Derivation
A non-rigorous but conceptually accurate derivation of the Hill radius can be made by equating the orbital angular speed of the orbiter around a body (i.e. a planet) and the orbital angular speed of that planet around the host star. This is the radius at which the gravitational influence of the star roughly equals that of the planet.
Where is the Hill radius, a is the semi-major axis of the planet orbiting the star. With some basic algebra:
Giving a Hill radius of:
Read more about this topic: Hill Sphere