A stellar halo is a region of space surrounding a galaxy, composed mainly of old stars, globular clusters, and dark matter. These halos extend far beyond the visible parts of galaxies and can contain a significant amount of mass, influencing the galaxy's formation and evolution. The stars in a stellar halo are typically older and have different chemical compositions compared to those in the galaxy's disk. Milky Way is an example of a galaxy with a prominent stellar halo, which helps astronomers study the history and dynamics of galaxies in the universe.