How Hot Is the Sun Really? The Answer Has a Plot Twist

How hot is the sun really? Most people guess a million degrees. Some say a hundred million. Both are wrong. The surface of the sun is around 5,500 °C (9,932 °F), cooler than a steel forge. The core sits at 15 million °C (27 million °F). And then comes the plot twist: the sun's atmosphere is up to 200 times hotter than the surface, and nobody on Earth can fully explain why. Here are the numbers, the comparisons, and the biggest unsolved mystery in solar physics.

How hot is the surface of the sun?

The visible surface of the sun is called the photosphere. It sits at about 5,500 °C (9,932 °F). That sounds extreme, but in cosmic terms it is almost boring. For context: lava from a volcano is around 1,200 °C (2,200 °F). Steel melts at 1,500 °C (2,732 °F). The sun's surface is roughly four times hotter than molten iron, but cooler than most people think. This is the layer that produces the light and heat we feel on Earth.

The photosphere also has cooler patches called sunspots. They sit at roughly 3,500 °C (6,332 °F), which is still hotter than anything you can build on Earth, but cool enough to look dark in photos. Strong magnetic fields slow down the hot plasma rising from the inside.

How hot is the core of the sun?

The core of the sun runs at about 15 million °C (27 million °F). That is roughly 2,700 times hotter than the surface. At those temperatures nuclear fusion happens: hydrogen atoms fuse into helium, and energy gets released. That energy is the reason the sun shines at all. Every second, the core burns 600 million tons of hydrogen. The pressure down there is around 250 billion times the air pressure at sea level.

Quick math: a single photon born in the core takes around 100,000 years to reach the surface. The sunlight hitting your skin right now was created in the sun's core long before the pyramids were built.

Why is the corona hotter than the surface?

The corona is the sun's outer atmosphere. It reaches between 1 and 3 million °C. By every rule of physics, that should be impossible. Heat flows from hot to cold. If the core is 15 million degrees and the surface is 5,500, the atmosphere should be cooler than the surface. But it is not. It suddenly heats up again. By a lot.

This is called the coronal heating problem and it has been unsolved for over 80 years. Researchers think tiny magnetic waves or small explosions in the sun's magnetic field reheat the corona. NASA's Parker Solar Probe has been flying ever closer to the sun since 2018 to figure it out. So far there is no final answer.

How old is the sun and how much longer will it burn?

The sun is about 4.6 billion years old. So it has already burned a brutal amount of hydrogen. But: it is only halfway through its life. The sun has roughly 5 billion years left. After that it will swell into what astronomers call a red giant and likely swallow Mercury, Venus, and probably Earth. Then it will fade into a white dwarf, a tiny and extremely dense stellar leftover.

For scale: the sun is about 1.39 million km (864,000 miles) across. That is roughly 109 Earths lined up side by side. And about 1.3 million Earths could fit inside the sun by volume. If you want more facts like this, try our science estimation questions.

How hot is the sun compared to other stars?

The sun is a yellow dwarf star, which makes it pretty average in the universe. Some stars are hotter, some are cooler. Red dwarfs have surface temperatures around 2,500 °C (4,532 °F). Blue giants like Rigel push past 12,000 °C (21,632 °F). The sun, at 5,500 °C, lands right in the middle. That is actually perfect for us: hot enough to deliver energy, but not so hot that it would burn out in just a few hundred million years.

If you like that kind of comparison, also check our related post on how much a cloud actually weighs. Same flavor of aha moment, with numbers your gut refuses to accept.

Take-away: the sun is much less hot than you think, at the surface anyway. The core is absurdly hot. And the atmosphere is hotter than either should logically be. If you want to test your gut feeling, try our science estimation questions or check how much a cloud actually weighs.