Here's why:
* Fusion is a slow process: Fusion involves smashing together atomic nuclei (like hydrogen) to create heavier ones (like helium). This requires overcoming the electrostatic repulsion between the positively charged nuclei, which takes an immense amount of energy and time.
* High temperatures and pressure: The Sun's core is incredibly hot (around 15 million degrees Celsius) and under immense pressure. These conditions are necessary to provide the energy and force to initiate fusion.
* Quantum tunneling: While the temperatures are high, the energy required to overcome the repulsion is still immense. Fusion relies on a quantum mechanical phenomenon called "tunneling" where particles can, in a sense, "borrow" energy to temporarily overcome the barrier and fuse. This process is still very slow, even at those extreme temperatures.
The timescale:
It takes on average billions of years for hydrogen atoms to fuse into helium in the Sun's core. While this might seem incredibly long, it's the reason why the Sun has been shining for billions of years and will continue to do so for billions more.
In summary: The slow, continuous process of nuclear fusion inside the Sun is what provides its energy and light. The extreme conditions and quantum tunneling are the key players in this incredibly long-lasting phenomenon.
