Question

An alien spaceship moving at 91% the speed of light measure the distance between two stars to be 3.6 light years. What is the proper distance between the stars ?

Answer 1

$$L_0=\frac{L}{\sqrt{1-(\frac{v}{c})^2}}$$

$$L_0=\frac{3.6}{\sqrt{1-0.91^2}}=8.7$$

The proper distance between the stars is 8.7 light years.