Fusion in stars produces energy, including visible light radiated from the surface of a star, and forms new elements.

Fusion occurs in the center or core of a star, where the pressure and temperature are highest. The high temperature and pressure ensure that particles — always nuclei, since the high temperature strips electrons from neutral atoms — come together with enough energy to overcome the electric forces that would otherwise keep them apart.
The minimum temperature to ignite fusion is about 10 million K. Inside the Sun, a typical star, the core temperature is about 15 million degrees K.

Credit: Wikipedia

In stars like the Sun, which is of average mass and about halfway through its 10 billion year life span, the fusion reaction is primarily the "burning" of hydrogen into helium.

Four hydrogen nuclei — that is, four protons — form one helium nucleus (two protons and two neutrons), plus neutrinos and the release of energy. The four hydrogen nuclei do not all join at once, however. The reaction proceeds in a chain called the proton-proton chain.

Here’s a simple explanation of the proton-proton chain sequence:

Two protons (hydrogen nuclei) come together. One of the protons becomes a neutron. (A positron and a neutrino are also produced.) We now have the nucleus of a deuterium atom (one proton and one neutron).
The deuterium nucleus joins with a proton to form a helium-3 nucleus (two protons and a neutron) and a gamma ray.
Two helium-3 nuclei come together to form helium-4 (two protons and two neutrons), with the release of two protons.

This diagram of the proton-proton-chain includes the timescales of the reaction. Credit: UTK Physics

$E = m c^{2}$

Einstein's famous equation, E= mc2, provides the framework for understanding energy release in nuclear fusion.

The mass of a helium nucleus — two protons and two neutrons — is slightly less than the combined mass of the initial four protons. That difference in mass (about 0.7% of the original four protons) converts to energy according to this equation. The energy is carried off by neutrinos and gamma rays. The neutrinos fly off into space without significantly interacting with matter. The gamma rays heat the Sun.

In more massive stars, elements heavier than helium (including carbon and oxygen) can be fused once the hydrogen fusion is complete. The nuclear reactions that produce the heavier elements also proceed in a chain.

Energy released during the steps of nuclear chain reactions in the core makes its way to surface mainly by radiation and convection. From the surface, energy is radiated in the form of visible light, as well as electromagnetic radiation from other parts of the spectrum, such as x-rays. Energy is also lost to subatomic particles and to a stellar wind of electrons and protons.

Where do the heavy elements in the Sun's atmosphere come from?

Spectroscopy shows that the Sun's atmosphere contains trace amounts of heavy elements, such as iron and nickel. These elements were not formed in the Sun's interior; the Sun is still fusing hydrogen to helium.

The heavier elements were present in the solar nebula that condensed to form the Sun some 5 billion years ago. The solar nebula was in turn enriched by material from stars that formed earlier in the history of the universe. These early stars were massive enough to form the heavy elements and exploded after a relatively short lifetime, dispersing the elements into interstellar space. Supernova explosions also provide conditions to generate the very heaviest elements such as lead, gold, and uranium.

Thus the existence of trace amounts of heavy elements in the Sun can be traced to the life and death of more massive stars.

Fusion/Fission Confusion!

Students often confuse nuclear fusion, which goes on inside stars, with fission, by which nuclear power plants produce energy.

In fusion, the nuclei of smaller, lighter elements (lighter than iron, which has 26 protons and 30 neutrons) come together to form more massive, stable nuclei, releasing energy in the process.

In fission, we start with a very massive nucleus, such as that of uranium, which (with 92 protons and 145 neutrons) is inherently unstable. An energetic neutron colliding with the massive nucleus causes it to split apart into more stable, smaller nuclei, again releasing energy in the process.

Visit this site for more information on how both fission and fusion can release energy.