STARS

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STARS

• A Life and Death Production

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Nebula

• A very large diffuse mass of interstellar dust
  and gas (mostly Hydrogen).
• This material starts to collapse in on itself
  due to gravitational forces.
• Eventually enough material gathers together to
  form a gas clump called a protostar.

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Examples of Star Forming Nebula
Cone Nebula
Eagle Nebula
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Protostar

• As a gas clump collapses, it heats up because the
  gas particles collide more often and it starts to
  spin (counterclockwise)
• The gas clump forms a disk with the protostar in
  the center. Extra material in the disk may form
  planets.

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Protostars in Infrared Light
Infrared Light
Visible Light
Dust clouds block visible light whereas infrared
light can pass through the dust clouds and be
seen as bright spots where new stars are forming.
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Nuclear Fusion

• When enough material collapses the temperature
  and pressure can build until nuclear fusion
  starts in the core.
• 4 H ? He
• The star settles down to spend about 90 of its
  life as a main sequence star.

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Main Sequence Stars
Depending on the size of the protostar a small or
medium sized star may be created such as our sun.
Small sized stars may last for 100 billion years.
Medium sized stars may last for 10 billion years.
Large stars (10 times the mass of our sun) use
their hydrogen fuel faster and may only last a
few million years. However they are 5000 times
brighter than our sun.
Extremely large stars (30 times the mass of our
sun) may only last for one million years and are
extremely bright (and are very rare).
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Old Age

• Eventually all the H2 is converted to He and the
  nuclear reactions stop.
• Gravity causes the core to shrink.
• As the layers collapse, fusion may start again
  using Helium as the fuel and the outside layers
  puff out to form a RED GIANT.

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Main Sequence Star

• When our Sun expands and becomes a red giant,
  Mercury, Venus and possibly Earth will be
  swallowed up by the Sun.
• If Earth is not swallowed up, the Sun will heat
  the Earth's surface so that the oceans will boil
  and the atmosphere will evaporate away.

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Final Farewell

• When the core fuel runs out again, the outer
  layers are ejected and a planetary nebula forms.

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Supernova

• The largest stars shed their layers in a massive
  explosion called a Supernova.
• The end result is also a planetary nebula.

 
Supernova are so bright that they can outshine
an entire galaxy for a period of time.
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Planetary Nebula from a Supernova
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Supernova 1987A

• In February 1987, a supernova exploded in a
  nearby galaxy. It was the first supernova to be
  clearly visible to the unaided eye in over 400
  years.

BEFORE
AFTER
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Supernova 1987a Remnants
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Death

• What remains after the outer layers of a star are
  blown off depends on the mass of the core .
• 1. Small and medium stars will shrink down to a
  white dwarf.
• 2. Large stars form a neutron star.
• 3.The largest of the large will collapse to a
  point called a black hole.

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White Dwarf

• The last stage of a small to medium sized star.

After losing its outer layers of gas as a
planetary nebula the rest of the star collapes to
about an Earth-sized object and will remain hot
enough so that it appears as white light.
These celestial objects are extremely dense (a
lot of matter in a very small volume).
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Neutron Star
The last stage of a large sized star (10 solar
masses).
After losing its outer layers of gas as a
planetary nebula the rest of the star collapes to
about 10 to 20 km in size and is even more dense
than a white dwarf.
One teaspoonful of a neutron star weighs about 2
billion tons.
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Black Hole
The last stage of a very large sized star (30
solar masses).
This collapsed core is so dense that not even
light can escape its gravitational pull!
It may only be a few kilometers in size.
http//imagine.gsfc.nasa.gov/docs/science/know_l2/
black_holes.html
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Star Colour
The colour of a star is dependent on its
temperature.
The order of colours from hottest to coolest
is Blue Bluish-white White Yellowish-white Yellow
(our sun) Orange Red
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Star Brightness
The brightness of a star as we see if from Earth
is dependent on two things
1. How big/hot the star is.
2. How far away the star is.
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Magnitude

• The brightness of stars are split into six
  categories of magnitudes.
• The brightest stars are called first-magnitude
  and the faintest stars visible with the unaided
  eye are called sixth magnitude.
• The rest of the stars are ranked in between.

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Apparent Magnitude

• This is the brightness of a star as it appears to
  someone looking at the night sky.
• This is not an indication of how bright the star
  actually is since the observed brightness is
  affected by how far away the star is.

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Brightness is affected by distance
If two stars of equal size and brightness were
observed, the closer of the two stars would
appear brighter to us and the further star
would appear dimmer.
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Absolute Magnitude

• This discrepancy in measuring the brightness of
  stars led to a new measurement called absolute
  magnitude.
• This is an actual measurement of the brightness
  of a star if it were located 3.26 light years
  from Earth (1 parsec).

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An example of Absolute Magnitude
Stars as they actually appear in the sky.
The same stars as they would appear if they were
all 3.26 light years from Earth.