The universe is everything that exists-all matter, space, and time.it extends as far as the largest telescopes can see, at least 10 billion light-years in all direction around us. The Universe is bought to have formed in a gigantic explosion, called the Big Bang, about 10 to 20 billion years ago and is still expanding. Within the Universe, matter is held together by gravity in structures of varying size.
Most of the visible matter in the Universe is grouped in huge aggregations of stars, gas, Sun, is an insignificant star in a galaxy of at least 100 billion others (usually known as “the Galaxy”). Although it is difficult to tell from our position inside it, the Galaxy is thought to be spiral in shape. There is some evidence indicating that there is a bar of material across the center. The Galaxy is about 100,000 light-years in diameter, and we are located about two-thirds of the way from the center to the rim, in one of its spiral arms. The stars that we see in spiral arms. The stars that we see in the sky are all part of the Galaxy. Those nearest to us appear in all directions in the sky, and from the constellations (seep.18). Since the Galaxy is flattened in shape, more distant stars mass into a hazy band known as the Milky Way.
THE MILKY WAY
The Milky Way, a pale band that can be seen arching across the sky on dark nights, consists of innumerable distance stars within our Galaxy. In this view of the Milky Way, looking toward the center of the Galaxy, apparent gaps are caused by clouds of dark dust that lie in the spiral arms, Obscuring the light from stars behind.
The Universe contains countless galaxies, some of which we can see from the Earth. Galaxies are divided into spirals, barred spirals, elliptical, and irregulars. Spirals have arms that consist of relatively young stars. In barred spirals, the arms extend from the ends of a central bar. Elliptical galaxies, consisting of old stars, have no arms and very little gas or dust. Stars have no arms and very little gas or dust.
Irregular galaxies have no regular shape or structure. The largest galaxies are elliptical, with more than time the mass of our Galaxy; some right giant elliptical may have formed by the merger of smaller galaxies. Far off in the Universe are objects called quasars that emit as much energy as a galaxy from an area not much larger than the Solar system. Quasars, and their less luminous relatives, known as Seyfert Galaxies and BL lacerate objects, are thought to be galaxies with massive black holes at their centers.