How hot is the sun?

The sun isn’t one temperature. It’s a giant ball of hot gas made of several layers, and each one has its own temperature. Some of them behave in ways that still puzzle scientists.

The core: where the heat is made

At the center of the sun, gravity squeezes hydrogen so tightly that atomic nuclei crash together and fuse into helium. This process, called nuclear fusion, releases a huge amount of energy and keeps the core at around 15 million°C.

Every second, the sun converts roughly 4 million tonnes of its mass into pure energy. That energy is what eventually reaches Earth as sunlight, but it takes a long time to get out. Light made in the core can bounce around inside the sun for tens of thousands of years before it reaches the surface.

The surface you can see

The part of the sun we see from Earth is called the photosphere. It’s the sun’s “surface,” although you couldn’t stand on it, because it’s made of gas. Its temperature is about 5,500°C, or around 5,800 kelvin.

That is hot enough to melt or vaporize any material known on Earth. For comparison, lava from a volcano is usually around 700 to 1,200°C, and the filament in an old incandescent light bulb glows at roughly 2,500°C.

The photosphere also has darker patches called sunspots. They look dark only because they are cooler than their surroundings, at roughly 3,500 to 4,500°C. They’re caused by intense magnetic fields that slow down the flow of heat.

The corona mystery

Here’s the strange part. Above the surface, the sun’s outer atmosphere, the corona, is far hotter than the surface below it. It reaches 1 to 3 million°C, hundreds of times hotter than the photosphere.

That’s a bit like walking away from a campfire and feeling the air get hotter instead of cooler. Scientists call this the coronal heating problem, and it is one of the biggest open questions in solar physics. The leading ideas involve the sun’s magnetic field, either through waves that carry energy upward or through countless tiny explosions called nanoflares.

NASA’s Parker Solar Probe, launched in 2018, has flown closer to the sun than any spacecraft before it, partly to help solve this puzzle.

Sun temperature at a glance

Why it matters to us

Even 150 million kilometers away, the sun’s heat drives Earth’s weather, ocean currents and the entire food chain through photosynthesis. Without that steady output from the core, Earth would be a frozen, lifeless rock.

So the next time you feel the warmth of sunlight on your skin, remember that it began as a nuclear reaction at 15 million degrees, deep inside a star.