megaparsec

A megaparsec (Mpc) is a unit of length used in astronomy to express distances between objects beyond the solar system. It equals one million parsecs, and a parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond. Since a parsec is about 3.26 light-years, a megaparsec is about 3.26 million light-years. In meters, 1 parsec is approximately 3.0857 × 10^16 m, making 1 Mpc about 3.0857 × 10^22 m.

Megaparsecs are convenient for describing distances to galaxies, clusters, and large-scale structures in the nearby universe. For example, the Andromeda Galaxy is about 0.78 Mpc away from the Milky Way, while the Virgo Cluster lies at roughly 16–20 Mpc. Distances to faraway galaxies and surveys can extend to hundreds of Mpc. In cosmology, redshift measurements are often converted to distances in Mpc using a cosmological model; at small redshifts, distance approximately equals velocity divided by the Hubble constant.

The unit is frequently used alongside the reduced Hubble constant h, with values expressed as h^(-1) Mpc to reflect uncertainty in H0. The megaparsec thus remains a standard unit for expressing extragalactic distances and for comparing scales in observational cosmology.