Black Hole gravity (Score 1)

I'm trying to understand the gravity of black holes. When a star dies and a black hole results the amount of mass that makes up the black hole can not be more than the mass of the original star but the black hole exhibits much more gravity than the original star. Is this the result of r approaching zero in the law of universal gravitation? F = G ( m1*m2/r**2 ) as r->zero, F = G*m1*m2