The Lives of Stars

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The Lives of Stars
EXAM 3 WILL NOT BE APRIL 5th it has been
re-scheduled for THURSDAY APRIL 12th 4/12
(Wednesday) Tailgate Party (aka exam review) For
Exam 3 Wed April 11th Observing Logs Due IN
CLASS May 1st
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Topics that will be on the test!! Formation of
the Solar System Sun Size Greenhouse Effect
Apparent and Absolute Magnitude HR
Diagram Stellar Formation and Lifetime Binary
Stars Stellar Evolution
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Recall from the HR- Diagram
Red Giants are Main Sequence stars are White
Dwarf stars are
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Stars condense from clouds of gas and dust (the
interstellar medium) that exist throughout the
disk of the galaxy

• Interstellar medium
• Gas Hydrogen
• Dust Carbon and Silicon

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Becoming a Star Step 1 Cloud collapses

• Why do these clouds of gas and dust collapse?
• One idea is that a shockwave from the explosion
  at the death of a star known as a supernova cause
  the gas and dust cloud to become unstable and
  start to collapse

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Becoming a Star Step 1 Cloud collapses

• As the cloud collapses, the center becomes very
  very hot and very very dense -

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Becoming a Star Step 2 Fusion

• As the gas cloud collapses due to gravitational
  forces, the core becomes hotter and the density
  inside the core increases
• Eventually, the temperature and density reach a
  point where nuclear fusion can occur

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Fusion is the combining together of light atoms,
into heavier atoms
For all Main Sequence stars, the temperature and
density in their cores are so great that Hydrogen
atoms combine to make Helium atoms and release
energy a process known as thermonuclear
fusion 4H ? He energy
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Becoming a Star Step 3 BalanceAll Main
Sequence stars are in hydrostatic equilibrium

• Fusion produces radiation (light) that creates an
  outward pressure
• During hydrostatic equilibrium there is a balance
  between the gravitational collapse of the star
  pushing inward and the outward pressure produced
  by photons from nuclear fusion in the core.

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Its a matter of balance.

• This balance is called hydrostatic equilibrium
• gravity ( ) wants to collapse the star,
  but pressure ( ) pushes outward against the
  collapsing material

Fusion 4H ? He energy(light)
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• All Main Sequence stars are in hydrostatic
  equilibrium because nuclear fusion of hydrogen is
  producing enough outward pressure to balance
  gravitational collapse.

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It takes a few million years to get there but -
stars spend most of their life time as a Main
Sequence star
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Stars spend most of their life time as a Main
Sequence star

• 90 of the whole life of all stars is spent on
  the Main Sequence
• 90 of all stars are found on the Main Sequence

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Stars often form in pairs called Binary Stars

• These stars can orbit each other much like a star
  and a planet, and in some cases the stars pass in
  front of each other - we call these Eclipsing
  Binary stars

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Tutorial Binary Stars LT (handout)

• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
  another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer,
  ask another group.

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In which case will you receive the most light on
Earth?
C
A
B
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Which of these is physically wrong if they are
all main sequence stars?
C
A
B
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In which case will you receive the most light on
Earth?

1. When a A spectral class main-sequence star is in
   front of a red giant.
2. When an M spectral class main-sequence star is in
   front of a O spectral class main sequence star.
3. When a M spectral class main-sequence star is in
   front of a G spectral class star.
4. When a K spectral class main-sequence star is in
   front of a red giant.

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STELLAR LIFETIMES

• Which will have a greater core temperature and
  density a high mass star or a low mass star?
• Which will then have a greater fusion rate?
• Which will use up its fuel more quickly?
• What is the fuel?

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STELLAR LIFETIMES

• Consider a main sequence star with 10 times the
  mass of the Sun
• It will
• have higher temps and pressures at the core
• have greater fusion rates - consumes fuel at 1000
  times the rate of the sun
• be 1000 times as bright and last 1/100 as long
• Burn bright, die young.

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LIFETIMES

• Bright O-type stars live very short lives (about
  10 million years)
• Very small stars live a long time (100 billions
  of years)
• Our SUN will live a total of about 10 billion
  years (half used up)

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The more massive a star, the faster it goes
through its main sequence phase
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Tutorial Star Formation and Lifetimes (p.81)

• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
  another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer,
  ask another group.

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How does the Sun produce the energy that heats
our planet?

1. The gases inside the Sun are burning and
   producing large amounts of energy.
2. Hydrogen is combined into helium, giving off
   large amounts of energy.
3. Gas inside the Sun heats up when compressed,
   giving off large amounts of energy.
4. Heat trapped by magnetic fields in the Sun is
   released as energy.
5. The core of the sun has radioactive atoms that
   give off energy as they decay.

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Consider the information given below about the
lifetime of three main sequence stars A, B, and
C. - Star A will be a main sequence star for
45,000 million years. - Star B will be a main
sequence star for 70 million years. - Star C
will be a main sequence star for 2 million
years.Which of the following is a true
statement about these stars?
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Star A has a mass of 5 solar masses and Star B
has a mass of 10 solar masses. How will the
fusion rate of Star A compare to the fusion rate
of Star B?

1. Star As fusion rate will be more than two times
   slower than that of Star B.
2. Star As fusion rate will be two times slower
   than that of Star B.
3. Star As fusion rate will be the same as that of
   Star B.
4. Star As fusion rate will be two times faster
   than that of Star B.
5. Star As fusion rate will be more than two times
   faster than that of Star B.

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Stars spend most of their life cycles on the Main
Sequence

• Main Sequence stars are in hydrostatic
  equilibrium because nuclear fusion is turning
  hydrogen into helium and producing enough outward
  pressure to balance gravitational collapse.
• 90 of all stars are found on the Main Sequence
• 90 of the whole life of all stars is spent on
  the Main Sequence
• BUT What happens when the hydrogen runs out?