Lecture 23 Flashcards
What does it mean what a star is in gravitational equilibrium?
That it is in balance
What does the stellar thermostat do?
Keeps the luminosity and temperature stable
What are the stages of the life of a low mass star?
- main sequence- hydrogen fuses to helium
- red giant- hydrogen shell fuses and the outer layers expand
2a.helium fusion- helium fuses into carbon - horizontal branch- stars return to the main sequence
- double-shell red giant- carbon core stops collapsing
- planetary nebula- shell fusion becomes violent
- white dwarf- small, hot carbon rock
What happens when all the hydrogen is fused?
- the core is helium “ash”
- without thermal pressure, gravity causes the core to collapse and core temperature rises
- the temperature (~50 million K) is hot enough for hydrogen fusion in the shell around the core
What happens in the red giant stage?
- the star becomes larger, cooler and brighter
- the core continues to shrink and get denser and hotter
What happens during helium fusion?
- “helium flash”
- the core expands
- in the core helium is converted to carbon and in the shell hydrogen is converted to helium
What happens during the horizontal branch stage?
- the core stabilises
- luminosity decreases
- outer layers stop being pushed out
- gravitational equilibrium is restored
What happens during the double-shell red giant stage?
- the carbon core stops collapsing and heats up
- the inner shell fuses helium to carbon
- the outer shell fuses hydrogen to helium
- energy generation increases
-outer layers puff up and cool - luminosity increases
What does electron degeneracy pressure describe?
Quantum mechanics says two particles cannot have the same position and energy- this restricts how tightly particles can be packed
Why can’t carbon fusion occur at 600 million K in small stars?
Electron degeneracy pressure
What occurs during the planetary nebula stage?
- temperature never gets high enough for carbon fusion
- shell fusion becomes so violent that outer layers are blown off- this forms planetary nebula
What causes helium hiccups?
The He shell is very sensitive to temperature and small variations can causes explosive reactions. Explosions then blow off large portions of hydrogen in the outer atmosphere
What occurs during the white dwarf stage?
- most of the mass is gone- decreasing core pressure
-temperature decreases - energy source is gone
How is fusion different in high mass stars?
It occurs much faster
How do the early stages of high mass stars compare to those of low mass stars?
The processes are similar but occur much faster
When do the lives of high-mass stars diverge from those of low mass stars?
During helium evolution
What happens after the double-shell red giant stage in massive stars?
- the core contracts and heats again
- massive enough to reach temperatures over 600 million K despite degeneracy pressure
- most elements are formed by helium capture
What is the onion model of high-mass stars?
Concentric shells of increasing temperature and pressure produce heavier and heavier elements
What element is the final element fused in the core of massive stars?
Iron
How do successive shells in massive stars differ?
They are hotter, denser and burn out faster
Why is iron the final fusion product?
Iron does not release energy through fusion or fission because there is not way iron can produce any energy to push back against the crush of gravity in the star’s core
What happens to the core as iron doesn’t fuse?
- no photons
- no heating
- lower thermal pressure
What happens when the core pressure (iron core) becomes too high?
- iron atoms are compressed into pure neutrons- regular matter becomes neutronium
- electron degeneracy pressure stops and the core collapses but eventually neutron degeneracy pressure stops the collapse
What happens when a neutron star collapses?
- a supernova
- the light atmosphere impacts on the heavy core and is bounced off in a huge explosion
- the energy of the explosion produces elements heavier than iron
- large energy release from neutrinos
