Well, not really. The mass of the black hole created equals the amount of mass that falls into it, of course, and to first order the mass of the BH will be equa

Well, not really. The mass of the black hole created equals the amount of mass that falls into it, of course, and to first order the mass of the BH will be equal the mass of the iron core of the star. That mass is however quite a bit smaller than the mass of the star, and very much smaller than the initial mass of the star. (A star has to start out with a mass > 8* the mass of the Sun to go supernova, and that will only guarantee that it forms a neutron star whose mass will be of order 1 Solar mass. The rest is lost first during the life of the star, as massive stars are so luminous they drive their own matter off from the surface, and then ejected in the explosion itself. Edit: The article has a number off, as it states that the black hole has a mass of 15 Solar masses and that the companion is 19 times larger. The original article says the companion is 19 Solar masses, which makes more sense.