The type of reactions considered to be the principal sources of energy in stars is `……………..` when hydrogen is converted to `………………` .

Answer

To solve the question, we need to identify the type of reactions that are the main sources of energy in stars and what element hydrogen is converted into during these reactions.

### Step-by-Step Solution:

1. **Identify the Type of Reaction**:
- The principal source of energy in stars is through nuclear reactions. Specifically, we are looking for the type of nuclear reaction that occurs in stars.
- In stars, the primary reaction is **nuclear fusion**. This is the process where two light atomic nuclei combine to form a heavier nucleus.

2. **Determine the Elements Involved**:
- In the case of stars, the most common element that undergoes fusion is **hydrogen**.
- During nuclear fusion, hydrogen nuclei (protons) fuse together to form a heavier nucleus.

3. **Identify the Product of the Reaction**:
- When hydrogen undergoes fusion, it is primarily converted into **helium**. This process releases a significant amount of energy, which is what powers the stars.

4. **Final Answer**:
- Therefore, the complete answer to the question is:
- The type of reactions considered to be the principal sources of energy in stars is **nuclear fusion reactions** when hydrogen is converted to **helium**.

### Summary:
- The type of reactions: **Nuclear fusion reactions**
- Hydrogen is converted to: **Helium**