Sketch of mass and momentum flow. Surrounding the central gravitating body of mass M is a large, imaginary sphere. The thin curves represent streamlines of background gas. After entering the sphere, most gas simply exits again, while a small portion joins directly onto the mass. In addition, some gas temporarily forms an overdense wake, also sketched here. The wake tugs gravitationally on the mass, thereby imparting momentum to it (broad arrow). All gas entering the wake also leaves it, either joining the mass or exiting the sphere.