Chapter 6 Flashcards
What is the key assumption about Main Sequence stars?
They are initially chemically homogeneous
Then stellar mass uniquely determines entire stellar structure
For low mass MS stars, what are their characteristic masses, temp and spectral type?
0.1 M < M < 2M
3000K < T< 10,000 K
M < spectral type < F
How is energy generated for a low mass MS star?
From PP chain
E proportional to T^4 ia not highly concentrated in centre
In core of low mass MS star what type of radiation takes place?
Radiative core
No convection in core
CNO cycle operates slowly in stars of mass 1-2 M
What happens in outer parts of low mass star due to Teff being less than 10,000K?
There will be neutral hydrogen and helium in the outer parts while there elements are ionized in the inner parts
Why do low mass stars not have convection zone?
They are too unstable and a convective envelope forms instead, which extends down to where H and He is fully ionized (T ~ 10^4 K)
What does a cooler surface temperature of a star mean?
Deeper convection zone
In the present day Sun what has happened in the centre of the star, in relation to hydrogen and heliun?
A large fraction of hydrogen has already been converted to helium
Carbon has been mainly converted to nitrogen
What happens to the isotopes destroyed by slow reactions?
They are enhanced
In the present day Sun where is all the luminosity generated?
In inner 10-20% in radius, containing 20% of mass
How much mass does the outer 25% of radius contain?
Few % of mass
4 billion years later, how would the sun change on MS?
Star would be a bit brighter
Hydrogen depleted in centre
Density much higher in centre
Luminosity generated outside the centre
What are the characteristics of high mass MS stars?
2M < M <150M
10,000K < T < 40,000K
A< spectral type <0
How is energy generated in high mass MS stars?
From CNO cycle
E is proportional to T^16 (large dT/dr which leads to convection in core)
Why do high mass stars not have a radiation zone?
Since T > 10,000K , the envelope is completely ionized and so no convection which leads to a radiative envelope
In high mass stars, what is in the core?
Central convection zone where the abundances are uniform. The convective core extends to 1 R
What is the structure of energy of a low mass star?
Most of the mass of the star is in radiative equilibrium including the energy generating regions at the centre.
But there is an outer shell in which convection carries most of the energy
What is the structure of energy of a high mass star?
Convection carries all the energy in the central, energy-generating core.
Radiation carries the energy from the core to the surface
What is stellar evolution driven by?
Abundance changes due to nuclear burning
Which timescales are faster than the timescale for changes in abundances?
Dynamical timescale
Thermal timescale
How are the effects of the changes in relative abundances measured?
Using mean molar mass
What does the mean molar mass (μ) effect?
E(μ): The way a star can run out of fuel or will need a different energy generation mechanism
k(μ): A change in opacity affects the temperature gradient dT/dr which can change energy transport to convection
How does the effect of abundance changes affect pressure?
P = ρRT/μ
The pressure decreases (μ increases) and causes gravitational contraction
What happens to the mean molar mass when hydrogen is converted into helium?
Results in a higher mean molar mass
What are the characteristics in the core on the “Zero Age Main Sequence” (ZAMS)?
The core is homogeneous with original composition e.g 70% H, 28% He and 2% metals by mass
What happens when H is burnt to He during the evolution on MS?
μ increases and P decreases
core contracts and T and ρ increase
Envelope expands slightly, while higher core temperature results in more nuclear reactions
NET RESULT: L increases and T decreases
Where does star move on HR diagram during the Zero Age Main Sequence?
Star moves slightly upwards and redwards in HR diagram
When do significant structural changes in the star take place?
When around 10% of the total H has been burnt (TAMS-Terminal Age MS)
What is used to track and account for changes in abundances?
A series of “static” stellar structure models
Where does the star move on HR diagram when its hydrogen is burnt in the core?
Moves across The Main Sequence from Zero Age Main Sequence to Terminal Age Main Sequence
What % of all hydrogen is burnt on the MS?
Typically 10%
This represents the mass of the core where the conditions are sufficient for fusion to take place
Which types of stars spend longer on MS
Low mass than high mass stars
