When Herschel was sweeping the heavens with his telescope, and saw but few stars, he often said to his assistant, "Prepare to write; the nebulæ are coming." They are most abundant where the stars are least so. A zone about the heavens 30° wide, with the Milky Way in the centre, would include one-fourth of the celestial sphere; but instead of one-fourth, we find nine-tenths of the stars in this zone, and but one-tenth of the nebulæ.
These immense masses of unorganized matter are noticed to change their forms, vary their light greatly, but not quickly; they change through the ages. "God works slowly." He takes a thousand years to lift his hand off.
Fig. 79.—Crab Nebula, near ζ Tauri. (See Frontispiece.)
There are many unsolved problems connected with these strange bodies. Whether they belong to our system, or are beyond it, is not settled; the weight of evidence leans to the first view.
Variable Stars.
Fig. 80.—The Ring Nebula.
Our sun gives a variable amount of light, changing through a period of eleven years. Probably every star, if examined by methods sufficiently delicate and exact, would be found to be variable. The variations of some stars are so marked as to challenge investigation. β Lyræ (Fig. 81) has two maxima and minima of light. In three days it rises from magnitude 4-1/2 to 3-1/2; in a week falls to 4, and rises to 3-1/2; and in three days more drops to 4-1/2: it makes all these changes in thirteen days; but this period is constantly increasing. The variations of one hundred and forty-three stars have been well ascertained.
