Post-Main-Sequence Evolution
When a star exhausts the hydrogen in its core it leaves the main sequence and undergoes a rapid succession of structural changes, swelling into a giant and igniting heavier fuels in a sequence determined chiefly by its mass.
Definition
Post-main-sequence evolution is the sequence of structural and nuclear-burning stages a star passes through after it has exhausted hydrogen in its core, leading from the giant branches to its terminal remnant.
Scope
The topic covers the subgiant and red giant branch phases that follow core hydrogen exhaustion, shell burning and core contraction, the helium flash in low-mass stars and the horizontal branch, the thermally pulsing asymptotic giant branch with its strong mass loss, and the divergent advanced evolution and final fates of low-, intermediate-, and high-mass stars.
Core questions
- What happens to a star when its core hydrogen runs out?
- Why does a star expand into a giant after the main sequence?
- How do helium and heavier elements ignite as a star ages?
- How does a star's mass determine its post-main-sequence path and end state?
Key concepts
- red giant branch
- shell burning
- helium flash
- horizontal branch
- asymptotic giant branch
- stellar mass loss
- planetary nebula
Key theories
- Shell burning and the giant branches
- After core hydrogen exhaustion the inert helium core contracts and heats while hydrogen burning continues in a surrounding shell; the envelope expands and cools, moving the star up the red giant branch, and later helium-shell and hydrogen-shell burning drive the asymptotic giant branch.
- Mass-dependent fate of stars
- Low-mass stars ignite helium in a degenerate flash and end as white dwarfs after the asymptotic giant branch and planetary-nebula ejection, while massive stars burn through successive heavier fuels to iron and end in core collapse, so initial mass sets the entire late evolution.
Mechanisms
When core hydrogen is exhausted the core contracts and heats while a hydrogen-burning shell forms around it; the resulting energy and structural readjustment inflate the envelope into a cool, luminous giant. Continued core contraction eventually ignites helium and, in massive stars, heavier elements, while strong stellar winds on the giant branches strip the envelope and set the stage for the final remnant.
Clinical relevance
Post-main-sequence stars dominate the light of old stellar populations, drive the chemical enrichment of galaxies through mass loss and supernovae, produce planetary nebulae and the standard-candle pulsating variables on the horizontal branch and beyond, and determine which stars leave white dwarfs, neutron stars, or black holes.
History
Mid-twentieth-century numerical models by Schwarzschild, Hoyle, and others traced the path off the main sequence, and the detailed phases of giant-branch, horizontal-branch, and asymptotic-giant-branch evolution were mapped through the influential calculations of Iben and collaborators from the 1960s onward.
Key figures
- Martin Schwarzschild
- Icko Iben
- Allan Sandage
- Fred Hoyle
Related topics
Seminal works
- kippenhahn2012
- iben1991
Frequently asked questions
- Why does a star become a red giant?
- When the core runs out of hydrogen it contracts and heats, intensifying burning in a shell around it; the extra energy and the star's adjustment cause the outer envelope to expand enormously and cool, so the star becomes large, luminous, and red.
- Will the Sun explode as a supernova?
- No, the Sun is too low in mass; it will expand into a red giant, lose its outer layers as a planetary nebula, and leave behind a white dwarf, whereas only stars well above about eight solar masses end their lives in core-collapse supernovae.