Units · Stellar & galactic scale
Parsec
Approximately 3.26 light-years or 206,265 AU. The standard unit of distance in professional astronomy, defined by the angle of stellar parallax: the distance at which 1 AU subtends 1 arcsecond.
Where the name comes from
Parallax + arcsecond = parsec. Coined in 1913 by Herbert Hall Turner.
Source: International Astronomical Union
Distance units compared
The parsec sits above the light-year in scale and is the preferred unit of professional astronomers because it emerges directly from measurements. At galactic scales, kiloparsecs (kpc) and megaparsecs (Mpc) are used; cosmological distances often reach gigaparsecs (Gpc).
Convert between AU, light-years, and parsecs instantly with our Distance Converter tool. The Sun is about 8.3 kpc from the centre of the Milky Way. The Andromeda Galaxy (M31) is about 0.78 Mpc away.
Frequently asked questions
- The parsec arises directly from the parallax method of measuring stellar distances — it is the distance at which 1 Astronomical Unit subtends an angle of 1 arcsecond. Because parallax is how stellar distances are actually measured, the parsec is the most natural unit for professional astronomy. Light-years are used in popular science because they communicate the concept of "looking back in time" intuitively. The IAU defines both units formally.
- Astronomers routinely use prefixed versions: a kiloparsec (kpc) is 1,000 parsecs (~3,260 light-years) — used for distances within the Milky Way. A megaparsec (Mpc) is one million parsecs (~3.26 million light-years) — used for distances to other galaxies and for cosmological measurements. The Hubble constant, for example, is expressed in km/s per Mpc. The ESA Gaia satellite measured parallaxes for over a billion stars, producing distances in parsecs for most of the Milky Way.
- Parallax is the apparent shift in a nearby object's position against a distant background when viewed from two different positions. For stars, the baseline is Earth's orbit: a star is observed in January and again in July, when Earth is on opposite sides of the Sun (a baseline of 2 AU). The apparent angular shift (the parallax angle, p) allows the distance to be calculated: distance in parsecs = 1/p (arcseconds). A star at 1 parsec has a parallax of exactly 1 arcsecond. The nearest star, Proxima Centauri, has a parallax of 0.768 arcseconds → distance of 1/0.768 = 1.30 parsecs = 4.24 light-years.
- 1 AU (Astronomical Unit) = 149,597,870.7 km (Earth–Sun mean distance). 1 light-year ≈ 63,241 AU ≈ 9.461 × 10¹² km (distance light travels in a year). 1 parsec ≈ 3.26 light-years ≈ 206,265 AU ≈ 3.086 × 10¹³ km. Convert between all three with our Distance Converter tool. See also: Light-Year in the dictionary.
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