Megaparsec

Megaparsecs and gigaparsecs

Astronomers typically express the distances between neighboring galaxies and galaxy clusters in megaparsecs (Mpc). A megaparsec is one million parsecs, or about 3.26 million light years. Sometimes, galactic distances are given in units of Mpc/h (as in "50/h Mpc", also written "50 Mpc h"). h is a constant (the "dimensionless Hubble constant") in the range 0.5 < h < 0.75 reflecting the uncertainty in the value of the Hubble constantH for the rate of expansion of the universe: h = ⁠H/100 (km/s)/Mpc⁠. The Hubble constant becomes relevant when converting an observed redshiftz into a distance d using the formula d ≈ ⁠c/H⁠ × z.