Why does the core of a high-mass star not become degenerate, as the core of a low-mass star does?

In the hearts of stars, 'degenerate matter' is a key concept. It usually shows up in low-mass stars as they age. When electrons are smashed together in extremely tight spaces, they start to behave according to quantum mechanical rules. One of these rules is the Pauli exclusion principle, which says no two electrons can occupy the same state at the same time. This principle stops the core from being compressed further, even if more gravitational pressure is applied. This resistance against further compression is known as electron degeneracy pressure. It's crucial because it marks a stage where normal gas behaviors, like increased pressure with higher temperature, no longer apply. So, degenerate matter in low-mass stars acts as a safeguard against gravitational collapse.

Nuclear fusion is the process that powers stars, including high-mass stars. At extreme temperatures and pressures found in their cores, lighter atomic nuclei such as hydrogen fuse to form heavier nuclei like helium. This process releases immense amounts of energy. In high-mass stars, the core temperatures are so high that fusion can continue to occur even as the star ages. This constant nuclear fusion generates significant energy and keeps the star stable. Unlike low-mass stars, high-mass stars can cycle through different types of nuclear fusion, fusing elements heavier than helium when the core temperature rises further. Each stage of fusion produces thermal pressure, which helps control the gravitational forces trying to compress the core.

Thermal pressure is the force exerted by particles moving at high speeds due to the intense heat in a star's core. In high-mass stars, this pressure is extraordinarily high because of the extreme temperatures caused by continuous nuclear fusion. The thermal pressure pushes outward, balancing the inward pull of gravity. This dynamic equilibrium keeps the star from collapsing in on itself.
Without sufficient thermal pressure, as happens in low-mass stars when their nuclear fuel depletes, the core starts to contract. This contraction raises the core's density until degeneracy pressure takes over. However, high-mass stars maintain high thermal pressure throughout much of their lifetimes, avoiding this degenerate state.

Gravitational forces are the inward-pulling forces exerted due to the mass of the star. For stars, gravity is always working to compress the core. In both high-mass and low-mass stars, gravity aims to pull the core material inward. However, the counterbalance to this force comes from the pressures inside the core.
In high-mass stars, the extreme temperatures from nuclear fusion create enough thermal pressure to push back against gravity. This prevents the core from reaching the point where it would become degenerate. Conversely, in low-mass stars, once the nuclear fuel is run out and thermal pressure drops, gravitational forces dominate, leading to core contraction and the emergence of electron degeneracy pressure to balance the gravitational forces once again.
This balancing act between gravitational forces and internal pressures shapes the life and fate of stars.