Jeff Shull, age 14, of Bessemer City, N.C., for his question:

WHAT IS A LIGHT-YEAR?

Get ready for some really big numbers.

First of all, light travels at the rate of 186,282 miles per second. The distance that light will travel in one year is called a light-year. And this adds up to a distance of 5 trillion 88 million miles.

Because distances in space are too vast to be measured conveniently in miles, astronomers use both the astronomical unit and the light-year. The astronomical unit is the average distance between earth and the sun: 93 million miles. Pluto, the most distant planet, is said to be about 40 astronomical units from the sun.

Farther out into space the light-year is used. Some stars are 300 light-years away.