How much mass do you need....

... for a star to form a black hole? Well, apparently more than previously thought.
When massive stars run out of fuel, their Iron core collapses under its own gravity and turns into a neutron star. If the energy released in this collapse is enough to expel the outer layers of the star, you are left with a neutron star. If too much more material falls onto the neutron star before the released energy blows away the rest of the star, you have a black hole. Stars with an initial mass more that 40 times that of the Sun were expected to be well above the threshold where you make a black hole. However, nature appears to have other ideas, as you can read about here and here and here. Maybe such stars lose enough mass before they explode that the initial collapse is sufficient. Hopefully we'll find out soon.