The temperature at which fusion of hydrogen into helium begins is approximately 10 million degrees Celsius (18 million degrees Fahrenheit). This is the temperature required for the hydrogen nuclei to overcome their electrostatic repulsion and fuse together.

It's important to note that this is just the ignition temperature. Sustaining fusion reactions requires a much higher temperature, typically around 100 million degrees Celsius (180 million degrees Fahrenheit), as well as extremely high pressures.

Here's why it's so hot:

* Electrostatic repulsion: Hydrogen nuclei (protons) have positive charges, which repel each other.

* Quantum tunneling: At these high temperatures, the hydrogen nuclei have enough energy to overcome this repulsion due to a quantum mechanical phenomenon called tunneling.

* Strong nuclear force: Once the nuclei are close enough, the strong nuclear force takes over, overcoming the electrostatic repulsion and binding them together.

This is why achieving sustained fusion on Earth is a very difficult engineering problem.