A term used in
astrophysics, the
critical circumference is a limit to the size an object of a given
mass can be, without
imploding to form a
black hole. It's given by the equation :-
Critical Circumference = 18.5 km x Mass
Where the mass is in
solar masses.
So if a mass the size of the sun ever shrinks so it's circumference is less than 18.5 km, it's
event horizon time!
This value is needed for the
hoop conjecture, to model the collapse of
non-spherical masses.