The concentration of stars then gradually declines out to an invisible boundary called the cluster's tidal radius
. Here the gravitational attraction of the Milky Way is stronger than that of the cluster itself, and stars become unbound and join the galaxy's stellar halo.
The theory assumes that it always fills its tidal radius; that is, it extends outward to the point where its attraction for nearby gas equals that of the sun.
After accretion stops, the planet settles down, contracting a little from its tidal radius. The contraction makes what is knwon as an infall zone between the tidal radius and the actual surface of the planet.