Stellar Streams & Galactic Archaeology: Observation, Measurement & Evidence

A stream can contain leading and trailing tails extending many degrees across the sky. Member stars share coherent orbital motion and often similar chemical abundances, although the original satellite may be partly or completely disrupted.

Wide-field surveys and Gaia reveal coherent six-dimensional phase-space structures and abundance patterns. Streams such as Sagittarius and GD-1 trace past accretion and show perturbations that may encode encounters with visible or dark substructure.

As a satellite or cluster orbits the Milky Way, differential gravity removes stars preferentially near tidal boundaries. Stripped stars occupy slightly different orbital energies and spread along the progenitor orbit, producing a narrow stream whose detailed path responds to the Galaxy's mass distribution.

Gaia positions and velocities, spectroscopy, metallicities, and wide-field imaging reveal streams as structures in phase space rather than merely faint sky overdensities.