Parsec (pc)
The parsec is a unit of distance used in professional astronomy, equal to approximately 3.2616 light-years or 30.857 trillion kilometers. Its name is a portmanteau of "parallax" and "arcsecond" - one parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. While the light-year is more popular with the general public, the parsec is the preferred unit among professional astronomers.
Definition
One parsec equals exactly 648,000/π astronomical units (approximately 206,265 AU), approximately 3.2616 light-years, or approximately 3.0857 × 1016 meters (30.857 trillion km). The parsec is defined geometrically: it is the distance at which one AU subtends one arcsecond. Larger multiples are common: kiloparsecs (kpc) for galactic distances and megaparsecs (Mpc) for intergalactic distances.
History
British astronomer Herbert Hall Turner coined the term "parsec" in 1913, combining "parallax" and "second" (of arc). The concept arose from the need to express stellar distances measured by parallax - the apparent shift of nearby stars against the distant background as the Earth orbits the Sun. A star at exactly one parsec would show a parallax angle of one arcsecond. No star is quite that close; Proxima Centauri, the nearest, is about 1.30 parsecs away. The parsec became the default unit in professional astronomy because parallax measurements yield distances directly in parsecs without conversion.
Common Uses
Professional astronomy papers report stellar and galactic distances in parsecs, kiloparsecs, and megaparsecs. The Hubble constant, which describes the expansion rate of the universe, is expressed in km/s per megaparsec. Star catalogs such as the Hipparcos and Gaia catalogs list parallax in milliarcseconds, yielding distances in parsecs directly. Galaxy cluster distances are expressed in megaparsecs. The parsec also appears in calculations of stellar luminosity, spectroscopic parallax, and cosmic distance ladders.
Did You Know? Facts About Parsec
- No star is within one parsec of the Sun - the closest, Proxima Centauri, is about 1.30 parsecs (4.24 light-years) away.
- In Star Wars, Han Solo claims the Millennium Falcon made the Kessel Run in "less than twelve parsecs" - famously using a distance unit where a time unit was expected.
- The center of the Milky Way is about 8.2 kiloparsecs (26,700 light-years) from Earth.
- The Hubble constant is approximately 70 km/s/Mpc, meaning a galaxy one megaparsec away recedes at about 70 km/s.
- The word "parsec" was coined in 1913 but didn't become widely used until stellar parallax measurements became routine in the mid-20th century.