Scientists claim that if an alien 65 million light-years away see the earth through a powerful telescope, they can see "dinosaurs". How can that be possible?
This question raises the fascinating issue of look-back times. Because of the finite speed of light, when you gaze up into the night sky, you are looking into the past. The bright star Sirius is 8.6 light-years away. That means the light hitting your eye tonight has been traveling for 8.6 years. Put another way: When you look at Sirius tonight, you see it as it was 8.6 years ago.
As you look at more distant objects, the effect becomes bigger and bigger. The stars of the Big Dipper range from 60 to 125 light-years away. When you look at Dubhe, the front star in the "bowl" of the Dipper, you are seeing light from before you were born.
Here's a fun way to think about it. Go to this list of the brightest stars, click on the "distance" column to rank them by how far away they are, and find the number closest to your age. That is your birth star—the one whose visible light is the same age you are. When you look at that star tonight, you see it as it was at the time of your birth. There is a birth star for everyone (roughly) if you are more than 4 years old.