What are lightyears and why do scientists use them?

Light years are the distance is takes light to travel in a year. Scientist may use light years to measure the distance of how far away a star is, or other celestial objects.

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Most objects in space are so far away, that using a relatively small unit of distance, such as an astronomical unit, is not practical. Instead, astronomers measure distances to objects which are outside our solar system in light-years.

Source: coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/distance.html