Topic 5 Evolution And Death Flashcards
What is shell hydrogen burning?
A nuclear fusion process occurring is the early stages of post-main sequence evolution in stars, in which a spherical shell of hydrogen surrounding the care fuses to helium after the core has exhausted its hydrogen supply.
What is the triple alpha process?
The nuclear reaction that forms one carbon is nucleus from three helium 4 nuclei (alpha particles).
4He2 + 4He2 > 8Be4
8Be4 + 4He2 > 12C6 + γ
What is degeneracy pressure?
The pressure excited by gos particles in a degenerate state, caused by quantum mechanical effects that increase typical particle energies at high density.
Electron degeneracy pressures occults in the cores of (red and asymptotic giant branches) giant stars and in white dwarfs while neutron degeneracy pressure occurs in neutron stars.
What is thermonuclear runaway?
A process where the nuclear fusion rate is not kept controlled by the ideal gas law. The nuclear fusion rate increases in a runaway fashion, essentially as a controlled nuclear explosion.
How does a Red giant form?
When a low mass star moves to hydrogen shell burning, energy is injected into the stars envelope. This causes the stars size and luminosity to increase while its surface temperature decreases.it is now a red giant.
What is a subgiant star?
The evolutionary phase between the main sequence and red giant branch where nuclear fusion shuts off in the stars corn and starts in shell surrounding it.
What is the red giant branch?
It is a region of the H-R diagram in which stars have a contracting inert helium core before the onset of helium burning. Hydrogen burning occurs in a shell around the core. Often abbreviated to RGB.
What is the horizontal branch?
The evolutionary phase between the red giant branch and the asymptotic giant branch where helium burning occurs in the stars core. Sometimes abbreviated to HB.
What is a helium flash?
The sudden onset of helium burning at the tip of the red giant branch, caused by thermonuclear runaway in the helium rich layers of the star’s core. It also occurs in the shells of asymptotic giant branch stars.
Only occurs in stars smaller than 2.25 solar masses.
When does thermonuclear runaway occur?
Stars with a mass less than 2.25 solar masses have degenerate gas cores as they collapse. Temperature increase has no effect on degenerate gas so the temperature can continue to increase.
The rate of energy release from burning helium has an extremely strong dependence on temperature, so fusion in degenerate gas leads to **thermonuclear runaway **.
As the core doesn’t expand and cool there is a runaway ignition of helium leading to a large release of energy known as a ** helium flash**.
Once a helium flash has occurred the degenerate gas becomes ordinary and helium burning becomes stable.
What is the asymptotic giant branch?
The final phase of most low mass stars evolution, before they become planetary nebulae. Here, hydrogen and helium are burned in concentric shells around the inert car of the star, which (for most stars) consists of carbon and oxygen. Often abbreviated to AGB.
How is oxygen formed in stars?
During the AGB stage of a stars life carbon can bond with an alpha particle to form oxygen in stars that are smaller than 8 solar masses.
12C6 + 4He2 > 16O8 + γ
What are thermal pulses?
Periodic thermonuclear runaway in the helium shells of asymptotic giant branch stars which creates s-process elements and brings nuclear enriched materials to the surface via convection.
Not to be confused with stellar pulsations on the surface of a star.
What is the s-process?
The slow neutron capture process for creating certain heavy elements up to bismuth, which occurs in environments like the shell burning regions of asymptotic giant branch stars.
Where do the neutrons come from for the s-process?
13C6 + 4He2 > 16O8 + n
Here carbon 13 is an intermediate from the CNO cycle.
The neutron can easily read with other particles as there are no forces of repulsion to overcome.
How do a variety of elements form once they have gained neutrons in the S-process.
Different elements can form due to the decay of neutrons to protons to form new elements.
How does neutron day occur?(beta minus decay)
The radioactive day of a neutron to a proton ejecting an election and an antineutrino.
What is alpha decay?
The radio active decay of a nucleus where an alpha particle is ejected.
What is the s-process?
The slow neutron capture process for creating certain heavy elements up to bismuth which occurs in environments like the shell-burning regions of asymptotic giant branch stars.
What is a supergiant star?
A star that lies along the top of the H-R diagram. Main sequence stars above 11 solar masses will evolve to become supergiants and later type II supernova.
Why does hydrogen shell burning occur?
When the care is depleted of fuel there is no longer enough pressie to counteract gravity. This releases energy into the gas surrounding the core until it is hot enough to burn hydrogen.
What is photodisentegration?
Nuclear fission (splitting of atoms) caused by interactions with very high energy photons (gamma ray).
What would the structure of a supergiant star look like on its last day?
It has an onion like structure with the following layers going from the iron core to the hydrogen surface:
Core: Fe
Si and S
C and O
He
Outer: H
How can carbon be burned to form magnesium?
12C6 + 12C6 > 24Mg12 + γ
What are Wolf - Rayet stars?
There are extremely luminous blue stars with little to no hydrogen present. They are thought to be the cores of high mass stars that have been subjected to high mass loss, causing the depletion of hydrogen.
What is a blue straggler?
A star that appears left behind on the main sequence when it should have evolved further.
This results from a binary system where one star transfers onto the other or the two stars merge. The new star behaves like a more massive star with delayed evolution.
What is a common envelope?
A situation in a binary star system where the envelope of one star expands so much that the two stars touch each other. The stars can stay attached like this while evolution is stable, but one will ultimately spiral in when one star expands, and the two stars will merge.
What are stellar pulsations?
Alternate expansions and contractions of the outer layers of a star. These fluctuations in stellar radius cause corresponding changes in luminosity and surface temperature of the star.
What is the instability strip?
A region that cuts diagonally across the H-R diagram, in which stars are unstable to pulsation via the kappa mechanism.
What is the kappa mechanism?
The mechanism that causes stellar pulsations on the instability strip. They are caused by a feedback loop between the opacity of the stars atmosphere and its temperature.
What are the 5 steps of the kappa mechanism?
- A gas layer moves inwards.
- The layer becomes compressed.
- This ionises the gas and increases the opacity of the layer, resulting in a build up of heat below the layer.
- The increase in pressure eventually causes it to expand again.
- The gas recombines and becomes more transparent allowing energy to flow through it more rapidly.
Therefore the pressure below the layer is reduced and it falls back in again restarting the cycle.
How can planetary nebulas form?
Planetary nebula are formed by repeated mass ejections of enriched materials from stars that are nearing the end of their lives.
What is a remnant?
The dead core of a star this is left once it has completed is nuclear fusion i.e. A white dwarf, neutron star or black hole.
How is a white dwarf formed?
At the end of a low mass stars life, the care of a star now stripped of its outer layers due to AGB and planetary nebula stages contracts until degeneracy pressures become strong enough to to balance further contraction. Once contraction is halted there is no further energy source and a stable white dwarf is formed.
What is the Chandrasekhar limit?
The maximum theoretical mass for a white dwarf beyond which degeneracy pressure cannot support the stellar remnant against gravity. The limit is close to 1.4 solar masses.
How does the mass:radius ratio differ for white dwarf stars compared to main sequence stars?
Unlike main sequence stars, white dwarf stars of a higher mass have smaller radii.
This occurs because as mass increases, higher densities are required to provide enough degeneracy pressure to balance inward gravitational forces.
