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Originally Posted by Regulus
Chris you rubber sheet analogy (which I also dislike but can't think of a better one) made me think thusly.
If a sun has thrown off mass and then collapsed to a point and is now a black hole, why wasn't the original sun with it's greater mass also a black hole??
Is it to do with the smaller area the mass is concentrated into?
Dave2042 thanks for the article link it was interesting.
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Regulus
You have correctly answered your own question. To elaborate, the Schwarzchild radius (ignoring spinning and charged matter) is simply defined for any point mass, right down to fundamental particles (leaving aside the fact that at this point QM is well in play). A non-point mass (eg a star) is a black hole iff it is so dense that it is all inside its Schwarzchild radius.
Glad someone enjoyed that link.