Question

The andromeda galaxy is located approximately 2.5 million light years from earth. Why is the light that reaches earth 2.5 million years old?

Answer 1

1 ly is a distance, defined as the distance travelled by light in a year. hence 2.5 mn lys away means it takes 2.5mn years for light to reach earth b/c light travels a distance of 1 light year in a year