Here's why:

* The Sun's primary fuel is hydrogen. Nuclear fusion in the Sun's core converts hydrogen into helium, releasing energy in the process.

* Hydrogen makes up about 70-75% of the Sun's mass, while helium makes up about 24-27%.

* Although helium is being constantly created through fusion, the process is slow, and the Sun has a vast supply of hydrogen.

Even though the Sun is continuously fusing hydrogen into helium, it still has a massive amount of hydrogen left, allowing it to continue burning for billions of years.