Red giant and asymptotic giant branch

Assembled from 13 blocks · 2 evidence · 16 related

1. Story
2. What happens when the core runs out of hydrogen. With no fusion to hold it up, the inert helium core contracts and heats, hydrogen ignites in a shell around it, and the enormous energy flux from that shell forces the envelope outward until the star is hundreds of times its former radius and, because that huge surface radiates at low temperature, red. In stars of roughly 0.5 to 2 solar masses helium ignition occurs abruptly in a degenerate core - the helium flash. Later, on the asymptotic giant branch, helium and hydrogen burn in alternating shells and the star loses between roughly forty and eighty per cent of its mass in slow dusty winds over a few hundred thousand years. The counter-intuitive part is worth stating plainly: a star gets larger and brighter, not smaller and fainter, when it exhausts the fuel in its core. Not a place.
3. Knowledge
4. Red giant and asymptotic giant branch
5. Connections
6. Main sequence (core hydrogen burning)
7. Betelgeuse (Alpha Orionis)
8. Planetary nebula
9. White dwarf
10. Betelgeuse fades by more than a magnitude
11. Evidence
12. Models the Sun to the tip of the red giant branch at 7.59 Gyr from now with 0.332 solar masses lost, and concludes the Earth is engulfed despite orbital expansion. Journal, volume, pages and DOI come from publisher and ADS listings via search; the paper was not fetched, so the model assumptions behind the engulfment result are not independently characterised here.
13. Supports the description of the planetary nebula phase as brief, on the order of ten thousand to twenty-five thousand years, following AGB mass loss of roughly 40 to 80 per cent of the star. The page was not fetched; figures come from a search summary and vary between sources.