Is a black hole heavier than the star from which it was created?

I did some googling around and couldn't get answer to a simple question of black hole mass compared to the star from which it was created. It is well known that giant stars burn very bright and relatively very quickly. They end up their lives as red giants and eventually collapse into a black hole.

Suppose that the star is mid-life and has some mass M. Over the time it burns its fuel, turns into a red giant and after supernova, turns into a black hole. This black hole has some mass B. Why B >> M?

According to some simple physics laws, mass could not be created nor destroyed.

• What made you think B > M in the first place? Nov 13, 2015 at 17:58
• @RussellBorogove The fact that when any black hole meets a star, it devours it. That was my misconception that the heaviest "eats" the lightest. Nov 13, 2015 at 18:00
• Aha. Density is the dominant factor here, rather than mass. Note that in a star vs black hole collision, stellar matter approaching the black hole gets ferociously compressed, which leads to a lot of the matter getting blown clear of the collision -- you'll get a slightly fatter black hole and a lot of radiation emission and loose gas. youtube.com/watch?v=thkKrW23vNU Nov 13, 2015 at 18:11
• Mass, no. Weight, yes, pressure, yes, density yes. The weight of a dead star grows exponentially as it's size shrinks, even though it loses mass in it's final stages. Mass really can't be created, the mass is just what's left over, but as it contracts the weight grows very fast, and that's why a stellar core can collapse in as little as 1 second once it gets going. Nov 13, 2015 at 18:41
• @xeon: it's energy that can be neither created nor destroyed. Mass is destroyed in something as simple as a fire. A very (very!) small percentage of the matter is converted into light. Newton knew about this, see Opticks query 30. Nov 15, 2015 at 11:51