How Hot Is The Sun?
QUICK ANSWER
The Sun's surface (the photosphere) is about 10,000°F (5,500°C). The core is far hotter at about 27 million°F (15 million°C), hot enough to fuse hydrogen into helium. Strangely, the Sun's outer atmosphere (the corona) is hotter than the surface, reaching temperatures of several million degrees.
The Sun is hot, but how hot depends on which part you're talking about. The surface is around 10,000°F, hot enough to vaporize anything. The core is 27 million°F, hot enough to fuse atoms together. And the corona, the Sun's outer atmosphere, is mysteriously hotter than the surface, a puzzle scientists are still trying to fully explain.
How hot is the Sun's surface?
About 10,000°F (5,500°C). According to NASA, the Sun's visible surface (the photosphere) averages 5,500°C, or about 10,000°F. This is the part we see when we look at the Sun, the layer where light escapes the Sun's interior and travels into space. The surface isn't solid; it's a layer of superheated plasma, the fourth state of matter. Anything that touched the Sun's surface would vaporize instantly. Even hardened spacecraft can only get close by carefully managing heat exposure.
How hot is the Sun's core?
About 27 million°F (15 million°C). The core is where the Sun's energy is generated through nuclear fusion. Under the enormous pressure and temperature at the center, hydrogen atoms fuse together to form helium, releasing the energy that ultimately becomes sunlight. The core makes up only about 25 percent of the Sun's radius but contains over 60 percent of its mass. The energy generated in the core takes hundreds of thousands of years to work its way to the surface, then just 8 minutes to reach Earth as sunlight.
Why is the corona hotter than the surface?
Nobody fully knows, and it's one of the longstanding puzzles of solar physics. The Sun's corona (the wispy outer atmosphere visible during total eclipses) reaches temperatures of 1.8 to 3.6 million°F, hundreds of times hotter than the surface below it. This is counterintuitive: you'd expect the atmosphere to be cooler than the source of heat. NASA's Parker Solar Probe is investigating this mystery by flying through the corona directly, the closest any spacecraft has ever come to the Sun. Magnetic field activity and tiny solar flares are leading suspects.
Could the Sun ever get hotter?
Yes, slowly. The Sun has been getting brighter over its lifetime. Compared to when it formed 4.5 billion years ago, the Sun is now about 30 percent brighter and slightly hotter. The trend continues: the Sun will keep growing brighter and hotter for billions of years until it expands into a red giant in about 5 billion years. By that point, the Sun will be hot enough to engulf Mercury, Venus, and possibly Earth. The Sun's gradual brightening means Earth's climate will eventually become uninhabitable far before then.
The Sun's surface is about 10,000°F, the core 27 million°F, and the corona several million degrees, each part hotter or cooler in surprising ways. The temperature gradients drive everything from sunlight to solar flares to the slow brightening that will eventually make Earth uninhabitable. The Sun isn't just hot; it's hot in different ways at different layers, and we're still figuring out why.
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