Chapter 8

Question 1

What happens as a star moves from Main Sequence to Red Supergiant?

Answer 1

In the convective core: increase in He abundance uniform over core
Reduction of H fuel causes core to contract and heat up causing a H burning shell
Core contraction leads to envelope expansion
R increases and T decreases

Question 2

Where does a star travel to reach Red Supergiant branch?

Answer 2

Stars move rapidly redwards in HR diagram at constant luminosity

Question 3

What is the structure of a Red Supergiant when it first reaches the RSG phase?

Answer 3

Helium core
Hydrogen fusion shell around core
Hydrogen shell (no thermonuclear reactions)

Question 4

When does a star become a red supergiant?

Answer 4

As T increases to 10,000K and a convective envelop develops

no significant luminosity increase as the more efficient energy transport by convection does not occur

Question 5

When does the red supergiant move to blue in HR diagram and become a blue supergiant (BSG)?

Answer 5

Degeneracy doesn’t set in
Core He burning begins smoothly
T increases and core expands and envelope contracts
R increases and T increases

Question 6

What does “blue loop” depend on?

Answer 6

Composition and mass

Question 7

After becoming a blue supergiant, how does the star return to RSG region?

Answer 7

He quickly becomes exhausted in core
He shell burning begins
Core contraction leads to envelope expansion

Question 8

What happens as further loops develop and a star moves from BSG TO RSG?

Answer 8

Core contraction continues and central T becomes high enough for C burning
Core expands and envelope contract leading back to BSG

core exhausts and shell burning begins and back to RSG

Question 9

How do further loops form?

Answer 9

New reactions begin burning in core moving star to the blue

the lifetime of these loops become shorter still and it is not very likely to see stars on such loops

Question 10

What can we not determine from observing stellar properties?

Answer 10

Which fusion process is at work

Question 11

In a RSG what does the central region contain?

Answer 11

Hydrogen-fusing shell
Helium-fusing shell
Carbon-fusing shell
Neon-fusing shell
Oxygen-fusing shell
Silicon-fusing shell
Iron core (no fusion)

Question 12

What does radiation pressure do in massive stars?

Answer 12

In outer layers it is high
It exerts outward force on atmosphere
This is scattering by electrons, which collide with atoms and impart momentum
Absorption of photons (and their momentum) by atoms (line driving)

Question 13

What results in mass loss from a star?

Answer 13

If radiation pressure exceeds gravity

Question 14

What is Thomson scattering?

Answer 14

Scattering by electrons whose momentum transferred to positive ions (mostly protons) by collisions
Then mass is lost if luminosity L at radius r obeys the Thomson scattering equation

Question 15

What exceeds gravity on a proton?

Answer 15

Radiation pressure on an electron

Question 16

What is the Eddington luminosity?

Answer 16

Opacity due to millions of ionic absorption lines raises the absorption cross-sections much higher than Thomson constant and a dense wind develops.

Question 17

What is observational evidence that massive stars lose mass in powerful stellar wind?

Answer 17

• UV P-Cygni lines
• Free-free radio emission from wind
• X-ray emission from intrinsic wind shocks
• Emission lines in some stars (eg WR stars, molecular emission lines in cool star winds)

Question 18

How is a P-Cygni Line Profile made?

Answer 18

Symmetric Emission + Blue-shifted Absorption

Question 19

What is the effect of mass loss?

Answer 19

It significantly reduces the mass of the star during its evolution
This alters its evolutionary path and the star follows the path of a lower mass star

Question 20

What happens if a star looses a lot mass?

Answer 20

The entire H envelope can be blown off, revealing the hot inner core and becomes a Wolf Rayet star

Question 21

After the entire H envelope is removed from mass loss of a star what happens?

Answer 21

This reveals the hot He-rich core and the star appears to left of the H-burning MS
Spectra consist of He and CNO emission lines

Question 22

How is a rapidly rotating object different to a slow rotating object?

Answer 22

It is hotter and brighter at the poles and cooler at equator

Equator bulges and darkens as star spins faster

Question 23

What is the effect of rotation?

Answer 23

Centrifugal acceleration expands star at equator
More efficient mixing of elements throughout envelope
larger core makes object brighter
Mass loss asymmetric

Question 24

What is brought to the surface by convection?

Answer 24

Nuclear burning products

Question 25

When do massive stars stop creating new nuclear reactions?

Answer 25

When they reach Iron and no further fusion reactions are exothermic

Question 26

What leads to different shells of different compositions in the core?

Answer 26

Progressively heavier elements have burned in the core of the star

Question 27

What happens after Si buring has exhausted and core is Fe rich?

Answer 27

Core contracts and T increases
Heavy nuclei photo-disintegrate
Energy is provided through the gravitational contraction, which releases GPE
Helium disintegrates back into protons and neutrons

Question 28

Why can’t completely relativistic electron degeneracy pressure resist gravity (if M > 1.4M)?

Answer 28

Due to extremely high densities and gravity overcomes the degenerate electron pressure and the core further contracts

Question 29

What happens after gravity overcomes the degenerate electron pressure?

Answer 29

Electrons are forced into the nuclei of atoms where they combine with protons to form neutrons:
p + e- = n + ne
Neutrinos carry off energy in a short burst
The electrons are now removed, these provided most outward pressure, which now suddenly disappears
Rapid core collapse to form a neutron star

Question 30

What happens in core of neutron star?

Answer 30

All Fe in core converts into He, then protons, then neutrons

Question 31

When does the collapse in the core of the neutron star stop?

Answer 31

When neutrons are degenerate

Densities are high

Question 32

When do neutrons become degenerate?

Answer 32

Their number density needs to be larger than that is required for electrons by a factor m^3/2 (violation of Pauli exclusion which is dependent on mass)

Question 33

What is the radius and upper mass limit of a neutron star?

Answer 33

10km (very high pressure)

3M

Question 34

How much energy is required to break up all iron in core?

Answer 34

As much as was released in all previous fusion reactions that led to the formed iron to begin with?