Light- year

A light- year is a unit of length, equal to just under 10 trillion kilometers (10¹⁶ metres, 10 pentameters or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.

To measure really long distances, people use a unit called a light year. Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light year is the distance that light can travel in a year, or:

186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year

A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers).

Using a light year as a distance measurement has another advantage, it helps you determine age. If a star is 1 million light years away, and the light from that star has traveled at the speed of light to reach us, then it has taken the star's light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, our sun is about 8 light minutes away. If the sun were to suddenly explode right now, we wouldn't know about it for eight minutes because that is how long it would take for the light of the explosion to get here.

http://science.howstuffworks.com/dictionary/astronomy-terms/question94.htm