Given:
Questions:
1.Why is the distance between stars expressed in light years?
2.What do you understand by the statement that a star is eight light years away from the earth?
Solution and answer
 Very big distances can not be measure in small units like meter and km
thus we have to use bigger scale
For example:
1. In your maths note book you use cm  and mm 
2. But length of your class can not be measured in mm ocm so we use meter or feet
3.Distance of your house is measured in km not in meter
4. Stars are at at very big height so we use ‘LIGHT YEAR’
NOW
WHAT IS LIGHT YEAR?
”one light year is equal to the distance the sunlight travels in one year”
 
One light-year = 5,878,625,370,000 miles = 9.5 x 1012 km
Now our question2:
A star is eight light years away from the earth
=8 x i light year
=8 x9.5 x 1012 km
=76 x 1012 km
So the distance of this star from earth =76 x 1012 km
OTHER IMPORTANCE BIG DISTANCES:
1 LIGHT YEAR= 63,240 AU
1 AU=93 million miles (150 million km)
1 AU= DISTANCE BETWEEN EARTH AND SUN
 
Mean distance from sun to each planet
Mercury: 0.387 AU
Venus: 0.723 AU
Earth: 1.000 AU
Mars: 1.524 AU
Jupiter: 5.203 AU
Saturn: 9.582 AU
Uranus: 19.201 AU
Neptune: 30.047 A
THANKS