eli5 how does matter behave inside of red giants?
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Stars are big fusion machines, with the extreme pressure of gravity causing hydrogen to fuse into helium, and in certain stars further fusion of that.
A red giant is a star relatively far into its life that is running out of hydrogen in its core. One thing that keeps a star large is pressure from the energy released during fusion. As this decreases, the star initially contracts until it can start fusing hydrogen outside of the core. For fairly complicated reasons, this causes the outer layers and corona of the star to expand and get less hot. As the star continues to burn hydrogen, it will eventually run out of enough of that to sustain the pressure needed to keep its size, and it will contract enough to start fusing helium, if it is massive enough. This process repeats until it fuses up to carbon and oxygen, or further for very large stars. Once it reaches a point where even after contraction, it can’t get hot enough to fuse any higher up the periodic table and thus generate outward pressure, it will collapse into a white dwarf, which is a very dense remnant of the star that is no longer undergoing fusion.
Highly massive stars can fuse beyond carbon to generate enough pressure to continue, with iron being the limit of where fusion outputs energy instead of taking energy. A large enough star won’t become a red giant, and will eventually collapse into either a neutron star or a black hole depending on the exact mass.
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