A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, equal to about 3.26 light-years or roughly 31 trillion kilometers. The name is a contraction of parallax and arcsecond, and the unit exists because it falls directly out of how stellar distances are measured. Professional astronomers use it in preference to the light-year, scaling up to kiloparsecs for galactic distances and megaparsecs for intergalactic ones. The nearest star system, Alpha Centauri, lies about 1.3 parsecs away.