Why doesn't the Sun burn out?

When we look at the Sun, it certainly looks like a giant ball of fire glowing in the sky. But the truth is that the Sun is not burning in any normal sense of the word. Fire on Earth is a chemical reaction called combustion, which requires oxygen, fuel like wood or gas, and heat. The Sun has none of these things in the way Earth does. Instead, it produces its enormous energy through a completely different process called nuclear fusion, which works deep inside its core under conditions that exist nowhere on our planet.

At the center of the Sun, temperatures reach about 15 million degrees Celsius and pressures are so extreme that hydrogen atoms are squeezed together until they fuse to form helium. This fusion releases an incredible amount of energy in the form of light and heat. The process is so efficient that the Sun converts about 600 million tons of hydrogen into helium every second. Even at that rate, the Sun has so much hydrogen in its core that it can keep this process going for billions of years. Fusion is not burning — it is the same process that powers every star in the universe.

The reason nuclear fusion can power the Sun for such a long time comes down to two things — the Sun's enormous mass and the staggering efficiency of fusion. The Sun contains around 330,000 times the mass of Earth, and the vast majority of that mass is hydrogen. Each fusion reaction also converts a tiny portion of matter directly into energy, following Einstein's famous equation E equals mc squared. This means even a small amount of fused hydrogen releases an enormous amount of energy. The Sun has been steadily fusing hydrogen for about 4.6 billion years and has enough fuel to keep going for roughly another 5 billion years.

Although the Sun will not burn out anytime soon, it will eventually run low on hydrogen in its core. When that happens, billions of years from now, the Sun will begin to change. It will swell into a red giant, expanding far enough to engulf Mercury and Venus and possibly Earth. After the red giant phase, it will shed its outer layers into space and leave behind a small, dense core called a white dwarf. The white dwarf will slowly cool and fade over trillions of years. So the Sun will not vanish suddenly — it will go through a long, dramatic transformation.

This article was written using information from Wikipedia.