Welcome to You Ask Andy

Walter Wood, age 12, of Silverbell, Ariz., for his question:

WHAT EXACTLY IS A LIGHT YEAR?

A million miles takes us around the earth's bulging equator about 41 times. To earthlings, this is a lot of traveling. But out in the vast oceans of outer space, a million miles is too small to be noticed. Obviously we need a unit much larger than the earth mile to measure the great distances between us and the stars. One of these astronomical units is the light year.

When traveling in Arizona, one might say that Tucson is about two hours from Phoenix. We mean that it would take us about two hours to drive there at about 55 miles per hour. On earth we are used to measuring distance in terms of travel time. The light year is the same idea  on a much grander scale.

On this grand cosmic scale, the speed limit zooms up to almost 700 million miles per hour. This is based on the speed of light, which ships across space at about 186,282 miles per second. Radio and certain other energies also travel at this speed, though as far as we know nothing in the universe can go any faster.

A light second is the distance that light travels in a second, and a light minute is the distance it travels in a minute. The sun's distance from the earth is eight light¬ minutes and 20 light seconds. This means that a sunbeam takes eight minutes and 20 seconds to get from there to here.

These small units may be used to measure the distances of our neighbors in the solar system. But they would be lost out there between the stars. For this we need the light year, which is the distance that light travels in one earth year. This stupendous distance equals about 6 trillion earth miles.

Such a distance sounds fantastic, yet the nearest star to our sun is at a distance of 4.3 light years. It is the triple star Alpha Centauri, seen south of the equator. Its light, traveling at about 186,000 miles per second, takes 4.3 years to reach us.

The sun and about 100 billion other stars belong in a great wheel shape system called the Milky Way galaxy. The big wheel is about 100,000 light years wide. Out in deep space there are at least a billion more big galaxies. Our neighboring Andromeda galaxy is at a distance of 3 million light years. And powerful telescopes have spotted other galaxies at distances of several billion light years.

 

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