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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

(1863-1941)

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

Fission

Fusion

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 …


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