Question

Why nuclear fusion is easy in stars and difficult in labs?

Answer 1

The fusion of two nuclei lighter than iron or nickel generally releases energy while the fusion of nuclei heavier than iron or nickel absorbs energy; vice-versa for the reverse process, nuclear fission.

Nuclear fusion of light elements releases the energy that causes stars to shine and hydrogen bombs to explode.

Nuclear fusion of heavy elements (absorbing energy) occurs in the extremely high-energy conditions of supernova explosions.