The Lethe stream is a stellar stream in the Milky Way Galaxy, consisting of approximately 1100 stars. It was discovered by Carl J. Grillmair in 2008 along with the Acheron, Cocytos and Styx streams,^[1] and moves with an average tangential velocity of ${\displaystyle v_{T}=113\pm 12.5~\mathrm {m/s} }$.^[2] The name of this stream stems from Lethe, the mythical river of unmindfulness in Hades.^[1]

The stars in the Lethe stream are not in the plane of the Milky Way and are proposed to have originated from the debris of a globular cluster. Evidence supporting this theory of origin is taken by comparing Lethe's stars to the color magnitude diagrams (CMDs) from the Sloan Digital Sky Survey (SDSS).^[1] The stream's stellar population consists of old, metal-poor stars with an approximate metallicity of ${\displaystyle [\mathrm {Fe/H} ]\sim -1.5}$. Along with it being dynamically cold and having a relatively narrow width of ${\displaystyle \sim 95~{\text{pc}}}$, these are indicators that the Lethe stream is composed of the remnants of an either extant or disrupted globular cluster instead of debris from a dwarf galaxy.^[1][3]

The stream was first described in 2009 by Carl Grillmair using data from the SDSS recorded at the Apache Point Observatory in New Mexico.^[1]

• List of stellar streams