As the giant telescope sweeps the abysmal depths of space over a distance of at least 4,000 billion miles, it reveals here and there among the hundred million stars, vast patches of cloud-like material. This material is called nebula, and it is the original substance of which all suns and planets are made. One hundred and twenty thousand of these nebulæ come within the range of the great Crossley Reflector at the Lick Observatory. The telescope photographs the nebula—celestial photography is one of the most interesting and instructive branches of modern astronomy—and when you look at a picture of a nebula, you see an actual photograph of an object that marks the beginning of Nature’s work. [Illustration: FIG. 3.—AN ELONGATED IRREGULAR NEBULA.] According to Sir Robert Stawell Ball, the nebula in Orion (Fig. 1), one of the most beautiful objects revealed by the telescope, covers an area more than a million times larger than that occupied by our entire solar system. Many of the nebulæ are of the spiral form, which shows their whirling motion. To the astronomer, the Great Spiral Nebula (Fig. 2) represents a mighty sun and system of planets in process of formation. Fig. 3 shows an elongated, irregular nebula, from the Constellation Cygnus. This sketchy streak of nebulous material, although it is still many millions of miles wide, is but the remaining thread of a once mighty nebula that has condensed and is still condensing into the surrounding stars. The nebulæ represent various stages of evolution into suns and worlds. Some resemble great clouds of rarified matter; some are distinctly spiral in form; some show advanced condensation into stars with attendant planetary systems. The stars, too, exhibit various degrees of progress from their birth in the nebula. Some are white, which shows that they are young; some are yellow, which indicates that they have reached middle age; some are red, which is the mark of their declining years. [Illustration: FIG. 4.—LUNAR CRATERS: HYGINUS AND ALBATEGNIUS.] So the stars, the glowing suns, grow old and die, and, lifeless, wheel in space, like mighty cinders cold and dark, reflecting, like the moon, the light that shines on them from brilliant orbs. The moon (Fig. 4) is a dead star. Its light and heat are gone. It wheels in space, an extinct cinder, and by the borrowed light which enables us to see the craters on its surface, it prophesies for us the fate that one day will overtake the earth. But that will be millions of years from now; so we need not worry!