Astronomers typically express the distances between neighbouring galaxies and galaxy clusters in megaparsecs (Mpc). A megaparsec is one million parsecs, or about 3,260,000 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 constant H for the rate of expansion of the universe: h = ⁠ H / 100 (km/s)/Mpc ⁠. The Hubble constant becomes relevant when converting an observed redshift z into a distance d using the formula d ≈ ⁠ c / H ⁠ × z.