Introduction

The vast surrounding that contains everything that exists is called universe. It includes the sun, the planets, constellation, comets, stars, etc. Many scientists believed that the universe is the result of an explosion called big bang which occurred billions of years ago. As the result of the huge explosion the matters were thrown out. These matters formed many galaxies. The galaxies are still moving farther away from each other due to this explosion. Thus the universe is still expanding.

The universe is so vast the distance related to it is not measured in meter or kilometer. For convenience, following units are used to measure the distance in universe.

Light year

The distance traveled by light in one year is called one light year. The speed of light in vacuum is 3×10⁸ m/s.

In one year, the light travels= 3×10⁸ ×365×24×60×60

                                                     =9.46×10¹⁵m

Therefore, 1 light year =9.46×10¹⁵m

• The nearest star from our solar system is Alpha Centaury which is about 4.3 light years far. It means light takes 4.3 years to reach from this star to our solar system.
• The nearest galaxy from our solar system is Andromeda which is about 2×10⁶m light years away.
• The sun is 480 seconds away from the earth.

Astronomical unit

1.  The average distance between sun and the earth is called astronomical unit.

1 AU=1.5×10¹¹m           

Parsec

The distance having the value 3.26 light years is called 1 parsec.

The Big Bang Theory

According to this theory, all the matters in the universe were originally concentrated in one vast lump called a primeval atom. About 15 billion years ago, this super dense and extremely hot body having neutrons and protons exploded with a big bang and the matters started flying in all directions through space leading to the formation of galaxies of the stars and other heavenly bodies.

Stars

The stars are heavenly bodies of glowing gases and are extremely hot. They are seen in the sky at night and have light of their own. They shine due to the nuclear fusion occurring there. Sun, Vega, Regal, Cuprous etc. are stars.

• The vast surrounding that contains everything that exists is called universe. It includes the sun, the planets, constellation, comets, stars, etc.
• The distance traveled by light in one year is called one light year.
• The nearest star from our solar system is Alpha Centaury which is about 4.3 light years far. It means light takes 4.3 years to reach from this star to our solar system.
• The nearest galaxy from our solar system is Andromeda which is about 2×10⁶m light years away.
• The stars are heavenly bodies of glowing gases and are extremely hot.