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In the core, gas is squashed by the pressure of the overlying material, so that it occupies a much smaller volume. At the very center, the Sun's density is about 150 g/cm³, 10 times greater than that of solid lead, and the temperature soars to more than 15,000,000°C. As described above, nuclear reactions in the core consume hydrogen to form helium.

Both the temperature and the density decrease with distance from the center, so that nuclear fusion more or less ceases beyond the outer edge of the core (about 25% of the distance to the surface or 175,000 km from the center). At that point the temperature is only half its central value and the density drops to about 20 g/cm³.

Table 2.2 Internal Zones of the Sun

Zone	Radial Distance from Center (%)	Temperature (K)	Density (g/cm3)	Energy Transport
Core	0–25	15 million–7 million	∼150–20	Radiative
Radiative zone	25–70	7 million–2 million	20–0.2	Radiative
Convective zone	70–100	2 million–5,700	0.2–0.0000002	Convective

The incredibly high temperature is a major factor in preventing the Sun from collapsing under its own gravity. Hydrogen nuclei near the center collide more frequently and at higher speeds than anywhere else in the Sun, so they exert a greater outward push against the overlying material. In addition to this gas pressure, the colliding particles also emit electromagnetic radiation which slowly escapes toward the surface, further helping to inflate the solar sphere. This latter force is called radiation pressure.

The main mechanism of energy transport in the inner 70% of the Sun is electromagnetic radiation in the form of photons which exhibit both wave‐like and particle‐like properties.

Although gamma ray photons are produced in the core, the photons that emerge from the Sun's surface are mainly in the form of visible light.

Each photon generated in the core travels only a very short distance before it encounters another particle. The photon is then scattered or absorbed, and then re‐emitted. The photons produced by these encounters may emerge in any direction, so their zig‐zag progress toward the surface via innumerable collisions may take hundreds of thousands of years.

As the photons diffuse outward, they move into regions where the plasma is cooler. As a result, they collide with electrons or ions that are generally moving at slower speeds, and the overall energy of the photon population is gradually reduced.

Above the core is the radiative zone, the Sun's largest internal “shell,” which reaches out to 70% of the star's diameter. From the bottom to the top of the radiative zone, the density drops from 20 g/cm³ (about the density of gold) to only 0.2 g/cm³ (much less than the density of water). Meanwhile, the temperature falls from 7 million degrees Celsius to about 2 million degrees over the same distance.