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       The vast surrounding space around us which consists of solar system, stars, galaxies, gases and dust particles is called universe. Our universe was formed about fifteen billions years ago by an explosion called Big-Bang. There are approximately 100 billion (1011) galaxies in the universe. Each galaxy further contains 100 billion (1011) stars. So, there are about 1022 stars in the universe.


Scale of the Universe

       The universe is so vast that the distances between various heavenly bodies cannot be measured in kilometers or miles. So, in order to measure the distances in universe we use units such as astronomical unit, light years and parsec.

Astronomical Unit

       The average distance between earth and sun is called one astronomical unit. As, the average distance of the earth from the sun is about 150000000 kilometers, so one astronomical unit is also equal to 150000000 kilometers. Thus,

1 Astronomical Unit = 150000000 km

or 1 Astronomical Unit = 15 × 107 km

or 1 A.U. = 15 × 1010 m

Light Years

       The distance traveled by light in one year is called one light year. Since light moves with a speed of 3 × 108 m/s, so the distance traveled by light in one year is equal to 3 × 108 m/s multiplied by total number of seconds in one year. Thus,

1 Light year = 3 × 108 × 365 × 24 × 60 × 60 = 9.46 × 1015 meters

Proxima centaury is the star which is closest to earth after the sun. The light emitted by proxima centaury takes 4.2 years to reach earth. So, proxima centaury is 4.2 light years away from the earth.


       Parsec is the unit used to measure distances between stars. One parsec is equal to 3.26 light years. Thus,

1 Parsec = 3.26 light years

Test Your Understanding and Answer These Questions:

  1. What units are used to measure distances in the universe?

  2. How are light years used to measure distances and sizes in the universe?

  3. What metric unit is used to measure distance?

  4. What is a parsec?

  5. One astronomical unit is equal to which distance?

  6. Which star is closest to earth after sun?

  7. What is 1 astronomical unit?

  8. What is 1 parsec?

  9. What is 1 light year?

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