A light-year is the distance that light (or any other form of electromagnetic radiation, such as radio waves) travels in a vacuum in one year. Since light travels at a velocity of 186,171.1 mi/s (299,792.5 km/s), one light-year equals 5,878,489,000,000 miles (9,460,530,000,000 km). The light-year is a convenient unit of measurement to use when discussing distances to the stars in the Milky Way galaxy and throughout the observable universe. When discussing distances within our solar system, the astronomical unit (the mean distance between Earth and the Sun) is commonly used. One light-year equals 63,239.7 astronomical units.
Since the distances between Earth and even the nearest stars are so enormous, a light-year can also be thought of as a measurement of time. Sirius, for example, is 8.57 light-years away. This means that when an observer on Earth looks at Sirius, they see light that left Sirius 8.57 years ago. The observer is therefore looking backward in time, seeing the star in the condition it was in more than eight years ago.
Alpha Centauri, the closest star to Earth, is 4.35 light-years distant. Among other stars, Barnard's star is 5.98 light-years away, 61 Cygni is 11.3 light-years away, and Antares is 400 light-years away. The center of the Milky Way galaxy is 27,000 light-years away, while the most distant clusters of galaxies are roughly estimated to be one million light-years away.