Distance between stars are very large in magnitude. Hence a new unit is required so that the value of distance is not too big and is sensible. A light year is defined as the distance travelled by light in one year. 1 ly = 9.461 × 1012 km. 8 ly = 7.57 × 1013 km. A star is eight light years away from earth means that light from the star takes 8 years to reach the earth's surface. As shown above, its approximate distance from the earth is 7.6 × 1013 km.