1. Stellar Evolution: The Live and Death of a Star
Star ch. 20

2. Standards
Understand the scale and contents of the universe, including stars
Describe how stars are powered by fusion, how luminosity and temperature indicate their age, and how stellar processes create heavier and stable elements that are found throughout the universe.

3. As a star begins to run out of fuel & die, its properties change greatly, and it moves along an evolutionary path on the H-R diagram that takes it off the main sequence.
A star’s ultimate fate depends on its mass.

4. Leaving the Main Sequence
Most stars spend most of their life on the main sequence.
The coolest M – type stars burn so slowly not one has yet left the main sequence.
The most massive O & B – type stars evolve from main sequence after only a few tens of millions of years
Most high mass stars that ever existed perished long ago

5. Structural Change
As hydrogen is consumed, balance between gravity and pressure begins to shift, both internal structure and outward appearance begin to change, and the star leaves the main sequence.
The end of a star’s life depends critically on its mass.
Low mass stars die relatively gently
High mass stars die catastrophically
The dividing line between the two is about 8 times the mass of the sun

6. Evolution of a Sun-like Star
A solar mass star does not experience sudden, large-scale changes in properties.
Its average surface temperature remains constant, while luminosity increases very slowly over time
After about 10 billion years of steady core fusion, a sun-like star begins to run out of fuel (like a car cruising down the highway at a constant 70 mph for many hours, only to have engine suddenly cough & sputter as the gas gauge reaches empty).

7. The Sub-Giant Branch Composition of the star’s interior changes:
It has increased helium and decreased hydrogen.
The helium content increases fastest in the center
When hydrogen becomes depleted in the center fusion moves to higher layers in the core

8. The Sub-Giant Branch
An inner core of non-burning helium starts to grow
The gas pressure weakens in the helium core and gravity causes the inner core to begin to contract

9. Hydrogen Shell-Burning Stage
Hydrogen burns at a furious rate in a shell surrounding the non-burning inner core of helium “ash”

10. Hydrogen Shell-Burning Stage
The hydrogen shell generates energy faster than the original main sequence fusion, & energy production continues to increase as the helium core continues to shrink
The star’s response is to get brighter

11. Hydrogen Shell-Burning Stage
After a lengthy stay on the main sequence, the star’s temperature and luminosity begin to change
The star evolves to the right on the H-R diagram to the subgiant branch

12. The Red Giant Branch
The star is now off the main sequence and no longer in stable equilibrium
The helium core is unbalanced and shrinking
The rest of the core is unbalanced & fusing at an increased rate

13. The Red Giant Branch
Gas pressure exerted by enhanced fusion forces star’s non-burning outer layers to increase in radius, and the overlying layers to expand and cool
Star is on its way to becoming a red giant
This change takes around 100 million years
A red giant’s luminosity is many hundreds of times the luminosity of the sun and its radius is around 100 solar radii

14. Helium Fusion
A few hundred million years after a solar-mass star leaves the main sequence helium begins to burn in the core
The helium fuses into carbon and the central fires reignite

15. Helium Flash
At highest densities in the core, gas enters a new state of matter governed by the laws of quantum mechanics
In this state, the Pauli exclusion principle prohibits electrons in the core from being squeezed too close together
This is called electron degeneracy and the pressure associated with it is called electron degeneracy pressure

16. Helium Flash
In the core’s degenerate state, helium burning becomes unstable with explosive consequences
When burning starts and temperature increases, there is no corresponding rise in pressure, no expansion of gas & no stabilization of core
The rapid temperature rise results in runaway explosion called the helium flash
The helium burns ferociously for a few hours, until equilibrium is reached and the stable core then fuses helium into carbon

17. Back to the Giant Branch
Whatever helium exists in the core is rapidly consumed (lasts a few tens of millions of years after helium flash)
As helium fuses to form carbon, a new carbon-rich inner core forms, surrounded by helium burning, hydrogen burning and non-burning shells
The non-burning layer expands and star becomes red giant or red supergiant

18. Core of Carbon Ash

19. Death of a Low-Mass Star
The inner carbon core becomes too cool for further fusion and continues to contract
The fires go out
Before the core attains the temperature necessary to fuse carbon, its density reaches a point where core can no longer be compressed
At this density, a cubic centimeter of core matter would weigh 1000 kg on Earth: a ton of matter compressed into a volume the size of a grape

20. Planetary Nebulae
Driven by increasing radiation and instabilities in the core and outer layers, all of the star’s outer envelope is ejected into space in less than a few million years at a speed of a few 10’s of km/s

21. Planetary Nebulae
The star now has two distinct parts: a core of carbon ash (a.k.a. white dwarf) and an expanding cloud of dust and cool gas spread over a volume roughly the size of our solar system
This is a planetary nebula (they have no association with planets)

22. Planetary Nebulae
It continues to spread out over time, and eventually disperses into interstellar space, enriching it with atoms of helium, carbon, oxygen & heavier elements
These elements eventually get swept up into nebulae (see ch. 18) and formed into new stars and planets

25. White and Black Dwarfs
The carbon core at the center of the planetary nebula continues to evolve
The core is very small, size of Earth or smaller
Its mass is about half the mass of the sun
It shines by stored heat, not nuclear reactions
The core’s temperature & size give it the name of white dwarf

26. White and Black Dwarfs
Once a star becomes a white dwarf, its evolution is over
It eventually becomes a black dwarf – a cold, dense, burned-out ember in space that remains about the size of Earth

27. Evolution of Stars More Massive than the Sun
High-mass stars evolve much faster than low-mass stars.
Its ravenous fuel consumption shortens its main sequence lifetime.
A solar mass star spends 10 billion years on the main sequence
A 5 solar mass B-type star is on main sequence for about a 100 million years
A 10 solar mass O-type star is on main sequence for about 20 million years

28. Evolution of Stars More Massive than the Sun
At 8 solar masses and larger, stars can fuse carbon, oxygen and even heavier elements. These stars die in violent explosions soon after leaving main sequence (next chapter!!!)