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?

Hint: A light year is defined as, how long the light can travel in one year. And the light is the unit of distance. Light can move with a velocity of about $3 \times {10^8}\,m{s^{ - 1}}$ which is equal to the $300000\,km$ per second. By using this velocity value, we can determine the total distance travelled by the light in one year.

Complete step by step solution
The distance between the stars is very large, So, that we cannot express it in normal value. Because the value is very large in number. Hence, the new term or unit is introduced here to express the distance between two stars. By the help of the new unit, the distance between the stars is expressed in simple form. So, the distance between the stars is expressed in light years. A light year is defined as the total distance travelled by the light in one year.

Generally, the speed of light in space is $3 \times {10^8}\,m{s^{ - 1}}$ that means it travels about $300000\,km$ per second. So, the calculation is done to find the total distance travelled by light in one year. For a year the light can travel about $9.4608 \times {10^{12}}\,km$ (Approximately ten trillion kilometres).

A star is eight light years away from the earth which means that the light from the star will take $8\,years$ to reach to the earth surface. For one year the light can travel about $9.4608 \times {10^{12}}\,km$ (Approximately ten trillion kilometre) and for eight years it can travel about $75.6874 \times {10^{12}}\,km$ (Approximately seventy-six trillion kilometre).

Note: The statement, star is eight light years away from the earth means the light from earth takes approximately eight years to reach the star. So, that statement defines that the star is eight light years away from the earth. The unit of light year is distance.