1. Stellar Evolution II

2. The Upper End of the Main Sequence: How massive can a star get?
Larger clouds of gas (GMCs) tend to fragment into smaller ones
before collapsing to form stars – very massive stars are rare
Stars with masses above 50 MSUN are unstable – nuclear reactions
in their core produce energy at such a fast rate that they blow off
their outer layers, losing mass.

3. Eta Carinae Mass: 100-150MSUN Giant eruption in 1843
Made it 2nd brightest star
in sky for a short time
Created two giant lobes
of hot gas expanding away
from the star

4. Evolution of Stars on the Main Sequence
Core starts with same fraction of hydrogen as whole star
Fusion changes H  He
For each reaction, the star loses 4 H and gains only one He, so pressure decreases and gravity squeezes the core more tightly
Core gradually shrinks and gets hotter, increasing the pressure to compensate
Energy generation rate gradually increases, so star gets more luminous and the surface gets hotter. Increased pressure of radiation increases the radius.
H > He

5. Evolution of Stars on the Main Sequence

6. Evolution of Stars on the Main Sequence
Lifetime of stars on the main sequence:
More massive stars burn their fuel faster, so will use it up
quicker > have smaller lifetimes
Lifetime T = 1/M2.5
O ,000,000 yr
A ,000,000 yr
G ,000,000,000 yr
K ,000,000,000 yr
M ,000,000,000 yr
(age of universe: 13,600,000,000 yr)

7. Main Sequence Stars: Energy generated by fusing H to He in their core
Luminosity and surface temperature increase as mass increases

8. Post-MS Evolution of Low Mass Stars (M < 8MSUN)
What happens when the core runs out of Hydrogen? H
H > He

9. Post-MS Evolution of Low Mass Stars (M < 8MSUN)
Core stops producing energy – gravity causes it to contract and heat up
Layer surrounding the core also contracts and heats up enough to start fusing H to He He
Outer parts of star expand
because star is H – burning layer
is producing more energy than is
required to balance gravity
H > He
Surface gets cooler because of
increased area > star becomes red
giant

10. Post-MS Evolution of Low Mass Stars (M < 8MSUN)

11. Post-MS Evolution of Low Mass Stars (M < 8MSUN)

12. Degenerate Gases Normal Gas:
Compress normal gases > particles move faster >
increased pressure and temperature
Heat a normal gas > pressure increases

13. Degenerate Gases Degenerate Gases:
If gas is dense enough, particles have no where to move – if you compress the gas, the particles cannot move faster; they simply ‘wiggle’ more energetically
Compress degenerate gas >
temperature increases but
pressure remains the same
Heat a degenerate gas >
pressure stays the same

14. Helium Flash
Matter in core is fully ionized – all electrons are free of their atoms
Most pressure in the core is from the electrons
As the core of Helium ash shrinks, it becomes degenerate – its
temperature will increase but its pressure will remain the same
Density now around 1000,000 grams/cubic cm (about 1000 tonnes/cc)
As the temperature of the core passes 100,000,000 K, it can start He
fusion into Carbon

15. Helium Flash
He ignites > produces energy > temperature increases, but pressure
stays the same – the core cannot respond to the increased temperature
by expanding
Increases temperature > increased He fusion rate > increased energy
production > increased temperature > increased He fusion rate >
increased energy production > increased temperature > increased He
fusion rate > increased energy production > increased temperature > ....
Explosion! – the Helium
Flash

16. Helium Flash
For a few minutes the core generates 100,000,000,000,000 times
more energy per second than the sun – 100 times more energy per
second than all the stars in the Milky Way combined
Does not destroy the star – energy is absorbed in outer layers. No
outward sign of explosion
Helium flash only occurs in stars between 0.5 and 3 MSUN
After a few minutes, the core becomes so hot that the gas becomes
normal again, and pressure increases

17. Helium Burning H still fusing to He in shell around core
He fused to C, O in core
H still fusing to He in
shell around core
He > C, O
H > He

18. Helium Burning Extra energy from He Fusion causes core to Expand
This forces H burning
shell to expand.
He > C, O
H > He

19. Helium Burning Expansion cools H burning shell, which then
absorbs heat from the
envelope, causing it to
shrink a little and get
hotter
He > C, O
H > He