The Hydrogen Battery
Our Sun is a middle-aged star known as a 'Yellow Dwarf.' For the last 4.6 billion years, it has been powered by a process called nuclear fusion. It takes hydrogen atoms and smashes them together to make helium, releasing a massive amount of energy.
Think of the Sun's hydrogen like a giant battery. Right now, that battery is about half-full. As long as it has hydrogen, the Sun stays the same size and the same temperature, providing the perfect light for life on Earth.
Running on Empty
In about 5 billion years, the hydrogen in the Sun's core will finally run out. When this happens, the 'balance' of the star will break. Gravity will start to crush the core even tighter, making it much hotter.
As the core gets hotter, it will start to burn the leftover helium. This new, intense energy will push the outer layers of the Sun outward, like a balloon being blown up too big. This is the beginning of the 'Red Giant' phase.
The Red Giant Expansion
As a Red Giant, the Sun will become much larger than it is now—nearly 100 times its current size! It will grow so big that it will likely swallow the planets Mercury and Venus.
Even though it's called a 'Red Giant,' the surface will actually be cooler than it is now, which is why it glows red instead of yellow. However, because it is so close to Earth, our planet would become a scorched, liquid rock.