Steradian is the solid angle subtended at the centre of the sphere by its surface whose area is equal to ___ of the sphere.

Definition. A steradian can be defined as the solid angle subtended at the centre of a unit sphere by a circular unit area on its surface. For a general sphere of radius r, any portion of its surface with area A = $r^2$ subtends one steradian at its centre. sr is the unit, steradian.