In the reactions occurring in the sun, some atoms collide with such force that their nuclei join together to make one new nucleus. Why must a new element be formed in reactions of this kind?
An element is defined by the number of protons existing in its nucleus. Atoms that collide with enough force to fuse their nuclei together to make one new nucleus will create a new element.
An element may have different numbers of electrons (called ions), or different numbers of neutrons (called isotopes). But, if the number of protons changes, a new element is formed.
The change in proton number may occur during nuclear decay and fission (loss of protons) or the process of fusion (gain of protons), as we have in this case.
The differences are described in detail here:
There are illustrations of the fusion reactions here: