The number take for the "radius" is normally taken to be the radius from the center to the event horizon, since nothing inside that can be observed, and since it is a point of no return. This radius...
A black hole is called that because when viewing it with the naked eye (visible light spectrum), the phenomenon appears to be a 2-D "hole" in space (or spacetime.) An actual Black Hole is much more...
When a huge star dies it's core becomes so dense it's own gravity starts to crush and collapse inwards the heart of a black hole is called the (Singularity)
When a Super Giant or Hyper Giant Star nears the end of it life, it goes through a process of shedding layers of itself, a little like onion layers cast off one after the other. When it reaches a...