Парсек: Астрономическая единица измерения расстояний.

Unit of length used in astronomy

The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i. e. The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 AU subtends an angle of one arcsecond ( of a degree). The nearest star, Proxima Centauri, is about from the Sun: from that distance, the gap between the Earth and the Sun spans slightly less than of one degree of view. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand parsecs, and the Andromeda Galaxy at over 700,000 parsecs. The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to simplify astronomers' calculations of astronomical distances from only raw observational data. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies. In August 2015, the International Astronomical Union (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly  au, or approximately 3.0856775814913673 metres (based on the IAU 2012 definition of the astronomical unit). This corresponds to the small angle definition of the parsec found in many astronomical references.

The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground based telescope measurements of parallax angle is limited to about 0.01, and thus to stars no more than 100 distant. This is because the Earth's atmosphere limits the sharpness of a star's image. Space based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground based observations. Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100000 stars with an astrometric precision of about 0.97, and obtained accurate measurements for stellar distances of stars up to 1000 away. ESA's Gaia satellite, which launched on 19 December 2013, is intended to measure one billion stellar distances to within 20s, producing errors of 10% in measurements as far as the Galactic Centre, about 8000 away in the constellation of Sagittarius.

Distances expressed in fractions of a parsec usually involve objects within a single star system. So, for example:
One astronomical unit (au), the distance from the Sun to the Earth, is just under 5. The most distant space probe, Voyager 1, was 0.0007897 from Earth as of 2024. Voyager 1 took 46 to cover that distance. The Oort cloud is estimated to be approximately 0.6 in diameter

Distances expressed in parsecs (pc) include distances between nearby stars, such as those in the same spiral arm or globular cluster. A distance of is denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to express distances between parts of a galaxy or within groups of galaxies. So, for example :
Proxima Centauri, the nearest known star to earth other than the sun, is about away by direct parallax measurement. The distance to the open cluster Pleiades is 130 (420) from us per Hipparcos parallax measurement. The centre of the Milky Way is more than from the Earth and the Milky Way is roughly across. ESO 383 76, one of the largest known galaxies, has a diameter of The Andromeda Galaxy (M31) is about away from the Earth.

To determine the number of stars in the Milky Way, volumes in cubic kiloparsecs (kpc3) are selected in various directions. All the stars in these volumes are counted and the total number of stars statistically determined. The number of globular clusters, dust clouds, and interstellar gas is determined in a similar fashion. To determine the number of galaxies in superclusters, volumes in cubic megaparsecs (Mpc3) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge Boötes void is measured in cubic megaparsecs. In physical cosmology, volumes of cubic gigaparsecs (Gpc3) are selected to determine the distribution of matter in the visible universe and to determine the number of galaxies and quasars. The Sun is currently the only star in its cubic parsec, (pc3) but in globular clusters the stellar density could be from 100. The observational volume of gravitational wave interferometers (e. g., LIGO, Virgo) is stated in terms of cubic megaparsecs (Mpc3) and is essentially the value of the effective distance cubed.

The parsec was seemingly used incorrectly as a measurement of time by Han Solo in the first Star Wars film, when he claimed his ship, the Millennium Falcon "made the Kessel Run in less than 12 parsecs". The claim was repeated in The Force Awakens, but this was changed in Solo: A Star Wars Story, by stating the Millennium Falcon traveled a shorter distance (as opposed to a quicker time) due to a more dangerous route through the Kessel Run, enabled by its speed and maneuverability. It is also used ambiguously as a spatial unit in The Mandalorian as opposed to a unit of distance. In the book A Wrinkle in Time, "Megaparsec" is Mr. Murry's nickname for his daughter Meg.