The number of stars in existence at any time varies to a slight degree, for occasionally a star disappears, or a new one makes its appearance. These, however, are rare phenomena, and new stars usually disappear or fade away after a short time, for which reason they are often called temporary stars. The greatest of these phenomena ever beheld was Tycho Brahe's star, which suddenly burst into view in the constellation Cassiopeia in 1572, and disappeared after a couple of years, although at first it was the brightest star in the heavens. Another temporary star, nearly as brilliant, appeared in the constellation Perseus, in 1901, and this, as it faded, gradually turned into a nebula, or a star surrounded by a nebula. It is generally thought that outbursts of this kind are caused by the collision of two or more massive bodies, which were invisible before their disastrous encounter in space. The heat developed by such a collision would be sufficient to vaporise them, and thus to produce the appearance of a new blazing star. It is possible that space contains an enormous number of great obscure bodies,—extinguished suns, perhaps—which are moving in all directions as rapidly as the visible stars. =2. The Nebulæ.= These objects, which get their name from their cloud-like appearance, are among the most puzzling phenomena of the heavens, although they seem to suggest a means of explaining the origin of stars. Many thousands of nebulæ are known, but there are only two or three bright enough to be visible to the naked eye. One of these is in the “sword” of the imaginary giant figure marking the constellation Orion, and another is in the constellation Andromeda. They look to the unaided eye like misty specks, and require considerable attention to be seen at all. But in telescopes their appearance is marvellous. The Orion nebula is a broad, irregular cloud, with many brighter points, and a considerable number of stars intermingled with it, while the Andromeda nebula has a long spindle shape, with a brighter spot in the centre. It is covered and surrounded with multitudes of faint stars. It was only after astronomical photography had been perfected that the real shapes of the nebulæ were clearly revealed. Thousands of nebulæ have been discovered by photography, which are barely if at all visible to the eye, even when aided by powerful telescopes. This arises from the fact that the sensitive photographic plate accumulates the impression that the light makes upon it, showing more and more the longer it is exposed. Plates placed in the focus of telescopes, arranged to utilise specially the “photographic rays,” are often exposed for many hours on end in order to picture faint nebulæ and faint stars, so that they reveal things that the eye, which sees all it can see at a glance, is unable to perceive.