**Nuclear Fusion**

Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus. This process is the opposite of nuclear fission, which involves the splitting of a heavy nucleus into lighter nuclei. Nuclear fusion is the primary process that powers stars, including our own Sun.

**Explanation:**

**1. Nuclear Fusion in Stars:**
In the core of stars, such as our Sun, extremely high temperatures and pressures exist. These conditions are necessary for nuclear fusion to occur. The most common fusion reaction in stars is the fusion of hydrogen nuclei (protons) to form helium nuclei.

**2. Proton-Proton Chain:**
The primary fusion process in stars, like our Sun, is known as the Proton-Proton Chain. In this process, four hydrogen nuclei (protons) combine to form one helium nucleus. The energy released during this fusion process is what powers the Sun and provides the Earth with light and heat.

**3. Fusion Reactions:**
Nuclear fusion can occur between various light nuclei. Some common fusion reactions include:

- Deuterium-Deuterium Fusion: Two deuterium nuclei combine to form a helium nucleus.
- Deuterium-Tritium Fusion: A deuterium nucleus and a tritium nucleus combine to form a helium nucleus.
- Carbon-Nitrogen-Oxygen (CNO) Cycle: This is another fusion process that occurs in stars and involves the fusion of carbon, nitrogen, and oxygen nuclei.

**4. Conditions for Fusion:**
Nuclear fusion requires extremely high temperatures and pressures to overcome the electrostatic repulsion between positively charged nuclei. These conditions are typically found in the core of stars, where temperatures can reach millions of degrees Celsius.

**5. Controlled Fusion on Earth:**
Scientists have been working on achieving controlled fusion reactions on Earth as a potential source of clean and abundant energy. However, recreating the conditions of the Sun on Earth is extremely challenging, and significant technological advancements are still required to achieve practical fusion power.

In summary, the process in which two light nuclei combine to form a heavier nucleus is known as nuclear fusion. This process is responsible for the energy production in stars, including our Sun. Scientists are actively researching fusion as a potential source of clean and sustainable energy for the future.