For the people that don't know what black holes are:
If a sun dies, it gets unstable because it consumed all of it's hydrogen. Then it falls together and ends up in a GIANT explosion (supernova, pl. supernovae). After the supernova, a white dwarf stays. It's mass is incredible, while it's even smaller then the moon. A spoon full of mass would weight more than 1,000,000 tons. As we all know, more mass means more gravity. The more mass the more escape speed. Escape speed is the speed you need to leave a planet, on the earth it's 40.000 km/h. That means a rocket has to travel with atleast 40.000 km/h, thats 11 metres per second. If it is slower than 11 m/s, it will fall back to earth. Okay, now if the sun had a REALLY high mass, the mass of the white dwarf is incredibly high too. That means it got a high gravity, with an escape speed so high, higher than light speed (~300.000.000 m/s). That means not even light can escape it. If there's no light, it's black.