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The region of the Sun that we can actually see, where all the light comes from, is known as the photosphere. This is the point where photons generated inside the Sun finally reach the vacuum of space, released from the inside of the Sun. As we look down into the Sun, it becomes less and less transparent. The point at which the Sun is opaque is called the photosphere.
The photosphere of the Sun ranges in temperature, from 4,500 kelvin to 6,000 kelvin. These various temperatures generate photons of different wavelengths. Lower temperature photons are redder, while higher temperature photons are bluer. When we look at the Sun (not with unprotected eyes!), we’re seeing the average color of all the photons released from the photosphere. That’s why it looks white/yellow from here on Earth.
If you look at a picture of the Sun, taken with a solar filter, you can see granules on the surface of the Sun. These are convective cells of hot gas, measuring about 1,000 km across. The centers contain rising hot gas, while the edges of the granules contain cooling gas that’s falling down into the Sun to pick up heat again. Granules only last for about 8 minutes, so when you see a video of the surface of the Sun, it looks like it’s constantly boiling, like water.
The photosphere is actually the densest part of the solar atmosphere, but it’s much less dense (0.1%) than the Earth’s atmosphere. Sunspots are dark regions in the photosphere where magnetic field lines pierce through the surface of the Sun. They appear dark because they have a lower temperature than the surrounding regions of the Sun, but they’re still thousands of kelvins hot.
We’ve also recorded an episode of Astronomy Cast about the Sun. Listen here, Episode 30: The Sun, Spots and All.