Lecture 23

Question 1

What does it mean what a star is in gravitational equilibrium?

Answer 1

That it is in balance

Question 2

What does the stellar thermostat do?

Answer 2

Keeps the luminosity and temperature stable

Question 3

What are the stages of the life of a low mass star?

Answer 3

1. main sequence- hydrogen fuses to helium
2. red giant- hydrogen shell fuses and the outer layers expand
   2a.helium fusion- helium fuses into carbon
3. horizontal branch- stars return to the main sequence
4. double-shell red giant- carbon core stops collapsing
5. planetary nebula- shell fusion becomes violent
6. white dwarf- small, hot carbon rock

Question 4

What happens when all the hydrogen is fused?

Answer 4

• 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

Question 5

What happens in the red giant stage?

Answer 5

• the star becomes larger, cooler and brighter
• the core continues to shrink and get denser and hotter

Question 6

What happens during helium fusion?

Answer 6

• “helium flash”
• the core expands
• in the core helium is converted to carbon and in the shell hydrogen is converted to helium

Question 7

What happens during the horizontal branch stage?

Answer 7

• the core stabilises
• luminosity decreases
• outer layers stop being pushed out
• gravitational equilibrium is restored

Question 8

What happens during the double-shell red giant stage?

Answer 8

• 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

Question 9

What does electron degeneracy pressure describe?

Answer 9

Quantum mechanics says two particles cannot have the same position and energy- this restricts how tightly particles can be packed

Question 10

Why can’t carbon fusion occur at 600 million K in small stars?

Answer 10

Electron degeneracy pressure

Question 11

What occurs during the planetary nebula stage?

Answer 11

• temperature never gets high enough for carbon fusion
• shell fusion becomes so violent that outer layers are blown off- this forms planetary nebula

Question 12

What causes helium hiccups?

Answer 12

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

Question 13

What occurs during the white dwarf stage?

Answer 13

• most of the mass is gone- decreasing core pressure
  -temperature decreases
• energy source is gone

Question 14

How is fusion different in high mass stars?

Answer 14

It occurs much faster

Question 15

How do the early stages of high mass stars compare to those of low mass stars?

Answer 15

The processes are similar but occur much faster

Question 16

When do the lives of high-mass stars diverge from those of low mass stars?

Answer 16

During helium evolution

Question 17

What happens after the double-shell red giant stage in massive stars?

Answer 17

• 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

Question 18

What is the onion model of high-mass stars?

Answer 18

Concentric shells of increasing temperature and pressure produce heavier and heavier elements

Question 19

What element is the final element fused in the core of massive stars?

Answer 19

Iron

Question 20

How do successive shells in massive stars differ?

Answer 20

They are hotter, denser and burn out faster

Question 21

Why is iron the final fusion product?

Answer 21

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

Question 22

What happens to the core as iron doesn’t fuse?

Answer 22

• no photons
• no heating
• lower thermal pressure

Question 23

What happens when the core pressure (iron core) becomes too high?

Answer 23

• 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

Question 24

What happens when a neutron star collapses?

Answer 24

• 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