![]() To describe such a huge range, astronomers need a way to avoid confusing terms like "a billion trillion" and "a millionth". The distances and sizes of of the objects astronomers study vary from very small, including atoms and atomic nuclei, to very large including galaxies, clusters of galaxies and the size of the universe. 1 pc = 3.09 x 10 13 km = 3.26 lyįor even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc). A parsec is the distance at which 1 AU subtends an angle of 1 arcsec. This is because its definition is closely related to a method of measuring the distances between stars. Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This means light from Proxima Centauri takes 4.2 years to travel to Earth. Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. ![]() A light-year is the distance that light travels in a vacuum in one year: 1 ly = 9.5 x 10 12 km = 63,240 AU To measure the distances between stars, astronomers often use light-years (abbreviated ly). The distance from the Sun to the center of the Milky Way is approximately 1.7 x 10 9 AU. Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. An astronomical unit is the average distance between the Earth and the Sun: 1 AU = 1.496 x 10 8 km = 93 million miles However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance.ĭistances in the solar system are often measured in astronomical units (abbreviated AU). They often use meters for length, kilograms for mass, and seconds for time. Astronomers use many of the same units of measurement as other scientists.
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