We saw that as the Sun's core is slowly converted to Helium from Hydrogen Fusion reactions, the center will collapse and heat up. The outer atmosphere will respond to this increased central temperature by expanding away from the hot core. This is the Red Giant Stage.

So first, the Sun will become enormous, expanding to the distance of Venus, and literally scorching everything on the surface of the Earth until it is nothing but a lifeless rock (no oceans or atmosphere).

As the atmosphere expands, it also becomes VERY thin. At some point, we will be able to peer directly THROUGH this outer atmosphere, and see the very hot central core of the Sun directly!