# Presentation: 161405

## Element Building in Large Stars

***Life Cycle of Stars - Part 2***

- Elements 2002 Workshop

**Notes:**
Imagine Life Cycle Poster Image

## 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.

**Notes:**
This is an important principle which governs the life stages of a star.

## All Types of Stars

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

## Nuclear Fusion !

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

- E = mc2

**Notes:**
Be sure audience members understand what the symbols mean.  Especially that the superscripts for H and He are the atomic weights.

The energy comes from the slight difference in mass between four H atoms and one He atom.  This excess mass gets converted to energy via Einstein’s famous equation.

## 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

- Sun-like Stars

- Massive Stars

**Notes:**
Stars are either low mass or high mass.  Their mass determines their fate.
One might note that stars are not quiescent even during the time they steadily fuse Hydrogen.  For example, our own sun is very active.

## The beginning of the end: 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 !

- At 100 million degrees Celsius, Helium fuses:
- Triple Alpha Process
**4**He + **4**He --> 8Be + energy
**4**He + 8Be --> **12**C + energy
- 3 (**4**He) --> **12**C + energy
- (Only 7.3 MeV produced)
- Energy sustains the expanded outer layers
- of the Red Giant

**Notes:**
Note that fusion of He requires a much hotter temperature than fusion of H.

## 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.

**Notes:**
After the red giant stage, there is a series of collapses and further nuclear burning.  Fusion creates heavy elements from light elements.

## Periodic Table

## The End of the Line for Massive Stars

- Massive stars burn a succession of elements.
- But the process stops at Iron.

**Notes:**
Fusion stops at Iron, and star collapses under its own weight.
The star contains products of the fusion processes.

## 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

**Fusion or Fission results in more tightly bound nuclei**
**(I.e. less stable --> more stable + energy)**

## 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 _s_low - thousands of years between captures.

## 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

- Heavy elements enter convection layers of star.
- Stellar winds then disperse them.

## S-Process Elements from Large Stars

_**S**_**low Addition of Neutrons**

## All Elements from Large Stars

- Dispersed via Stellar Winds

**Notes:**
This combines all the elements dispersed during the lifetime of the main sequence stars.  These are primarily emitted via stellar winds. 

## The Next Step ...

- Once Iron core forms,
- Energy Source disappears
- Gravity takes over ...

**Notes:**
This leads us to Supernovae ...