NCERT Question 14 - Chapter 17 Class 8 - Stars and the Solar System

Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?

Answer

The distance between the stars is so large that Kilometers becomes a very small unit.

Hence, the distance between stars and planets are expressed in light years .

One Light Year means Distance travelled by light in 1 Year.

Calculating 1 Light Year

Speed of Light = 3 × 10 ⁸ m/s

     Time = 1 Year

   = 365 days

                        = 365 × 24 hours

                        = 31536000 seconds

Now,

1 Light Year = Speed × Time

         = 3 × 10 ⁸ × 31536000 m

         = 94608000 × 10 ⁸ m 

         = 94608000 × 10 ⁸ × 1/000 km 

         = 94608 × 10 ⁸ km 

                   = 9.4608 × 10 ⁴ × 10 ⁸ km 

         = 9.4608 × 10 ¹² km 

Now,

A star located 8 light years away means

Distance between star and earth = 8 light years

           = 8 × 9.4608 × 10 ¹² km 

           = 8 × 9.4608 × 10 ¹² km 

           = 75.6874 × 10 ¹² km

           = 7.56874 × 10 ¹³ km 

           = 7.57 × 10 ¹³ km