Parsec

Parsec (pronounced pahr-sek)

A unit of astronomical length, based on the distance from Earth at which a star would have a parallax of one second of arc which is equivalent to 206,265 times the distance from the earth to the sun or 3.26 light-years.  Its lineal equivalent is about 19.1 trillion miles (30.8 trillion km).

1913: The construct was par + sec, derived from parallax + second.  Parallax is from the Middle French parallaxe, from the Ancient Greek παράλλαξις (parállaxis) (alteration) from παραλλάσσω (parallássō) (to cause to alternate) from ἀλλάσσω (allássō) (to alter) from ἄλλος (állos) (other).  Second, in the sense of time, is from the Middle English secunde & seconde, borrowed from the Old French seconde, from the Medieval Latin secunda, short for secunda pars minuta (second diminished part (of the hour)). Parsec is a noun; the noun plural is parsecs.

Distance in space

The parsec (pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System.  A parsec is defined as the distance at which one astronomical unit subtends an angle of one arc second, which corresponds to 648000 astronomical units.  The nearest star to Earth, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun.

The parsec unit appears first to have been suggested in 1913 by the English astronomer Professor Herbert Hall Turner (1861-1930).  It was invented to permit calculations of astronomical distances using only raw observational data and is thus the default unit of measure in astronomy and astrophysics although the more-easily understood light-year is preferred by lay-persons.  While useful to describe the comparatively short distances within the Milky Way, multiples of parsecs are needed to map the wider universe including kiloparsecs (kpc) for objects immediately beyond the Milky Way and megaparsecs (Mpc) for more distant galaxies.  Gigaparsecs (Gpc) are used to measure the quasars and galaxies which exist at the extreme edge of the known universe; the particle horizon (the boundary of the observable universe) has a radius of about 14.0 Gpc (46 billion light-years).

Although in use for more than a century, the parsec wasn’t formally defined until August 2015 when the International Astronomical Union (IAU) which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, noted the existing explicit definition of the parsec as exactly 648000 astronomical units, or approximately 3.08567758149137×10¹⁶ metres.

Hubble telescope image of star-forming region in the Carina Nebula.  As photographed, the gas and dust cluster is about .889 of a parsec (2.9 light-years) tall.