Hydrogen fusion into helium occurs in the core of the Sun.

Here's why:

* High Pressure and Temperature: The Sun's core has immense pressure and extremely high temperatures (around 15 million degrees Celsius). These conditions are necessary to overcome the electrostatic repulsion between hydrogen nuclei (protons) and allow them to fuse.

* Proton-Proton Chain: The fusion process in the Sun primarily occurs through a series of reactions known as the proton-proton chain. This chain starts with two protons colliding and fusing to form deuterium (an isotope of hydrogen), releasing a positron and a neutrino. This process continues until four protons ultimately fuse to form a helium nucleus.

So, the Sun's core is the nuclear furnace where hydrogen is converted into helium, releasing the energy that powers the Sun and sustains life on Earth.