I assume you mean the event horizon of a black hole. As I understand it, the explosion itself won't have any significant effect on the black hole. The black hole is capable of absorbing both matter and energy - both will increase the black hole's mass (since energy has a mass equivalent). Please note that a supernova explosion, for example, is an event that takes a few days - much longer than it would take the star to fall into a black hole of any known size. In any case, any mass (and energy) that gets inside the event horizon won't get out again. Light (and very fast-moving particles) that are slightly outside of the event horizon have a small chance of escaping.
No, they do not.
If you mean a giant star, no.
No. Only black holes have event horizons.
No. An event horizon is an area where even light cant escape so only black holes have it
That refers to a black hole - but a black hole is not exactly a star.
An event horizon is the boundary beyond which nothing, not even light, can escape the immense gravitational pull of a black hole.
they become super novas and explode
If a dwarf star crashed into a planet,the planet would likely explode.
A planet cannot explode on it's own. A star would experience very little changes if a planet did explode, even one as large as Jupiter.
What happens if a firework hits the star is it will explode anyway it is Impossible for a firework to hit the sky, that because a star is way higher then a firework.
It seems you are referring not to any collapsed star, but a black hole. The "event horizon" is the area from which nothing can escape.
The size of a black hole, as defined by the size of the event horizon, depends on the mass of the black hole and its electrical charge. The diameter of the event horizon is directly proportional to the black hole's mass. Adding electrical charge decreases the size of the event horizon.