Final answer:
Proton-proton fusion in the Sun's core converts one proton into a neutron through the emission of a positron, creating deuterium rather than He-2. This initial step is part of a chain reaction leading to helium formation and requires extremely high temperatures for nuclei to overcome repulsive forces and fuse.
Step-by-step explanation:
The fusion of a proton and a proton in the Sun's core does not directly produce the He-2 isotope, but instead leads to the formation of deuterium (H-2). During this fusion, one of the protons is converted into a neutron, which combines with the other proton to form the deuterium nucleus. This process conserves electric charge by emitting a positron, which is essentially antimatter equivalent of an electron, and carrying away the positive charge. After this initial step, the proton-proton chain continues, and deuterium further reacts to eventually form helium.
In terms of thermonuclear fusion, it's important to note that extremely high temperatures are necessary to overcome the repulsive forces between positively charged nuclei. Isotopes with low binding energies, like deuterium (H-2) and tritium (H-3), are more suitable for fusion reactions. These conditions allow for the nuclei to collide with enough kinetic energy to fuse.