How hot is the Sun?

The visible surface of the Sun, called the photosphere, has a temperature of about 5,500 degrees Celsius, or roughly 5,800 kelvin. Deep in the core, nuclear fusion turns hydrogen into helium at around 15 million degrees Celsius. Strangely, the Sun’s outer atmosphere, the corona, is far hotter than the surface, at 1 to 3 million degrees Celsius, and scientists are still working out exactly why. NASA’s Parker Solar Probe, launched in 2018, flew through the corona and in December 2024 passed just 6.1 million kilometres from the surface, closer than any human-made object. Sunspots look dark only because they are about 1,500 degrees cooler than the surrounding surface.