megaparsecs

Megaparsec (Mpc) is a unit of length used in astronomy to express distances between astronomical objects outside the Milky Way. It equals one million parsecs, or about 3.086 × 10^22 meters, and about 3.26 million light-years.

A parsec is defined as the distance at which 1 astronomical unit subtends an angle of 1 arcsecond; 1 parsec ≈ 3.26 light-years. Therefore 1 Mpc = 10^6 parsecs.

Megaparsecs are commonly used to denote distances to galaxies and clusters and to describe the large-scale structure of the universe. Distances to nearby galaxies such as the Andromeda Galaxy are about 0.78 Mpc; the Virgo cluster lies at about 16.5 Mpc. The observable universe has a comoving distance of about 14,000 Mpc (roughly 46 billion light-years). Hubble's law relates recession velocity v to distance via v ≈ H0 d, with H0 commonly quoted in km/s per Mpc.

In cosmology, several distance measures (luminosity distance, angular diameter distance, comoving distance) depend on the cosmological model; Mpc remains a convenient and widely used unit for expressing these distances in the local and extragalactic universe.