1. Prepare your scantron:
Use
a pencil,
not a pen!
Fill in your name and fill the bubbles under
your name.
LAST NAME FIRST, First name second
Put your 4-digit code instead of “IDENTIFICATION NUMBER”.
--- (The last 4 digits of your OleMiss ID.)
Question # 1: answer A
Question # 2: answer D
Question # 3: answer A
Setup:
Recall reading assignment:
Chapter 15 (Stars): pp. 486 – 510
Please take a moment to mute your cell phone!

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Question 4
What determines the length of life of a star?
A All stars live the same length of time.
B The temperature in its center.
C Its chemical composition.
D Its mass.
E Its magnetic field.
Next question coming …

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Question 5
Which of the following uses the same physical process as the Sun to generate heat?
A A gas stove.
B A wood stove.
C A hydrogen bomb.
D An atomic bomb.
E A nuclear reactor.

4. Nuclear processes Decay (occurs in nature) Fission (Reactors & A-bomb)
Fusion (H-bomb)
Nuclear processes
Don’t confuse with chemical processes!
They don’t change one atom into another.

5. Hydrogen to helium fusion
This reaction needs
heat > 1 million K
(only in very center of star!)
4 H  He + heat
(Call it H ‘burning’ - it is really not!)
Mass of
1 proton units
4 protons units
1 He units
Missing units = 0.8%
Becomes energy:
E = m c2

6. What if all hydrogen is gone?
All stars start on the main sequence
All stars become giants (when hydrogen is used up):
This produces heat and keeps the red giant shining.

7. A-bomb does this Stars do this
• More processes - helium fusion and beyond
He + He + He  C + heat
He + C  O + heat , ...
• These processes need
hotter and hotter temperatures
Not in the Sun yet!
• Energy is produced by fusion
of heavier elements up to iron
”Metals” build up
A-bomb does this
Stars do this

8. Questions coming …

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Question 6
What keeps a main-sequence star shining?
A It burns hydrogen into water.
B It produces helium from hydrogen.
C It produces heavier elements from helium.
D It is hot and cooling slowly as it gives off its heat.
Next question coming …

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Question 7
What keeps a red giant shining?
A It burns hydrogen into water.
B It produces helium from hydrogen.
C It produces heavier elements from helium.
D It is hot and cooling slowly as it gives off its heat.
Next question coming …

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Question 8
Which one comes first, the main sequence stage,
or the red giant stage?
A The main sequence stage.
B The red giant stage.
C It varies, depending on the chemical composition of the
gas the star is born from.
D It is not in order: heavy stars are born as red giants,
light stars are born as main sequence stars.
Next question coming …

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Question 9
Which star lives longer, one with a small or a large mass?
A The one with a large mass.
B The one with a small mass.
C All stars live the same length of time.
D Not known: there is no-one who could observe newly formed or dying stars.

13. The life of a star: a losing fight against gravity
Gravity pulls star together  contracts and is heated up  ignites
Radiation pressure balances gravity:
4 H  He + heat
* Small mass stars use hydrogen at slow pace
- live a lot longer
lifetime can be up to 100 billion years
* The Sun is average: 10 billion years (halfway now)
* Large mass stars use hydrogen at fast pace
- live a lot shorter
lifetime can be as short as 10 million years
Inside a
main-sequence
star
A losing fight 1

14. When all the hydrogen is gone
What happens after all the hydrogen is gone?
• Star collapses: inside heats up very hot
• Helium “ignites”: heavier elements build up
• Excess heat blows up the star large
- becomes a red giant (cold outside)
• On HRD - star moves off the main sequence
When all the hydrogen is gone
Why very hot inside? Needs extra high temperature to run against increased Coulomb barrier.
Why small core? Needs to be small to increase gravity in order to maintain extra high temperature.
Why bright? Needs extra power to hold up against increased gravity.
Why large? Extra radiation pressure blows out atmosphere.
Why cold atmosphere? Large size effectively cools outer layer.
In the process the star
blows off some of
its stuff - explosions

15. More processes 3 4He 12C + heat 12C + 4He  16O + heat ….
…  56Fe + heat
These processes last
shorter and shorter time
(100 million yrs for the Sun)
More processes
At the end of that, no more heat  contraction again  center hot
Either stops or blows up
Not too heavy stars - strong enough to withstand gravity: WHITE DWARF
Too heavy stars - not strong enough to withstand gravity: SUPERNOVA
(Sun will stop at C and O mixture)

16. Questions coming …

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Question 10
What keeps a normal (main sequence) star from collapsing under its own weight?
A The strength of the rock it is made of.
B The gravity of the star.
C Radiation pressure due to the heat leaving the star.
D The magnetic field of the star.
Next question coming …

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Question 11
In which of the two stages do stars spend more time
of their life, as main sequence stars or as red giants?
A About 100 times more time as red giants than as
main sequence stars.
B About 3 times more time as red giants than as
C About equal.
D About 3 times less time as red giants than as main sequence stars.
E About 100 times less time as red giants than as