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The Lives of Stars. From studying nearby stars and stellar clusters most stars are on the main sequence stars become red giants after leaving the main sequence How does this relate to the internal structure of the stars and their nuclear fusion reactions?. Fusion reactions.

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the lives of stars
The Lives of Stars
  • From studying nearby stars and stellar clusters
    • most stars are on the main sequence
    • stars become red giants after leaving the main sequence
  • How does this relate to the internal structure of the stars and their nuclear fusion reactions?

Our Evolving Universe

fusion reactions
Fusion reactions
  • Generate energy up to iron
  • But, need to get two positively charged nuclei close enough to fuse together
    • need fast movement
    • high temperature (and high density)
  • Converting hydrogen-1 to helium-4 is the easiest and most efficient fusion reaction
    • 0.7% of initial mass converted to energy


Our Evolving Universe

stellar structure and fusion
Stellar structure and fusion
  • To keep star stable need pressure to increase downwards
    • temperature increases
    • density increases
    • fusion most likely in central core of star
  • Stars are mainly hydrogen
    • expect main sequence stars to fuse hydrogen to helium in core

Our Evolving Universe

hydrogen fusion reactions
Hydrogen fusion reactions
  • Reaction rateincreases astemperatureincreases
    • more massive starshave higher fusionrates
  • Reaction can be direct or use carbon-12as catalyst
    • this tends to increase abundanceof nitrogen and oxygen

pp chain

CNO cycle

Our Evolving Universe

getting the heat out
Getting the heat out
  • Energy generated in stellar core has to be transported to surface
  • Two options:
    • radiation
      • absorption and re-emissionof photons
    • convection
      • hot gas rising towards surface, cool gas falling
  • Giant stars have convective outer layers
    • transports out the heavy elementsproduced by fusion

Our Evolving Universe

what happens when the core hydrogen runs out
What happens when the core hydrogen runs out?

star expands and cools,becoming luminousred giant (1000x Sun)

1.76 Gyr

core shrinks under gravityuntil hydrogen outside starts to fuse

1.65 Gyr

1.69 Gyr

1.61 Gyr

star now has helium core (not hot enough to fuse)

on main sequence10x Sun

Our Evolving Universe

stages of hydrogen fusion
Stages of hydrogen fusion
  • Main sequence stars fuse hydrogen to helium in core
  • Red giants (and subgiants) fuse hydrogen to helium in shell outside helium core
  • Stars have nearly constant luminosity on main sequence, but red giants get brighter as they age
  • Red giant stage lasts only 10% as long as main sequence

establishedfusion inshell

starting to fuse out-side core

core hydro-gen fusion

Our Evolving Universe

helium fusion
Helium fusion
  • Neither beryllium-8 nor boron-8 is stable
    • need to combine three helium nuclei to get stable carbon-12
    • beryllium-8 serves as intermediate stage
    • need high temperature and density (else 8Be decays before it gets converted to 12C)

Our Evolving Universe

stages of helium fusion
Stages of helium fusion

carbon core with heliumfusion outside: starbecomes a giant again

1.86 Gyr

core heliumstarts to fuse

helium fusion in core:star is smaller andhotter, but less bright

1.82 Gyr

1.76 Gyr

Our Evolving Universe

helium fusion on the hr diagram
Helium fusion on the HR diagram
  • Helium fusion is much less efficient than hydrogen fusion (0.07% instead of 0.7%)
    • helium fusion stage lasts for a much shorter time

helium corefusing stars:the hori-zontal branchof a globularcluster

Our Evolving Universe

side effects of helium fusion
Adding more helium nuclei to carbon can produce the alpha-process elements

oxygen-16, neon-20, etc.

Adding helium to carbon-13 or neon-22 produces free neutrons

which can easily combine with nuclei (no charge) to produce different elements

Why does helium fusion make mostly carbon?

because carbon nuclei have an energy level at exactly the right place

otherwise carbon would be a rare element

and we would not exist!

Fred Hoyle, 1953

Side effects of helium fusion

Our Evolving Universe

stellar evolution
Stellar evolution
  • Note step is in log (age): each frame is 60% older than the one before
    • massive stars evolve very quickly
    • post-main-sequence life of star is always comparatively short
    • massive stars change colour a great deal, but don’t change brightness much
    • less massive stars become much brighter as red giants

Our Evolving Universe

after helium fusion
Fusion of heavier elements gets more difficult

higher mass means lower speed at given temperature

higher charge means more electrostatic repulsion

Stars like the Sun never get beyond helium fusion

More massive stars (>8 MS) can fuse elements up to iron

What happens to Sun-like stars when the helium is used up?

What happens to massive stars when they reach iron?

fusion beyond iron requires energy

How are the heavy elements formed in stellar cores dispersed into space?

…next lecture!

After helium fusion

Our Evolving Universe