The sun will increase in size as it begins to burn off its fuel and the overlying (mostly hydrogen) layers would fall toward the core due to decreased radiation pressure if nuclear reaction is no longer in balance as the fuel burns away.
What is nuclear fusion reaction?
In the core of the sun hydrogen is being converted into helium. This is called nuclear fusion. It takes four hydrogen atoms to fuse into each helium atom. During the process some of the mass is converted into energy.
The sun formed about 4.6 billion years ago in a giant, spinning cloud of gas and dust known the solar nebula. As the nebula collapsed under its own gravity, it spun faster and flattened into a disk.
When the sun will start to die it will expand into a red giant star, becoming so large that it will engulf mercury and venus, and possibly earth as well.
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