Chapter 6

Question 1

What is the key assumption about Main Sequence stars?

Answer 1

They are initially chemically homogeneous

Then stellar mass uniquely determines entire stellar structure

Question 2

For low mass MS stars, what are their characteristic masses, temp and spectral type?

Answer 2

0.1 M < M < 2M
3000K < T< 10,000 K
M < spectral type < F

Question 3

How is energy generated for a low mass MS star?

Answer 3

From PP chain

E proportional to T^4 ia not highly concentrated in centre

Question 4

In core of low mass MS star what type of radiation takes place?

Answer 4

Radiative core

No convection in core

CNO cycle operates slowly in stars of mass 1-2 M

Question 5

What happens in outer parts of low mass star due to Teff being less than 10,000K?

Answer 5

There will be neutral hydrogen and helium in the outer parts while there elements are ionized in the inner parts

Question 6

Why do low mass stars not have convection zone?

Answer 6

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)

Question 7

What does a cooler surface temperature of a star mean?

Answer 7

Deeper convection zone

Question 8

In the present day Sun what has happened in the centre of the star, in relation to hydrogen and heliun?

Answer 8

A large fraction of hydrogen has already been converted to helium
Carbon has been mainly converted to nitrogen

Question 9

What happens to the isotopes destroyed by slow reactions?

Answer 9

They are enhanced

Question 10

In the present day Sun where is all the luminosity generated?

Answer 10

In inner 10-20% in radius, containing 20% of mass

Question 11

How much mass does the outer 25% of radius contain?

Answer 11

Few % of mass

Question 12

4 billion years later, how would the sun change on MS?

Answer 12

Star would be a bit brighter

Hydrogen depleted in centre

Density much higher in centre

Luminosity generated outside the centre

Question 13

What are the characteristics of high mass MS stars?

Answer 13

2M < M <150M
10,000K < T < 40,000K
A< spectral type <0

Question 14

How is energy generated in high mass MS stars?

Answer 14

From CNO cycle

E is proportional to T^16 (large dT/dr which leads to convection in core)

Question 15

Why do high mass stars not have a radiation zone?

Answer 15

Since T > 10,000K , the envelope is completely ionized and so no convection which leads to a radiative envelope

Question 16

In high mass stars, what is in the core?

Answer 16

Central convection zone where the abundances are uniform. The convective core extends to 1 R

Question 17

What is the structure of energy of a low mass star?

Answer 17

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

Question 18

What is the structure of energy of a high mass star?

Answer 18

Convection carries all the energy in the central, energy-generating core.
Radiation carries the energy from the core to the surface

Question 19

What is stellar evolution driven by?

Answer 19

Abundance changes due to nuclear burning

Question 20

Which timescales are faster than the timescale for changes in abundances?

Answer 20

Dynamical timescale

Thermal timescale

Question 21

How are the effects of the changes in relative abundances measured?

Answer 21

Using mean molar mass

Question 22

What does the mean molar mass (μ) effect?

Answer 22

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

Question 23

How does the effect of abundance changes affect pressure?

Answer 23

P = ρRT/μ

The pressure decreases (μ increases) and causes gravitational contraction

Question 24

What happens to the mean molar mass when hydrogen is converted into helium?

Answer 24

Results in a higher mean molar mass

Question 25

What are the characteristics in the core on the “Zero Age Main Sequence” (ZAMS)?

Answer 25

The core is homogeneous with original composition e.g 70% H, 28% He and 2% metals by mass

Question 26

What happens when H is burnt to He during the evolution on MS?

Answer 26

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

Question 27

Where does star move on HR diagram during the Zero Age Main Sequence?

Answer 27

Star moves slightly upwards and redwards in HR diagram

Question 28

When do significant structural changes in the star take place?

Answer 28

When around 10% of the total H has been burnt (TAMS-Terminal Age MS)

Question 29

What is used to track and account for changes in abundances?

Answer 29

A series of “static” stellar structure models

Question 30

Where does the star move on HR diagram when its hydrogen is burnt in the core?

Answer 30

Moves across The Main Sequence from Zero Age Main Sequence to Terminal Age Main Sequence

Question 31

What % of all hydrogen is burnt on the MS?

Answer 31

Typically 10%

This represents the mass of the core where the conditions are sufficient for fusion to take place

Question 32

Which types of stars spend longer on MS

Answer 32

Low mass than high mass stars