on account of their being situated nearly in the line of vision; these are said to be optically double. Others are actually connected, and revolve one around the other; these are said to be physically double. Binary Systems, are double stars, one of which revolves around the other;-or both revolve around their common centre of gravity. Binary Systems sometimes exhibit Stars, of different colors. Between forty and fifty have been discovered. Their periods of revolution, vary from forty to sixteen hundred years. The Galaxy or Milky Way, is a faint zone of light encompassing the heavens, and discovered, by the telescope, to consist of vast numbers of Stars. A Cluster is a number of stars collected in a certain space. Nebulæ are certain cloudy appearances, seen in the heavens; most of them supposed to consist of vast numbers of Stars. They may be divided into two classes; viz., Resolvable, and Irresolvable. Resolvable Nebula, are those which are supposed to consist of vast numbers of Stars, so far distant as to appear like spots of cloud. Irresolvable Nebula, are those which the most powerful telescopes, have failed to resolve into Stars. The latter are supposed to be luminous matter, condensing into solid bodies, like the Sun. What are binary systems? What do they sometimes exhibit? How many have been discovered? What are their periods of revolution? What is the galaxy or milky way? What is a cluster? What are nebule? How many classes are there? What are resolvable nebula ? What are irresolvable nebulæ? What are the latter supposed to be? Nebula which resemble the disc of a planet, are called Planetary Nebulæ. Those, which have the appearance of a Star, surrounded by luminous matter, are called Stellar Nebula. The Galaxy, or Milky Way, is supposed to be a nebula, or cluster, of which the Sun is a member. Its shape is supposed to resemble an immense wheel, the Sun being situated comparatively near the centre. All the Stars in this cluster, including the Sun, are believed to revolve around their common centre of gravity. The Universe is supposed to consist of an infinite number of Clusters similar to that, of which the Sun is a member, and situated at immense distances from each other. CHAPTER XXV. CONSTELLATIONS. A CONSTELLATION is a number of Stars, included in a certain space. There are ninety-three Constellations set down on most globes. The Constellations are divided into three classes, viz., Northern, Southern, and Zodiacal. What are planetary nebulæ ? What are stellar nebula? What is the galaxy supposed to be? What is its shape? What revolution have the stars of this cluster? Of what does the universe consist? What is a constellation? How many are there? Into how many and what classes are they divided? The Northern Constellations, are those which lie north of the Zodiac. The Southern Constellations, are those which lie south of the Zodiac. The Zodiacal Constellations, are those which lie within the Zodiac. The Northern are thirty-four in number, the Southern forty-seven, and the Zodiacal, twelve. The names of the Constellations of the Zodiac, are the same as those of the Signs. In consequence of the precession of the equinoxes, the Signs have fallen back of the Constellations, about thirty-one degrees. They corresponded twenty-two centuries ago. Stars are classified, according to their apparent size or brightness. The brightest stars, are said to be of the first magnitude, and the least that are visible to the naked eye, are of the sixth. Telescopic Stars, are those which can only be seen with a telescope. What are the northern constellations? What are the southern? What are the zodiacal? What is the number of each? What are the names of the zodiacal? Do the signs and constellations correspond? When did they correspond? How are stars classified? How are the brightest stars distinguished? What are telescopic stars? |
