In stars. As far as I know this is the only place.
When hydrogen stocks run out
The fusion of Hydrogen into Helium.
When two hydrogen atoms fuse to become one helium atom.
The fusion of Hydrogen into Helium causes heat and radiation to occur.
Hydrogen fusion does not occur in the corona of the sun. Fusion reactions occur in the sun's core where conditions are hot and dense enough for hydrogen nuclei to combine to form helium, releasing energy in the process. The corona is cooler and less dense than the core, so fusion cannot take place there.
In the core of the Sun, hydrogen atoms fuse to form helium in a process known as hydrogen fusion. This is the primary fusion process occurring in the Sun. As the core hydrogen is depleted, helium fusion into heavier elements like carbon and oxygen will occur in later stages of the Sun's evolution.
The helium in the Sun's core does not undergo fusion because it primarily consists of helium-4, which is a product of hydrogen fusion. While temperatures and pressures in the core are extremely high, the conditions required for helium fusion—such as even higher temperatures (around 100 million Kelvin) and sufficient density—are not met until much of the hydrogen has been fused into helium. Helium fusion, known as the triple-alpha process, will occur later in the Sun’s life cycle, once it exhausts its hydrogen fuel.
Nuclear fusion in the sun occurs when hydrogen atoms combine to form helium atoms. This process releases large amounts of energy in the form of photons. The intense pressure and temperature in the sun's core create the conditions necessary for nuclear fusion to occur.
Helium is formed in the universe through nuclear fusion processes that occur in stars. During the fusion of hydrogen atoms in the core of a star, helium is produced as a byproduct. This process releases a large amount of energy and is responsible for the creation of helium in the universe.
If there is no hydrogen left at the core of star then hydrogen fusion cannot occur. What happens in the core of a star before that happens is that helium begins to fuse, and then the other elements going up the periodic table until carbon. And then if the star explodes into a supernova, traces of the higher elements are fused as well.
Nuclear fusion occurs in the solar core.
In this process, hydrogen-1 is converted into helium-4. Energy is freed up in the process.