Question

How hot is the Sun?

Answer 1

Short answer: it depends what part you mean. The Sun’s temperature varies enormously from its cool “surface” to its blazing core and outer atmosphere.

Typical values
- Core (where nuclear fusion occurs): ≈ 15 million K (≈ 15 million °C).
- Radiative zone / interior away from core: a few million K, declining outward.
- Photosphere (the visible “surface”): ≈ 5,770 K (≈ 5,500 °C). This is the often-quoted “surface temperature.”
- Sunspots: ≈ 3,000–4,500 K (cooler than the surrounding photosphere).
- Chromosphere: ≈ 4,000–25,000 K (varies with height and activity).
- Corona (the outer atmosphere seen in X‑rays/EUV and during eclipses): ≈ 1–3 million K in quiet regions, up to 10+ million K in active regions.
- Solar flares: tens of millions of kelvin (large flares can produce temperatures of several 10s of MK, and extreme events can be even hotter).

Why the corona is hotter than the surface
- The corona’s high temperature is believed to come from magnetic processes (magnetic reconnection, waves, and related energy transport) that heat the outer layers; the details are an active area of solar physics research.

How these temperatures are measured
- Photospheric temperatures: spectroscopy and blackbody fits.
- Interior (core) temperatures: inferred from models and helioseismology constrained by neutrino observations and solar structure models.
- Corona and flares: X‑ray and extreme ultraviolet observations (space telescopes).

If you meant “How hot is the Sun’s surface?” the succinct answer is about 5,800 K.