Stellar Life Stages

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Stellar Life Stages

• Star Birth and Death

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Background info.

• Since early human history, people have looked up
  at the night sky in amazement.
• What is out there?
• What are stars?
• What are planets?

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Background info.

• Ancient civilizations imagined that groups of
  stars formed pictures called constellations.
• Many constellations are named after animals or
  mythological characters.

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Background info.
Orion, The Hunter
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Background info...
Astronomers recognize 88 constellations
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Birth of a Star

• stars are born in a nebula
• a nebula is a dark, cool cloud of dust and gas
• our galaxy has mostly hydrogen and a little
  helium gas

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Horsehead is a dark nebula in front of the pink
emission nebula.
Horsehead is about 1,600 light years away.
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• nebula becomes dense (thick and heavy) and starts
  to contract
• gravity squeezes dust particles to the center,
  gravitational energy changes into heat energy

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Contracting Nebula
draw this
HOT
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Protostar (developing)

• gravity squeezes for millions of years and
  temperature slowly rises
• when its hot enough it emits red light
• still too cool for nuclear fusion...

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Protostar
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• contraction continues and the core reaches up to
  10 million Kelvins
• pressure in the core is high enough for nuclear
  fusion to start and a star is born
• nuclear fusion when atomic nuclei join together
  to form a heavier nucleus, energy is released,
  aka nucleosynthesis

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Nuclear Fusion
draw this
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the sun
the atomic bomb
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• heat from hydrogen fusion causes gas pressure
  inside the star to increase

Gas molecules are always moving in random
directions. What happens to them when gravity
pushes down? What happens when the temp.
increases?
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• when pressure inside balances with pressure
  outside, it is called a main-sequence star

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Main Sequence Star
Gravity
Gas Pressure
draw this
Balance of two forces
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Main-Sequence Star

• hydrogen fusion continues for billions of years
  until it runs out of hydrogen
• stable stars like our sun will burn hydrogen for
  about 10 billion years
• our sun is about 5 billion years old

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Main Sequence Stars
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Red Giants

• hydrogen fusion in the core ends and the core
  contracts (gravity)
• hydrogen fusion still occurs in the outer layers
  and they expand
• surface temp. cools and turns red but the core
  stays hot and changes helium into carbon
  (nucleosynthesis!!!)

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Red Giant
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Burnout and Death

• all stars eventually run out of fuel and collapse
• low-mass stars and medium mass stars use up their
  hydrogen and collapse into a white dwarf

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...the fate of a star depends on its mass
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Massive Death

• massive stars explode in a supernova, debris is
  ejected as stardust...eventually gravity pushes
  it in
• dying stars that use up hydrogen produce helium
  (fusion), over time helium fuses to become
  heavier elements like lithium through iron
  (nucleosynthesis)

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• black holes are remains of massive stars that
  have exploded, they are hot, dense and have
  strong gravity
• when matter is pulled into it, the matter
  disappears forever

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Bend light around them Not even light can escape
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Stellar Evolution
(look at page 710)
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60 second summary
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Review 1 Where are stars born?

• nova
• nebula
• nuclear fusion
• hollywood

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• nebula

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Review 2 What happens to a nebula?

• gravity pushes out and it gets cold
• gravity only pushes out
• gravity pushes in and it gets hot
• gravity only pushes in

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• gravity pushes in and it gets hot

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Review 3 When a nebula is hot and enough
pressure is present, what happens next?

• nuclear combustion
• nuclear fission
• nuclear fusion
• nuclear expansion

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• nuclear fusion

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Review 4 What type of star is our sun?

• protostar
• main sequence star
• red giant
• white dwarf

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• main sequence star

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Review 5What is the next life stage for the sun?

• nebula
• nova
• red giant
• black hole

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• red giant

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Review 6 How do massive stars die?

• in a nova or supernova
• in a nebula
• in a circle
• in a pulsar

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• in a nova