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Recessional velocity is the rate at which anextragalacticastronomical object recedes (becomes more distant) from an observer as a result of theexpansion of the universe.[1] It can be measured by observing the wavelength shifts ofspectral lines emitted by the object, known as the object'scosmological redshift.
Hubble's law is the relationship between a galaxy's distance and its recessional velocity, which is approximately linear for galaxies at distances of up to a few hundredmegaparsecs. It can be expressed as
where is theHubble constant, is theproper distance, is the object's recessional velocity, and is the object'speculiar velocity.
The recessional velocity of a galaxy can be calculated from theredshift observed in its emitted spectrum. One application of Hubble's law is to estimate distances to galaxies based on measurements of their recessional velocities. However, for relatively nearby galaxies the peculiar velocity can be comparable to or larger than the recessional velocity, in which case Hubble's law does not give a good estimate of an object's distance based on its redshift. In some cases (such as theAndromeda Galaxy, 2.5 millionlight-years away and approaching us at 300 km/s, or evenMessier 81 at 12 million light-years away and approaching at 34 km/s) is negative (i.e., the galaxy's spectrum is observed to be blueshifted) as a result of the peculiar velocity.
