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.