far beyond our comprehension. We take it that when the crack of doom comes to this earth of ours it will be in this fashion. Some great dark star will strike our sun fair and square, and then in the twinkling of an eye, before the inhabitants of earth know what has taken place, sun and moon and planet will be wrapped up and dissolved in an atmosphere of fire. We can in a certain rough way compute the increase in temperature that would arise from the collision of two great orbs. Thus, let us suppose that Nova Persei was moving onward through space with a velocity of ten miles a second—a moderate velocity, be it noted, for a star—when it collided with the body that wrought its destruction. The impact would be terrific, and the result of it would be not only the complete disintegration of both stars, but a sudden rise in temperature of about five hundred thousand degrees, an increase sufficient to vaporize the hardest adamant. The second theory which has been suggested as explanatory not only of Nova Persei, but of all new stars, is a modification of the foregoing. This theory is that the new star in its flight through space suddenly plunged into a nebula, or into some portion of space denser than that through which it had already passed. This explanation is not only intelligible but reasonable. If the new star plunged into a region filled with matter even as rare as air, the friction would immediately set the star on fire. We see the same phenomenon every night when a meteor hustles through our atmosphere. The meteoric rocks, with the chill of empty space in and around them, dash into our upper air. A few seconds are ample for the practical annihilation of most of them: in that brief space of time they have been subjected to a heat many times greater than that of a Bessemer furnace. We can imagine Nova Persei as some monster meteor, a meteor larger than the sun, plunging into a gaseous mass somewhat like our air. In a few hours its temperature would be increased a million-fold. This increase would fill the surrounding space with fire, and there would be an immense and ever-increasing area at fervent heat. To the mind of the writer this explanation has most to commend it. It is the one that is most in harmony with the information which has been gathered by hundreds of observers aided by the finest of modern scientific equipment. But there are other explanations. There will always be other explanations so long as the world lasts.