Galaxy is very large in size and form. Three types of galaxy have been identified so far, namely spec elliptical and irregular galaxies. The earth lies in a spiral galaxy called the Milky Way. It has hundreds of billions of stars.

Universe and the big bang theory- 

The universe is the boundless expanse in which all matter, stars, planets, satellites, etc. exit. That was accepted theory related to the birth of the universe is called the Bing bang theory. According to this theory, in the beginning, all matter-substances like gases, dust particles, all elements were contained in a huge big burning ball of fire which later exploded with a big bang. Matter from it was thrown out with great force in all directions. From this matter many groups of stars were formed which we call galaxies. Our sun belongs to a galaxy called the Milky Way.

Stars-

Stars are very big celestial bodies, but they look small because they are very far from us. They are a ball of hot glowing gases. They produce their own heat and light. The best known example of a star is sun.

The solar system-

In Roman mythology, sol means the “sun goal” the world “solar is related to the sun. the family of the sun is, therefore, called the solar system. The family of the sun includes eight planets and more than 160 satellites, besides these, three are many comets and countless asteroid. All the planets revolve around the sun in an anti-clockwise direction and elliptical paths called orbits.

Sun-

The sun is the biggest member of the solar family. The sum, which is actually a star, is about 150 million km away from the Earth. Its diameter is about 100 time that of the Earth. Even though light travels at a tremendous sped of about 300000 km per second. It takes about 8 minutes to reach the Earth from the sun. The surface temperature of the sun is 6000°C which increases to 20 million degrees Celsius in the in the interior. Its weight is nearly 300000 times that of the earth.

Planets-

The word ‘planet’ is originally a Greek word which means ‘wanderer’. There are eight planets in our solar system. They according to their distance from sun, are- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury is the hottest planet because it is nearest to the sun. in the ascending order in size, they are Mercury, Mars, Venus, Earth, Neptune, Uranus, Saturn, and Jupiter. In this way the largest planet in our solar system is Jupiter while the smallest planet is Mercury.

The planets revolve round the sun in fixed paths which are called their orbits. They also rotate on their own axis. All the planets move round the sun in the same direction.

Earth- A Unique Planet-

The Earth is geoid (Earth shaped) in shape, i.e. it is bulge the Equator and flat near the poles. This is because of the speed at which it rotates on its axis (15° per hour). The Earth takes nearly 24 hours (actually 23 hours 56 minutes 4 sec.) to complete one rotation,

Unique Features of our Earth-

1. It is the only known planet of the solar system where life exists.
2. It is an active planet with volcanoes and earthquakes.
3. The Earth is the only planet on which water is found in liquid, solid and gaseous from. About 71% of the Earth’s surface is covered with water. Earth is also called a Blue Planet.