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SESK: Stuff Everyone Should Know · sesk.info/astro-photosphere-definition

What is the Sun's photosphere?

The visible surface, the thin layer where the Sun's gas turns from transparent to opaque.

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  1. The visible surface where the Sun becomes opaque ✔
  2. A solid crust over the core
  3. The outermost layer of the corona

Why: The Sun has no solid surface. The photosphere is a layer only about 400 km thick where the gas changes from nearly transparent to nearly opaque, so it marks the deepest layer we can see.

Huh, didn't know that: Photosphere comes from the Greek for light sphere, and its temperature is about 5800 K.

The dad joke
The Sun has no solid surface. It's all just a lot of hot air.
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References

Answer: The visible surface where the Sun becomes opaque. The Sun has no solid surface. The photosphere is a layer only about 400 km thick where the gas changes from nearly transparent to nearly opaque, so it marks the deepest layer we can see.

  1. Astronomy 2e, 15.1 The Structure and Composition of the Sun openstax.org/books/astronomy-2e/pages/15-1-the-structure-and-composition-of-the-sun
    The photosphere is the layer where the Sun becomes opaque and marks the boundary past which we cannot see
    Checked 2026-10-10.

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