Chapter 8 Flashcards
What happens as a star moves from Main Sequence to Red Supergiant?
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
Where does a star travel to reach Red Supergiant branch?
Stars move rapidly redwards in HR diagram at constant luminosity
What is the structure of a Red Supergiant when it first reaches the RSG phase?
Helium core
Hydrogen fusion shell around core
Hydrogen shell (no thermonuclear reactions)
When does a star become a red supergiant?
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
When does the red supergiant move to blue in HR diagram and become a blue supergiant (BSG)?
Degeneracy doesn’t set in
Core He burning begins smoothly
T increases and core expands and envelope contracts
R increases and T increases
What does “blue loop” depend on?
Composition and mass
After becoming a blue supergiant, how does the star return to RSG region?
He quickly becomes exhausted in core
He shell burning begins
Core contraction leads to envelope expansion
What happens as further loops develop and a star moves from BSG TO RSG?
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
How do further loops form?
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
What can we not determine from observing stellar properties?
Which fusion process is at work
In a RSG what does the central region contain?
Hydrogen-fusing shell Helium-fusing shell Carbon-fusing shell Neon-fusing shell Oxygen-fusing shell Silicon-fusing shell Iron core (no fusion)
What does radiation pressure do in massive stars?
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)
What results in mass loss from a star?
If radiation pressure exceeds gravity
What is Thomson scattering?
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
What exceeds gravity on a proton?
Radiation pressure on an electron
What is the Eddington luminosity?
Opacity due to millions of ionic absorption lines raises the absorption cross-sections much higher than Thomson constant and a dense wind develops.
What is observational evidence that massive stars lose mass in powerful stellar wind?
- 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)
How is a P-Cygni Line Profile made?
Symmetric Emission + Blue-shifted Absorption
What is the effect of mass loss?
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
What happens if a star looses a lot mass?
The entire H envelope can be blown off, revealing the hot inner core and becomes a Wolf Rayet star
After the entire H envelope is removed from mass loss of a star what happens?
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
How is a rapidly rotating object different to a slow rotating object?
It is hotter and brighter at the poles and cooler at equator
Equator bulges and darkens as star spins faster
What is the effect of rotation?
Centrifugal acceleration expands star at equator
More efficient mixing of elements throughout envelope
larger core makes object brighter
Mass loss asymmetric
What is brought to the surface by convection?
Nuclear burning products
When do massive stars stop creating new nuclear reactions?
When they reach Iron and no further fusion reactions are exothermic
What leads to different shells of different compositions in the core?
Progressively heavier elements have burned in the core of the star
What happens after Si buring has exhausted and core is Fe rich?
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
Why can’t completely relativistic electron degeneracy pressure resist gravity (if M > 1.4M)?
Due to extremely high densities and gravity overcomes the degenerate electron pressure and the core further contracts
What happens after gravity overcomes the degenerate electron pressure?
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
What happens in core of neutron star?
All Fe in core converts into He, then protons, then neutrons
When does the collapse in the core of the neutron star stop?
When neutrons are degenerate
Densities are high
When do neutrons become degenerate?
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)
What is the radius and upper mass limit of a neutron star?
10km (very high pressure)
3M
How much energy is required to break up all iron in core?
As much as was released in all previous fusion reactions that led to the formed iron to begin with?
