The celestial bodies are generally very far from one another. The sun,our nearest star is a distance of about 150,000,000 km from the earth.Other stars are much more father. To measure such large distance , a unit called ' light year is used. A light year is the distance travelled by the speed of light in one year. Since the speed of light is 300,000 km/s ,the value of one light year is about 9.46 x 10/12 km.