It was long ago noticed that, the more powerful a telescope was, the greater was the number of these hazy spots of light which it would resolve into clusters of stars. Consequently the opinion was formed that all the hazy little clouds or nebulæ which are so prevalent throughout a large part of the sky were simply clusters of stars, so far away that their light merged into a single impression on the eye. A great number of these nebulæ were only resolved by large telescopes; many were found to be irresolvable by any telescope. It was simply concluded from this that they were still more distant than the clusters which had yielded to the resolving powers of the telescope; and it was further supposed that each of these clusters of stars might be a separate universe or galaxy, comparable in extent and importance with our own universe, bounded by the vast girdle of the Milky Way.
The Nebular Hypothesis. This grand conception of innumerable universes scattered throughout space was speedily destroyed by the spectroscope, which distinguishes with entire certainty between the light sent to us from a solid star and that emitted by a gas. When it was turned upon the nebulæ which had been supposed in reality to be star-clusters so distant that no telescope could resolve them, it showed unmistakably that these nebulæ were not star-groups, but simply masses of incandescent gas.
Besides, nebulæ vary greatly in form and appearance; some are clearly clusters of stars, others are perfectly hazy. A round or oval form is sometimes exhibited, with a gradual condensation towards the center, and a number of stars standing in the center of a nebulous haze can be observed. Such observations on nebulæ caused Kant and Laplace to suggest a theory—now known as the nebular theory—as to the formation of worlds. They considered that the solar system, for example, originally existed as uncondensed nebulous matter. This gradually condensed towards the center, forming the nucleus of the sun, and later the outer parts separated into distinct parts, each part condensing into a planet. The different forms of nebulæ observed in the heavens are then supposed to be systems in different stages of development.
THE VARIED COLOR
OF THE STARS
Many of the stars shine with colored light, as red, blue, green, or yellow.
These colors are exhibited in striking contrast in many of the double stars. Combinations of blue and yellow, or green and yellow, are not uncommon; while in fewer cases we find one star white and the other purple, or one white and the other red. In several instances each star has a rosy light.
The following are a few of the most interesting colored double stars:
| Name of Star | Color of Larger One | Color of Smaller One |
|---|---|---|
| γ Andromedæ | Orange | Sea-Green. |
| α Piscium | Pale Green | Blue. |
| β Cygni | Yellow | Sapphire Blue. |
| η Cassiopeiæ | Yellow | Purple. |
| σ Cassiopeiæ | Greenish | Bright Blue. |
| ζ Coronæ | White | Light Purple. |
| ι Cancri | Orange | Blue. |
| α Herculis | Orange | Emerald Green. |
Single stars of a fiery red or deep orange color are common enough. Of the first color may be mentioned Aldebaran, Antares and Betelgeuse. Arcturus is a good example of an orange star. Isolated stars of a deep blue or green color are very rarely found; among the conspicuous stars, β Libræ appears to be the only instance.