What is the photosphere?

• A. The outermost part of the sun from which the Sun's radiation escapes outward.
• B. The hottest place in the Sun's interior.
• C. The layer where the solar wind originates.
• D. The inner boundary of the corona.

Answer: (A)

The photosphere is the Sun’s visible surface—the thin shell from which most of the Sun’s light escapes into space. It’s the layer where the gas becomes just transparent enough for photons to leave, so it effectively forms the disk we see. It doesn’t have to be a solid surface; rather, it’s the lowest, optically thick layer of the solar atmosphere, where the light we perceive is emitted. Its temperature is about 5,500–6,000 K, which is why we see a bright, yellowish-white glow and phenomena like limb darkening as we look toward higher, cooler layers.

Why the other descriptions don’t fit: the hottest part of the Sun’s interior is the core, where fusion happens. The solar wind originates from the hotter, tenuous outer atmosphere called the corona, not from the photosphere. The inner boundary of the corona lies above the photosphere, at the transition into the corona, so the photosphere is not that boundary.