Introduction
It was once believed that the earth was the center of the universe and that everything moved around it. It was later discovered that the earth moves around the sun.
The Solar System
The solar system consists of the sun and all the objects that travel around it. The solar system includes the earth and eight other planets, along with the satellites (moons) that travel around most of them; planet like objects called asteroids; chunks of iron and stone called meteoroids; bodies of dust and frozen gases called comets and drifting particles called interplanetary dust and electrically charged gas called plasma that together make up the interplanetary medium.
The Sun
The sun is at the center of the solar system. Its mass is about 740 times as great as that of all the planets combined. The huge mass of the sun creates the gravitation that keeps the other objects traveling around the sun in an orderly manner.
The Planets
Planets are the largest objects in the solar system except for the sun. Unlike the sun, the planets do not produce their own energy. They reflect the heat and visible light produced by the sun. The four planets near the Sun - Mercury, Venus, Earth and Mars are called terrestrial (earth-like) planets because they are somewhat similar in size and composition to the earth. They appear to consist chiefly of iron and rock. The terrestrial planets and Pluto are the smallest planets. The earth has one satellite, Mars has two, and Pluto has one. Mercury and Venus have no satellites.
Asteroids, Meteoroids and Comets
Asteroids are very small planets of rock and metal, which revolve round the sun mainly between the orbits of Mars and Jupiter. This area is known as the asteroid belt. Most of the asteroids have a diameter of only 1 kilometer. But some of them have diameters of more than 100 kilometers.
Satellites (Moons)
A satellite is a solid heavenly body that revolves round a planet. The moon revolves round the earth, so moon is a satellite of the earth. The inner planets have few moons. Earth has one, and Mars has two tiny satellites.
Formation of Our Solar System
Many scientists believe that our solar system was formed from a giant, rotating cloud of gas and dust known as the solar nebula. According to this theory, the solar nebula began to collapse because of its own gravity. Some astronomers speculate that a nearby supernova (exploding star) triggered the collapse. As the nebula contracted, it spun faster and flattened into a disk.
Evolution
Exploration of the planets by space probes has expanded our understanding of the solar system. Modern theories about the earth's origin explore how the earth fits into the solar system, the Milky Way galaxy, and the universe as a whole. Most scientists agree that the earth was probably formed at the same time as the rest of the solar system.
Structure of the Earth
In outward appearance, the earth is a nearly spherical ball with a radius of 6350 kilometers. Internally, the earth consists of three major layers.
Stars
A star is a dense cloud of hydrogen and helium gas contracting under its own gravitational pull. In all the stars hydrogen atoms are continuously converted into helium atoms and this process is accompanied by a large amount of energy. This energy is radiated in the form of heat and light.
The Life of a Star
The life cycle of a star begins with the accretion of hydrogen and helium gas at -173oC into smaller dense clouds, which contract under their own gravity. This dense cloud of gas contracting under its own gravity is called a Protostar. A cloud of gas from which a star is born is known as Nebula.
Constellations
Constellation is a group of stars visible within a particular region of the night sky. The stars that appear in the form of closed groups and form recognizable shapes and patterns are known as constellations. There are about 88 different constellations across the whole sky. But no one can see them all. This is because people in the northern part of the world see different stars from people living in the south. There are far more bright stars in the southern skies than in the north.
Galaxy
The sun and all the stars that we see are part of our Galaxy. A galaxy is a congregation of millions or billions of stars held together by gravity. Apart from the stars, a galaxy also contains dust and hydrogen gas. Galaxies are the building blocks of this universe. There are millions of galaxies besides ours in the universe. Some are huge while other, are dwarf galaxies.
Universe
Universe consists of all matter and all light and other forms of radiation and energy. It consists of everything that exists anywhere in space and time.
Summary
The solar system consists of sun, the nine planets and their satellites and thousands of other smaller heavenly bodies such as asteroids, comets and meteors.
