On the evening of November 11th, 1572, Tycho Brahe was astounded to see a brilliant star-like point of light shining in the constellation Cassiopeia. He was certain that no star had been visible before, in that exact position. Although he had no conception of the terrific intensity of the explosion that we now know had taken place, he realized at once the supreme importance of what he saw. At first, indeed, he could not believe his own eyes, but as soon as he found that others also saw the star, he judged its appearance to be the greatest miracle that had occurred in the whole range of nature since the beginning of the world.

Similar outbursts must occasionally have aroused wonder ever since man took an interest in the starry skies, but, until comparatively recent times, the majority of these passed unrecorded. We have no account of any seen in Europe before the one which blazed out in Scorpio in 134 B.C., which is said to have induced Hipparchus to make a catalogue of the stars. The Chinese, however, tell of bright stars appearing in the sky in 2679, 2255 and 2238 B.C.

But, as far as we know, Tycho was the first to make a scientific study of such a phenomenon. He recorded all the variations in the brightness of the strange light, and proved that the object, whatever it might be, was far beyond the limits of the solar system and somewhere in the region of the stars.

Thus was introduced into astronomy a fascinating problem, to which three and a half centuries of astronomical research, with all the help that physics and chemistry can give, has failed to find a solution which has won universal acceptance.

There is, at last, fairly general agreement as to what happens during the outburst, but an extraordinarily wide divergence of opinion still exists with regard to the cause.

It was not easy to find even an appropriate name for these mysterious objects. They were at first called “New Stars” or “Novae Stellae.” But in their behaviour, during the brief period of their vivid and spectacular life, they differ completely from normal stars. The word “stellae,” therefore, has been discarded and we call them simply “Novae.”

It is doubtful to what extent even this epithet is applicable, but undoubtedly the message borne on the wings of light, though it may have been a thousand years on its way, is “news” when it reaches us. It must be read the very moment it arrives, or it will pass on and be lost for ever.

Our problem is to find out what a Nova really is, and probably the most convenient method of approach will be to consider how it resembles, and how it differs from, a normal star.