As a matter of fact, I was learning stellar astrophysics where I couldn't understand the chain of events at the time of death of stars,
Once the hydrogen fuel core is exhausted, the stars start shrinking until the helium nuclei starts fusing under immense gravitational pressure. Due to powerful radiation pressure the star starts expanding. Like our sun is expected to stretch 200 times it current radius, gobbling up the Mercury and Venus planets.
Here I couldn't understand that helium fusion starts when star is shrinking, but after star starts expanding, so how could star sustain fusion $?$ I mean helium fusion requires even more pressure or temperature than hydrogen, but once star starts expanding, it become cooler and less denser, which should make fusion impossible. And during this phase of Red giant, not only helium, but more heavier nuclei are formed like oxygen which should require even more pressure and temperature.
Is there any flaw in my understanding,then please tell$!$ Please help and thanks in advance $!$