Q&A: General Astronomy and Space Science
What are light years and why are they used in astronomy?
In astronomy, distances are measured in units of light years,
where one light year is the distance that light travels in a
year—10 trillion kilometers. For historical reasons having
to do with measuring distances to nearby stars, professional
astronomers use the unit of parsecs, with one parsec being equal
to 3.26 light years.
For a complete discussion on light years and cosmic distance
please see http://chandra.harvard.edu/photo/cosmic_distance.html