But not all variable stars are of this kind. There is a star in the constellation Cetus which is sometimes of the second magnitude, at which brightness it may remain for about a fortnight. Then it will gradually diminish in brightness for nine or ten weeks, until it is lost to the unassisted sight, and after six months of invisibility it reappears and increases during another nine or ten weeks to another maximum. "Mira," _i.e._ wonderful star, as this variable is called, is about 1000 times as bright at maximum as at minimum, but some maxima are fainter than others; neither is the period of variation always the same. It is clear that variation of this kind cannot be caused by an eclipse, and though many theories have been suggested, the "+long-period variables+," of which Mira is the type, as yet remain without a complete explanation. More remarkable still are the "NEW STARS"--stars that suddenly burst out into view, and then quickly fade away, as if a beacon out in the stellar depths had suddenly been fired. One of these suggested to Hipparchus the need for a catalogue of the {88} stars; another, the so-called "Pilgrim Star," in the year 1572 was the means of fixing the attention of Tycho Brahe upon astronomy; a third in 1604 was observed and fully described by Kepler. The real meaning of these "new," or "temporary," stars was not understood until the spectroscope was applied to astronomy. They will therefore be treated in the volume of this series to be devoted to that subject. It need only be mentioned here that their appearance is evidently due to some kind of collision between celestial bodies, producing an enormous and instantaneous development of light and heat. These New Stars do not occur in all parts of the heavens. Even a hasty glance at the sky will show that the stars are not equally scattered, but that a broad belt apparently made up of an immense number of very small stars divides them into two parts. THE MILKY WAY, or GALAXY, as this belt is called, bridges the heavens at midnight, early in October, like an enormous arch, resting one foot on the horizon in the east, and the other in the west, and passing through the "+Zenith+," _i.e._ the point overhead. It is on this belt of small stars--on the Milky Way--that New Stars are most apt to break out. The region of the Milky Way is richer in stars than are the heavens in general. But it varies itself also in richness in a remarkable degree. In some places the stars, as seen on some of the wonderful photographs taken by E. E. BARNARD, seem almost to form a continuous wall; in other places, close at hand, barren spots appear that look inky black by contrast. And the +Star Clusters+, stars evidently crowded together, are frequent in the Milky Way.