At the centre of the Sun, the entirely ionised gas is, to within a couple of percent, an ideal perfect gas, with pressure $P = nk_B T$. Here, $n$ is the number density of all particles and $T$ is the temperature.

Nuclear fusion releases energy. The energy escapes from the fusing nuclei in the form of gamma/X-ray photons (98%) and neutrinos (2%).

Whilst the neutrinos can easily escape from the Sun, taking that energy with them, the photons are absorbed within a cm or so of where they are created. Thus their energy is deposited into the gas and that provides it with heat.

The interior of the Sun has evolved into an equilibrium, where the heat deposited by the fusion reactions equals the heat escaping from the surface.

The heat keeps the interior temperature high, and that leads to the gas pressure that supports the Sun.

At the centre of the Sun, the entirely ionised gas is, to within a couple of percent, an ideal perfect gas, with pressure $P = nk_B T$. Here, $n$ is the number density of all particles and $T$ is the temperature.

Nuclear fusion releases energy. The energy escapes from the fusing nuclei in the form of gamma/X-ray photons, the kinetic energy of the resultant particles and neutrinos (2%).

Whilst the neutrinos can easily escape from the Sun, taking that energy with them, the photons are absorbed within a cm or so of where they are created and the particles collide with other particles on similarly tiny length scales (compared with the size of the Sun). Thus their energy is deposited into the gas and that provides it with heat.

The interior of the Sun has evolved into an equilibrium, where the heat deposited by the fusion reactions equals the heat escaping from the surface.

The heat keeps the interior temperature high, and that leads to the gas pressure that supports the Sun.