The Bondi radius is analogous to the Hill radius, both giving an idea of how far from an object, a medium (gas, dust) is likely to be drawn in and accreted. The Bondi radius takes into account the (relative) speed of the object through the medium, and the medium's density and sound-speed. Accretion due to material falling within the Bondi radius is called Bondi accretion. The Bondi radius and Bondi accretion were developed to model the accretion of a compact object (black hole or neutron star) passing through a medium. The concept has since been applied to planet formation, i.e., protoplanets passing through the material of a circumstellar disk.