Here's why:
* Light travels at a finite speed: Light travels at approximately 299,792,458 meters per second.
* Distance and time: Since light takes time to travel, the light we see from a distant object left that object some time ago. The further away the object, the longer it takes for its light to reach us.
* 100 light-years: A light-year is the distance light travels in one year. So, if a star is 100 light-years away, the light we see from it today left the star 100 years ago.
Therefore, any observation you make of a star 100 light-years away will show you the star as it was 100 years ago.
