Remember when we talked about sun-sized stars? We said that at the end of their lives these stars expand, taking up much more space than before. This is exactly what a Giant Star is.
As a sun-sized star gets old, it starts to run out of its hydrogen fuel. When the process of burning hydrogen in the star's core begins to slow down, the core gets more compact and dense. This means all the stuff in the middle of the star gets really close together. As the center gets smaller and smaller it starts to heat up again. When it gets hot enough it will start to burn a new fuel called helium.
Once ignited, helium burns much hotter than hydrogen. The additional heat pushes the outer layer of the star out much further than it used to be, making the star much larger. Imagine a hot air balloon. As the air inside the balloon gets hotter, it stretches the balloon out further and further. As the giant star gets hotter, its outside stretches out further and further. When our own sun begins to stretch into a giant star, it will engulf Mercury, Venus, Earth and Mars.
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