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Element Building in Large Stars. Life Cycle of Stars - Part 2. Elements 2002 Workshop. Re: Principles of Stellar Evolution. The bigger it is, the hotter and the faster a star burns its life away. Energy released from nuclear fusion counter-acts inward force of gravity.

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element building in large stars
Element Building in Large Stars

Life Cycle of Stars - Part 2

Elements 2002 Workshop

re principles of stellar evolution
Re: Principles of Stellar Evolution
  • The bigger it is, the hotter and the faster a star burns its life away.
  • Energy released from nuclear fusion counter-acts inward force of gravity.
    • Throughout its life, these two forces determine the stages of a star’s life.
all types of stars

Annie J Cannon


All Types of Stars

Oh Big And Ferocious Gorilla, Kill My Roommate Next Saturday !

nuclear fusion
Nuclear Fusion !
  • At 15 million degrees Celsius in the center of the star, fusion ignites !
  • 4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy
  • Where does the energy come from ?
  • Mass of four 1H > Mass of one 4He

E = mc2

stellar winds in high mass stars
Stellar Winds in High Mass Stars
  • Wolf-Rayet stars may have winds up to thousands of km/s
    • These stars are hot, massive O stars
    • But are only 1-2% of normal stars in galaxy
  • Mass loss equivalent to ~1/3 mass of earth per year.
  • Elements pushed by intense photon radiation emitted by the star.
  • We observe He, N, C, O, Si, Fe
reprise the life cycle
Reprise: the Life Cycle

Sun-like Stars

Massive Stars

red giants
Red Giants
  • After Hydrogen is exhausted in core,
  • Core collapses, releasing energy to the outer layers
    • Outer layers expand
  • Meanwhile, as core collapses,
    • Increasing Temperature and Pressure ...
more fusion
More Fusion !
  • At 100 million degrees Celsius, Helium fuses:
  • Triple Alpha Process
  • 4He + 4He --> 8Be + energy
  • 4He + 8Be --> 12C + energy
  • 3 (4He) --> 12C + energy
  • (Only 7.3 MeV produced)
  • Energy sustains the expanded outer layers
  • of the Red Giant
fate of high mass stars
Fate of high mass stars
  • After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen.
    • 12C + 12C --> 24Mg

OR 12C + 4H --> 16O

  • Through a combination of processes, successively heavier elements are formed and burned.
periodic table

Light Elements Heavy Elements

4 (1H) 4He + energy

C-N-O Cycle

3(4He) 12C + energy

4He + 16O 20Ne + energy

16O + 16O 32S + energy

28Si + 7(4He) 56Ni + energy 56Fe

4He + 12C 16O + energy

12C + 12C 24Mg + energy

Periodic Table
the end of the line for massive stars
The End of the Line for Massive Stars
  • Massive stars burn a succession of elements.
  • But the process stops at Iron.
the stop light at iron
The Stop Light at Iron
  • Iron is the most stable element and cannot be fused further.
    • Instead of releasing energy, it uses energy.
  • Why ?

Binding energy

  • Mass of atomic nucleus < Sum of masses of individual neutrons and protons
  • Difference is energy which holds nucleus together.
  • (= amount of energy to break nucleus apart)
binding energy nucleon



Binding Energy/Nucleon

Fusion or Fission results in more tightly bound nuclei

(I.e. less stable --> more stable + energy)

s processes going beyond iron
S-Processes - Going beyond Iron
  • In Asymptotic Giant Branch Stars, He burning occurs in a shell around Carbon core.
  • Here reactions occur which release neutrons.
  • These neutrons help build heavy isotopes:
  • Fe56 + n --> Fe57
  • Fe57 + n --> Fe58
  • Fe58 + n --> Fe59
  • This process is slow - thousands of years between captures.
s process creating heavy elements
S-Process: Creating Heavy Elements
  • Beta Decay of unstable nuclei produce new elements:
  • 26Fe59 --> 27Co59 + e- + neutrino
  • (Decay occurs before another neutron can be captured)
  • Then Co59 + n --> Co60 --> Ni60 + e-
  • All the way up to 82Pb208 and 83Bi209
dispersal of s process elements
Dispersal of S-Process Elements
  • Heavy elements enter convection layers of star.
  • Stellar winds then disperse them.
s process elements from large stars
S-Process Elements from Large Stars

Slow Addition of Neutrons

all elements from large stars
All Elements from Large Stars

Dispersed via Stellar Winds

the next step
The Next Step ...
  • Once Iron core forms,
    • Energy Source disappears
    • Gravity takes over …