1. ASTR 1020 – February 23 Learning Assistant Meeting 5pm in UMC 235
Monday Feb 27 .
Website

2. MS Lifetime
What determines amount of time a star stays on Main Sequence?
Just like a kerosene heater: Amount of fuel and rate of burn.
More Mass = More Fuel
More Luminosity = Greater Burn Rate
We can scale from the Sun: M = 1M L = 1L
Sun lasts 1010 years
M in solar masses
L in solar luminosities

3. Some Lifetimes Mass Luminosity Lifetime in Billion Years Sun 1 1 10
Sirius
Prox Cen
Rigel 8 10,
Dinky little stars like Prox Cen will last trillions of years
Huge stars like Rigel are gone in a few million
There aren’t many large stars out there, because they don’t last.
10,000 O stars of the 100,000,000,000 Milky Way stars

4. Estimate the main sequence lifetime of a 10 solar-mass star.
A. 100 billion years
B. 10 billion years
C. 10 million years
D. 1 million years

5. Estimate the main sequence lifetime of a 10 solar-mass star.
A. 100 billion years
B. 10 billion years
C. 10 million years
D. 1 million years

6. Chemical Energy for Sun?
Chemical Energy Generates 2eV per atom in forming molecule (burning)
2eV = 3x10-19 Joules
Number of Atoms in Sun:
Available Energy
Time it can run:

7. Gravitational Energy? Available Gravity Energy: Time it can run:
Sun can only run 30million years on gravity.
It does this during formation
Best understanding of Sun until Einstein.

8. Nuclear Energy for Sun?
Nuclear Energy Generates 2MeV per atom in forming molecule (burning)
2MeV = 3x10-13 Joules
Number of Atoms in Sun:
Available Energy
Time it can run:
Sun can run 20 Billion years on nuclear energy
Which is what it does.

9. What Happens When Hydrogen Starts to Deplete?
Percentage of Helium in Core Rises
Nuclear Reaction Rate Slows
Energy Leaks Away and Pressure Drops
Pressure from above squeezes core,
making it smaller and hotter.
Nuclear Reaction Rate Increases
Star Gets Brighter!
The less fuel a star has left, the faster it burns it….

10. Effect on Star M Spectral Type Core Shrinks and Gets Hotter
Star gets brighter and bluer? -5
Rigel ?
Giants
Capella
Sirius M
Procyon
Sun +5
Main Sequence
a Cen B
White Dwarfs
+10
Sirius B
Prox Cen
+15 O B A F G K M
Spectral Type
Actually Goes Up and To Cooler Side
Why??

11. Outer Envelope of Star Expands
Force of photon flux from below lifts outer parts of star.
Core gets smaller and hotter, but
Surface gets larger and cooler.

12. Path on H-R Diagram
Over Time a Star becomes
a red giant.

13. What’s Happening in Core?
H->He
H->He He
In the center, H is depleted, but He too cool to burn.
“Shell Burning” describes source of nuclear power around dead center

14. He Compressed In Middle
H->He
Temperature and Density
Rise in Center
Star gets more luminous He

15. Electron Degeneracy Electron Degeneracy Helium Density Rises
Center of Sun has density of 10g/cc (H2O = 1g/cc)
When density in center of star reaches ~30,000g/cc
a new phenomenon kicks in
Electron Degeneracy
A purely quantum mechanical phenomenon

16. A Metaphor for Degeneracy

17. Electron Degeneracy Pressure
Because of electron degeneracy pressure, core stops shrinking.
Gains Mass, Temperature Rises, Stays Same Size
Has all the characteristics of a bomb.
Burnable Material
Confined Space
What ignites it?

18. Helium Flash When degenerate He core reaches about 108K, the nuclei
can burn by the Triple Alpha Reaction. (He+He+He->C)
They start to burn and release energy. Pressure rises and temperature
rise, but volume does not increase. P and T rise some more.
Finally, P gets so great it lifts the degeneracy and thermal pressure
equilibrium is re-established.
This is the “He Flash”
Could blow a star apart, but it doesn’t.

19. He Flash
Star changes L and R
in just 10,000 years!

20. After the Flash H H H->He H->He He->C
Helium Burns in the Center
Starts to Development Dead C Core

21. Red Giants

22. Nucleus
Earth Orbit

23. Fate of The Earth We will be swallowed by the Sun
In 5 billion years it will start to swell.
T will rise on Earth
Oceans will boil and then evaporate into space
Sun will cover the sky. Giant ruddy ball.
Then it will engulf us.
Mountains will melt.
Planet should survive.
Will look like a polished bowling ball.