How hot is the sun?

The photosphere, the visible surface, runs about 5,500 degrees Celsius or roughly 10,000 Fahrenheit. The core, where hydrogen fuses into helium and releases the energy that powers everything, reaches about 15 million degrees Celsius, or roughly 27 million Fahrenheit, under pressures hundreds of billions of times Earth's atmospheric pressure. Energy produced in the core takes an extraordinarily long time to work its way outward, with estimates ranging from thousands to over a hundred thousand years, because photons are absorbed and re-emitted constantly in the dense interior before finally escaping the surface and crossing to Earth in eight minutes.

The genuinely puzzling part is the corona, the sun's outer atmosphere, which reaches one to three million degrees Celsius despite sitting above a surface that is far cooler. Heat flowing outward from a hot interior should produce steadily decreasing temperatures, so something is depositing energy in the corona directly. Leading explanations involve magnetic reconnection releasing energy in small flare events and waves travelling along magnetic field lines. NASA's Parker Solar Probe, launched in 2018, has flown repeatedly through the corona to measure conditions directly and has returned data narrowing the possibilities.

Two related figures worth having. The sun accounts for about 99.86 percent of the mass of the entire solar system, and it converts roughly four million tonnes of matter into energy every second through fusion. It is a middle-aged star at around 4.6 billion years old, with enough hydrogen for several billion more years, after which it will expand into a red giant and eventually shed its outer layers to leave a white dwarf. It is not massive enough to become a supernova or a black hole.