Here's why:
* Nuclear Fusion: Hydrogen fusion is the process where two hydrogen nuclei (protons) fuse together to form a helium nucleus. This process requires immense energy to overcome the electrostatic repulsion between the positively charged protons.
* Temperature and Energy: Temperature is a measure of the average kinetic energy of particles. At extremely high temperatures, hydrogen nuclei move with enough energy to overcome their mutual repulsion and fuse.
* Quantum Tunneling: Even at temperatures slightly below 10 MK, some protons can fuse via a process called quantum tunneling. This is a probabilistic event where particles can "tunnel" through potential barriers, but it becomes increasingly likely as temperature increases.
Important Note: The exact temperature for hydrogen fusion depends on the density and composition of the stellar core. Larger and denser stars have higher core temperatures.
