Question

I got a question, are Red Giant, Hyper, and Superguant Stars hotter than the Sun, on the outside, but likely much cooler on the inside

Answer 1

Short answer: No. Whether a giant or supergiant’s surface is hotter than the Sun depends on its color/type, but their interiors (cores) are generally much hotter than the Sun’s, not cooler.

More detail:
- Surface temperatures:
- Red giants and red supergiants (e.g., Betelgeuse) have cooler photospheres than the Sun (≈3,000–4,000 K vs. the Sun’s ≈5,780 K).
- Blue supergiants and hypergiants (e.g., Rigel) have much hotter surfaces (10,000–30,000+ K).
- Interior (core) temperatures:
- The Sun’s core ≈15 million K.
- Low‑mass stars that become red giants develop contracting/heating cores that reach tens to ~100 million K to ignite helium.
- Massive supergiants have even hotter cores and go on to burn heavier elements; late stages reach hundreds of millions to billions of kelvin.
- Why the difference: giants/supergiants have huge, low‑density envelopes. Expansion cools the photosphere (so a red giant can be very bright but cool), while core contraction and/or advanced fusion stages make the core far hotter than the Sun’s.

So: red giants are cooler at the surface but hotter in the core; blue supergiants are hotter at the surface and also have hotter cores.