what is nuclear fission?

what is nuclear FUSION?

what are the two fissionable substances in common use in nuclear reactors?

what must happen to the radioactive atom before fission can occur?

what happens to the nucleus undergoing fission?

what is a chain reaction?

how is energy released in stars?

what conditions are needed for stars to form?

how are planets formed?

why is a star stable during the 'main sequence' period of its life

a stars life cycle is determined by what?