Well one's definition of "cool" may definitely vary greatly from others but....
In stars like our Sun fusion occurs via a process called the Proton-Proton chain, the basics is ionized hydrogen fuse together over a series of events and ultimately 4 hydrogen turn into one helium, this take a lot of heat AND pressure this is because the hydrogen ions are just protons and two protons don't want to be near each other at all so the heat makes everything wiggle around really fast and the pressure holds it close so that you can get collisions that eventually "stick" hence fusion. In stars that are heavier than our Sun when they fuse hydrogen they use a different method called the CNO cycle, now the net result is the same 4 hydrogen turn into 1 helium but the way it does it involves Carbon, Nitrogen and Oxygen, without boring you of the details this occurs it heavier stars because the cores are hotter and there's even more pressure, and instead of 2 protons that don't want to be near each other, 1 proton (hydrogen) doesn't want be near the 6 protons of a carbon (6 times the repulsive force at similar distances).
Now when stars stop fusing hydrogen into helium in their core if they have enough mass they will enter their giant phase of their life, and one particular stage of this giant phase is helium fusion in the core (from all that hydrogen fusion earlier), now helium has 2 protons, so squeezing 2 and 2 together seems like it would require less effort than squeezing 1 proton and 6 protons together and you'd be 100% right, however the temperatures required for helium fusion are tremendously greater than what is needed for the CNO cycle.
The reason for this is my interesting fact, when 4 hydrogen atoms fuse into 1 helium atom the reason why that makes energy is because the 1 helium atom has less mass than the 4 hydrogen atoms that went into it, this tiny amount of mass gets turned into energy (E=mc²), however when 2 helium atoms fuse what the turn into is beryllium atom there actually is MORE mass after the fusion, meaning that it takes energy to do (translation: star would get cooler) as a result the beryllium will decay back into helium with a half-life of about 10^-16 seconds (aka super quick), however the magic is in the high temperature, because temperatures exceed 100 million Kelvin in these reactions (compared to the puny 15 million Kelvin that our Sun does) if another helium fuses with the beryllium before the process can decay it will form carbon which has less mass than 3 helium ions, and a net energy gain is made by sequence.
And much like one of my classese that I teach this in, I can imagine quiet bored looks staring at the screen with the occasional blink.
