The star is cooler, so it is redder, but it is larger so it is more luminous. The result is a giant star. Let s return to the core. The helium core is really hot, so hot that every atom is completely ionized. It also has a huge amount of weight pressing down on it, causing it to become really dense.
Electrons can only occupy certain energy levels in the core.
This is similar to the situation with electrons in an atom, except now the electrons are a sea of electons among helium nuclei. No more than two electrons per energy level . These two laws are generally true, and in low density gas, there is no problem with this, because there are plenty of energy levels available. However, the problem comes when the gas becomes very dense. Then electrons cannot change their energy easily because all of the energy levels are filled.