In many stories on TV and in science-fiction books, men travel to faraway stars．They have quick, easy journeys．But so far, men have been able to reach only the earth's own moon.
Suppose a man wanted to reach a distant star．Even if he traveled his whole life, he would have to move faster than the speed of light．Nothing can move that fast except light itself.
Strange things happen to an object when it moves rapidly．The object weighs more．An object moving at 86 percent of the speed of light is twice as heavy as it is at rest．A stick appears shorter．A clock runs more slowly．A man would not age so fast as he would on the earth.
Light travels more than 186, 000 miles a second, or about 11 million miles a minute．In one year, light travels six trillion(万亿) miles．That great distance is called a light-year．It is used to measure distances in space.
The star closest to our sun is Alpha Centrauri (半人马座a星)．It is more than four lightyears away．If one traveled at the speed of light, he could make a round trip to Alpha Centauri in nine years．But, even at that speed, he could not reach Alcaid (北斗星)in the handle of the Big Dipper．A one-way journey to Alcaid would take almost 200 years!
Why do we measure the distance in light-years instead of miles?