Question

If fission reactions give off energy, then do fusion reactions absorb energy? How does a fusion reaction still give off energy?

Answer 1

Only the fission of large atoms give off energy, and only the fusion of small atoms give off energy, look at this diagram http://www.daviddarling.info/encyclopedia/B/binding_energy.html

Answer 2

For example a star will gain energy by joining together (fusing ) hydrogen atoms, making larger atoms , once the star has a core of Fe Iron , no energy is released from fusion .

Answer 3

Once the star forms the core of Fe , thermonuclear fusion still proceeds, however since iron is THE most stablest element in the universe ( highest binding energy per nucleon), fusion of iron to form other heavier products will not RELEASE energy, but will start ABSORBING energy.. This is when the star's fuel dies out fast and explodes