1. 20. Stellar Death Dredge-ups bring red-giant material to the surface
Low-mass stars undergo two distinct red-giant stages
Dredge-ups bring red-giant material to the surface
Low -mass stars die gently as planetary nebulae
Low -mass stars end as white dwarfs
High-mass stars synthesize heavy elements
High-mass stars die violently as supernovae
Supernova 1987A
Supernovae produce abundant neutrinos
Binary white dwarfs can become supernovae
Detection of supernova remnants

2. Low-Mass Stars: Three Red Giant Phases
Low-mass definition
< ~ 4 MSun during main-sequence lifetime
Red giant phases
Initiation of shell hydrogen fusion
Red giant branch on the H-R diagram
Initiation of core helium fusion
Horizontal branch of the H-R diagram
Initiation of shell helium fusion
Asymptotic giant branch of the H-R diagram

3. The Sun’s Post-Main-Sequence Fate

4. Interior of an Old Low-Mass AGB Star

5. Stellar Evolution In a Globular Cluster

6. Dredge-Ups Mix Red Giant Material
Main-sequence lifetime
The core remains completely separate
There is no exchange of matter with overlying regions
Decreasing H Increasing He in the core
Overlying regions retain cosmic chemical proportions
~ 74 % H ~ 25% He ~ 1% “metals”
Red giant phases
Three possible stages
Stage 1 dredge-up After core H fusion ends
Stage 2 dredge-up After core He fusion ends
Stage 3 dredge-up After shell He fusion begins
Only if MStar > 2 MSun
One possible result
A carbon star
Abundant CO ejected into space
Same isotopes of C & O that are in human bodies

7. Low-Mass Stars Die Gently
He-shell flashes produce thermal pulses
Caused by runaway core He fusion in AGB stars
Cyclical process at decreasing time intervals
313,000 years
295,000 years
251,000 years
231,000 years
All materials outside the core may be ejected
~ 40% of mass lost from a 1.0 MSun star
> 40% of mass lost from a >1.0 MSun star
Hot but dead CO core exposed
At the center of an expanding shell of gas
Velocities of ~ 10 km . sec-1 to ~ 30 km . sec-1
Velocities of ~ 22,000 mph to ~ 66,000 mph

8. A Carbon Star & Its CO Shell

9. Thermal Pulses of an 0.7 MSun AGB Star

10. One Example of a Planetary Nebula

11. The Helix Nebula: 140 pc From Earth

12. Making An Elongated Planetary Nebula

13. Low-Mass Stars End As White Dwarfs
UV radiation ionizes the expanding gas shell
This glows in what we see as a planetary nebula
Name given because they look somewhat like planets
No suggestion that they have, had, or will form planets
This gas eventually dissipates into interstellar space
No further nuclear fusion occurs
Supported by degenerate electron pressure
About the same diameter as Earth ~ 8,000 miles
It gradually becomes dimmer
Eventually it will become too dim to detect

14. The Chandrasekhar Limit
White dwarf interiors
Initially supported by thermal pressure
Ionized C & O atoms
A sea of electrons
As the white dwarf cools, particles get closer
Pauli exclusion principle comes into play
Electrons arrange in orderly rows, columns & layers
Effectively become one huge crystal
White dwarf diameters
The mass-radius relationship
The larger the mass, the smaller the diameter
The diameter remains the same as a white dwarf cools
Maximum mass degenerate electron pressure can support
~ 1.4 MSun After loss of overlying gas layers
This white dwarf upper mass limit is the Chandrasekhar limit

15. Evolution From Giants To White Dwarfs

16. White Dwarf “Cooling Curves”

17. High-Mass Stars Make Heavy Elements
High-mass definition
> ~ 4 MSun as a ZAMS star
Synthesis of heavier elements
High-mass stars have very strong gravity
Increased internal pressure & temperature
Increased rate of core H-fusion into He
Increased rate of collapse once core H-fusion ends
Core pressure & temperature sufficient to fuse C
The CO core exceeds the Chandrasekhar limit
Degenerate electron pressure cannot support the mass
The CO core contracts & heats
Core temperature > ~ K
C fusion into O, Ne, Na & Mg begins

