A star's core consists mostly of hydrogen. As the star ages, the amount of helium, carbon and other elements in the core increases as they are the result "ash" resulting from the consumption of the hydrogen fuel.
A stars core temperature will be several million kelvins
Nuclear Fusion occurs in the core of stars.
Nuclear Fusion at the Stars' Core.
A white main sequence star has the hottest core.
These are stars that have exhausted their core's supply of hydrogen by switching to a thermonuclear fusion made of hydrogen in a shell that surrounds the core.
They are called Core Burning Stars the smallest one is OGLE-TR-122b.
What happens to a stars nuclear energy generation change if the core decreases in temperature?
Nucleosynthesis in the core of stars.
nuclear fusion
Some stars end up as neutron stars because they exploded as a supernova and leaves a dense core behind. The strong gravity of the core compresses itself until protons and electrons are crushed into neutrons, this forms a neutron star. If the core is too massive, it would collapse into a black hole.
Spiral galaxies tend to have older stars towards the core and younger stars in the outer arms
Neutron stars could form in places where there are high-mass stars. After the star runs out of fuel in its core, the core collapses while the shell explodes into the space as supernova. The core would then become a neutron star, it might also become a black hole if it is massive enough.