Jillian's Guide to Black Holes: Forming - Types - Outside - Inside - Finding - References - Websites
The speed of light and how telescopes are time machines
While the speed of light is very, very fast, it is not infinite. It takes light a certain amount of time to cross a distance. For example, it takes light 8 minutes to get from the Sun's surface to us on Earth and it takes light about 5 hours to get from the Sun to Pluto. Thus, on Earth we see light that was emitted 8 minutes ago and on Pluto we would see light that was emitted 5 hours ago. That's just in our own back yard, our solar system!
The further away an object is from us, the longer it takes for its light to get to us, and the "older" we see it. The nearest stars to the Sun are 4 light-years away, which means it takes light 4 years to get to us from the Alpha Centauri triple star system. An astronomer looking at Alpha Cen through a telescope is actually studying how it looked 4 years ago. Our Sun is about 2.4 x 1020 meters (1.49 x 1017 miles) from the center of the galaxy. It takes light 26,100 years to travel from the galactic center to us. Telescopes let astronomers see more and more distant objects, so telescopes are the "time machines" allowing astronomers to look into the past!
Since astronomers study things which are very large, they found it handy to develop new units of distance to make the numbers easier to handle. A useful unit of distance is called the "light year". This sounds like a unit of time, but it is the distance that light travels in one year, 9.46 x 1015 meters (5.88 x 1012 miles). As you can see, light years turn large and unwieldy numbers into something more handy! Another cool thing about the unit of light years is that it also is telling you how long ago the light was emitted. If the galactic center is 26,100 light years away, then the light we see right now was emitted 26,100 years ago.
By looking at really distance objects, astronomers can look way back in to the past to study the early universe. To help keep things straight, astronomers refer to something called "lookback time". For any object the lookback time is the age of the universe when the light was first emitted. The lookback time for the light we see from the Sun is the age of the universe (~13.7 billion years) minus 8 minutes ... not much of a difference for nearby objects. However, more distant objects have more impressive lookback times. Astronomers can study galaxies with lookback times ranging 4 to 1 billion years --- that's between 9 and 13 billion years ago!