18. Synthesis of Even Heavier Elements
Very-high-mass definition
> ~ 8 MSun as a ZAMS star
Synthesis of still heavier elements
End of core-C fusion
Core temperature > ~ K
Ne fusion into O & Mg begins
Percentage of O & Mg increases in the core
End of core-Ne fusion
Core temperature > ~ K
O fusion into S begins
End of core-O fusion
Core temperature > ~ K
Si fusion into S & Fe begins
Additional layers of shell fusion begin

19. The Interior of an Old High-Mass Star

20. Consequence of Multiple Shell Fusion
Core changes
Core diameter decreases with each step
Ultimately about the same diameter as Earth ~ 8,000 miles
Rate of core fusion increases with each step
Energy changes
Each successive fusion step produces less energy
All elements heavier than iron require energy input
Core fusion cannot produce elements heavier than iron
All heavier elements must be produced by other processes

21. Evolutionary Stages of a 25-MSun Star

22. High-Mass Stars Die As Supernovae
Basic physical processes
All thermonuclear fusion ceases
The core collapses
It is too massive for degenerate electron pressure to support
The collapse rebounds
Luminosity increases by a factor of 108
As bright as an entire galaxy
> 99% of energy is in the form of neutrinos
Matter is ejected at supersonic speeds
Powerful compression wave moves outward
Appearance
Extremely bright light where a dim star was located
Supernova remnant
Wide variety of shapes & sizes

23. Supernova: The First 20 Milliseconds

24. Supernova 1987A Important details Basic physical processes
Located in the Large Magellanic Cloud
A companion to the Milky Way ~ 50,000 parsecs from Earth
Discovered on 23 February 1987
Near a huge H II region called the Tarantula Nebula
Visible without a telescope
First naked-eye supernova since 1604
Basic physical processes
Primary producer of visible light
Shock wave energy < 20 days
Radioactive decay of cobalt, nickel & titanium > 20 days
Dimmed gradually after radioisotopes were gone > 80 days
Luminosity only 10% of a normal supernova

25. Unusual Feature of SN 1987A Relatively low-mass red supergiant
Outer gaseous layers held strongly by gravity
Considerable energy required to disperse these gases
Significantly reduced luminosity
Unusual supernova remnant shape
Hourglass shape
Outer rings Ionized gas from earlier gentle ejection
Central ring Shock wave energizing other gases

26. Supernova 1987A: Three-Ring Circus

27. White Dwarfs Can Become Supernovae
Observed characteristics
No spectral lines of H or He
These gases are gone
The progenitor star must be a white dwarf
Strong spectral line of Si II
Basic physical processes
White dwarf in a close-binary setting
Overcontact situation Companion star fills Roche lobe
The white dwarf may exceed the Chandrasekhar limit
Degenerate electron pressure cannot support the mass
Core collapse begins, raising temperature & pressure
Unrestrained core C-fusion begins
The white dwarf blows apart

28. The Four Supernova Types
Type Ia
Type Ib
Type Ic
Type II
No H or He lines
Strong Si II line
No H lines
Strong He I line
Strong H lines

29. Type Ia & II Supernova Light Curves

30. Gum Nebula: A Supernova Remnant

31. Pathways of Stellar Evolution

32. Important Concepts Death of low-mass stars Death of high-mass stars
ZAMS mass < 4 MSun
Red giant phases
Start of shell H fusion
Start of core He fusion
Start of shell He fusion
No elements heavier than C & O
Gentle death
Dead core becomes a white dwarf
Expelled gases become planetary neb.
Death of high-mass stars
ZAMS mass > 4 MSun
Red supergiant phases
No elements heavier than Fe
Catastrophic death
Dead core a neutron star or black hole
Supernova remnant
Elements heavier than Fe produced
Pathways of stellar evolution
Low-mass stars
Produce planetary nebulae
End as white dwarfs
High-mass stars
Produce supernovae
End as neutron stars or black holes