What is a light-year?
Space is so vast that astronomers don’t use miles to measure how far stars are away from us. The numbers of miles would be so large that you would have a difficult time using them.
Instead, astronomers use what they call “light-years” to measure great distances. A light-year is the number of miles that light can travel in a year’s time. Light travel very fast—about 186,000 miles every second!
If we multiply this speed by the seconds in a year, we find that it adds up to roughly 6,000,000,000,000 (six million) miles.
Most stars are so far away that the light they give off takes several years to reach the earth. For example, the nearest visible star beyond our sun. Alpha Centauri, is a little over 4 light-years away, about 26 trillion miles.
When you look at this star, the light has been speeding toward you for more than four years. The farthest stars may be billions of light-years away. – Dick Rogers