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 is a calculated radius that additionally approximates the effects of 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.