Parsec (pc)

The parsec is a unit of distance equal to about 3.0857 × 10¹⁶ metres, or 3.26 light years. The symbol is pc. It is the standard distance unit in professional astronomy, used in preference to the light year in research literature.

The name is a contraction of parallax second, and the definition follows directly from how stellar distances are actually measured. As the Earth orbits the Sun, nearby stars appear to shift slightly against the more distant background. Half of that annual shift is the star's parallax angle. A star with a parallax of one arcsecond lies at a distance of one parsec. The relationship is a simple reciprocal: distance in parsecs equals one divided by parallax in arcseconds, which is why the unit is convenient for observers. No conversion is needed between the measurement and the result.

Formally the parsec is defined as 648,000 divided by pi astronomical units, a value the International Astronomical Union fixed exactly in 2015. Herbert Hall Turner proposed the name in 1913.

No star is close enough to have a parallax of a full arcsecond. Proxima Centauri, the nearest, has a parallax of about 0.77 arcseconds and lies 1.3 parsecs away. The angles involved are minute, which is why parallax was not successfully measured until 1838 despite being sought since antiquity. The Gaia spacecraft has since measured parallaxes for over a billion stars with microarcsecond precision.

Multiples handle larger scales. The Milky Way is about 30 kiloparsecs across, and distances between galaxy clusters run to megaparsecs. The Hubble constant is conventionally quoted in kilometres per second per megaparsec.

The unit is widely known from a line in Star Wars in which the Millennium Falcon completes a route in under twelve parsecs, a distance where a time would be expected.

Cosmology works in megaparsecs and gives the unit a role in one of its central quarrels. The Hubble constant, the rate at which the universe expands, is quoted in kilometres per second per megaparsec: a galaxy one megaparsec away recedes at about seventy kilometres a second, one two megaparsecs away at twice that. Measurements from the cosmic microwave background give a value near 67, while those from supernovae and variable stars give about 73, and the gap between them has resisted a decade of effort to close it. Whatever resolves the disagreement, it will be argued over in this unit.

One parsec equals 3.26156 light years, 206,265 astronomical units, or 30.857 trillion kilometres.

Parsec conversion table