The Sun's core is 15 million °C and fuses by quantum tunnelling

The Sun’s core is about 15 million °C (27 million °F), the hottest part of the Sun, and it is where hydrogen fuses into helium to power everything the Sun emits. Its matter is squeezed to about 150 grams per cubic centimetre, roughly eight times the density of gold.

Most summaries stop there, but by the rules of classical physics 15 million degrees is not enough. Hydrogen nuclei are positively charged and repel each other, and at the core’s temperature too few protons have the energy to get close enough to fuse. The Sun keeps shining because of quantum tunnelling: there is a small chance that a proton slips through that electrical barrier instead of climbing over it. With an enormous number of protons in the core, those unlikely events happen constantly. Without tunnelling, fusion in the Sun and other stars would not be sustainable.

The output is still vast. Every second the Sun turns about 600 million tonnes of hydrogen into 596 million tonnes of helium; the missing 4 million tonnes is released as energy, following Einstein’s E=mc².

For scale, here is how the core compares with the layers above it:

Layer	Temperature
Core	15 million °C
Corona (outer atmosphere)	about 2 million °C
Photosphere (visible surface)	5,500 °C

The corona being hotter than the surface is one of the Sun’s unsolved puzzles, but even it reaches only a fraction of the core.

The energy is also slow to leave. In the radiative zone above the core, the plasma is so dense that photons travel only a few millimetres before being absorbed and re-emitted in a random direction. Energy takes around 170,000 years to work its way out to the convection zone, so the energy in today’s sunlight was released by fusion many thousands of years ago.