Final answer:
Scientists use light-years or parsecs to measure distances outside our solar system, with a light-year being the distance light travels in one year and a parsec being 3.26 light-years.
Step-by-step explanation:
Outside of our solar system, distances are so vast that the astronomical unit (AU) is too small to be practical for measurements. Therefore, scientists use either light-years or parsecs to express these distances. A light-year is the distance light travels in one year, and it's a particularly intuitive unit because it not only conveys the notion of space but also time - the light we see from distant objects shows them as they were when that light left them, not as they are today. One parsec equals 3.26 light-years and is defined as the distance to a star with a yearly parallax of one arcsecond. Although parsecs are still in use, particularly when measuring distances derived from parallax measurements, the light-year is more commonly understood and emphasizes the time aspect of observing distant celestial objects.