Stars

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Stars
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What is a star?

• A ball of very hot hydrogen and helium gas
• So hot that nuclear fusion takes place in it
  (hydrogen atoms fusing to make helium)
• Produces and emits electromagnetic radiation

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How hot?

• Stars that are blue or white are hottest 6000C
  to 25000C
• Stars that are yellow or red are coolest 3000C
  to 6000C
• The bigger the star, the greater the surface
  temperature.

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A star is born

• When hydrogen gas and dust (nebulae) is pulled
  together by gravitational attraction
• As the mass of this protostar increases, its core
  temperature increases because it is squeezed by
  gravity.
• When the hydrogen is hot enough nuclear fusion
  starts.

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nebulae
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Size matters

• If a young star has a very large mass (more than
  20X bigger than the Sun) it becomes a Blue or
  White star.
• If a young star has a smaller mass (the same size
  as the Sun) it becomes a Red or Yellow star.
• The different kinds of stars have different life
  cycles.

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Small Stars

• The size of our Sun, or smaller.

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Yellow dwarf

• Our Sun is a yellow dwarf star.
• It has very strong forces of gravity pulling it
  together, which are balanced by strong forces
  trying to make it expand because of its high
  temperature.
• Yellow dwarfs are stable until all the hydrogen
  in the core is fused into helium.
• When the hydrogen runs out, the star becomes
  unstable. (10,000 million yrs)

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Yellow dwarf
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Yellow dwarf to red giant to white, red, black
dwarf)

• When a yellow dwarf star has run out of hydrogen,
  the star expands, cools, and turns into a red
  giant.
• After that, it contracts under its own gravity to
  become a white dwarf.
• White dwarf stars are very dense.
• White dwarfs cool to red dwarfs, then black
  dwarfs.

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Red giant
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Red giant
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White dwarf
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White dwarf
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Red dwarf
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black dwarf
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Large Stars

• 20 X bigger than our Sun...

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Blue Star

• Blue stars are only stable for 100 million
  years.
• When its hydrogen has been used up, it expands
  into a blue supergiant.
• It then cools down to be a red supergiant.
• It may then become unstable and explode, making a
  supernova.

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Blue star
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Blue supergiant
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Red supergiant
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supernova
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supernova
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Supernova to Neutron Star

• The centre of a smaller supernova collapses to
  make a very dense neutron star. In this
  explosion huge clouds of gas are sent out into
  space, called supernova remnants.
• Neutron stars spin rapidly and send out pulses of
  radio waves (pulsar).

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Neutron star
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Neutron Stars to Black Holes

• The core of a big neutron star can collapse even
  more.
• If this happens, it becomes extremely dense.
• So dense, that its gravitational force stops
  anything from escaping from its surface, even
  light.
• This is called a Black Hole.

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Black hole
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Life CYCLE

• Supernova remnants (clouds of dust, gas,
  materials thrown out from the explosion) expand
  outwards into space
• they bump into and merge with other gas and dust
  in space
• which eventually make new nebulae, which turn
  into new stars.
• The Solar System is made from recycled
  materials.

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Life Cycle
gravity
nebulae
black hole
supernova
nuclear fusion
small star
adult star
large star
black dwarf
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Life Cycle
neutron star
gravity
nebulae
black hole
supernova
protostar
nuclear fusion
small star
red supergiant
adult star
large star
red giant
white dwarf
blue supergiant
black dwarf