Hoop Conjecture

The Hoop Conjecture is a notion proposed in the early 1970s by Kip Thorne. The conjecture supposes that an imploding object forms a black hole when, and only when, a circular hoop with a specific circumference could be placed around the object and rotated. The critical circumference is 2 times Pi times the Schwarzschild radius for the object. This sounds rather arcane, but the idea is quite simple. What Thorne did was figure out the effects of gravity on different shapes, and decided that the only way gravity could make a black hole was if the object could be compressed in all 3 directions. Its somewhat akin to a magician claiming their floating assistant is not held up by wires, and demonstrating as much through a hoop conjecture: if the hoop can pass all the way over the assistant and be rotated at the ends, there are no wires. While nobody's made a hoop big enough to spin around a star so the conjecture remains a conjecture, most illusionists are also just illusionists and the pretty frock distracts you from the incompleteness of the demonstration. 
In short - objects only collapse into black holes when they are small enough in all directions (an infinite cylindrical star won't).