WHERE

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WHERE STARS ARE BORN
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The Interstellar Medium (ISM)
The space between the stars looks empty . .
. But its not!
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Mainly hydrogen helium Avg density ? 1
10 atoms/cm3 99 of ISM
Tiny grains (smoke)
Silicates, carbon, ice (?) Avg density ? 1
grain/million m3
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Whirlpool Galaxy
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Looking at a galaxy from the inside.
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Looking for Gas
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Emission Nebulae
Ultraviolet photons from hot stars are
absorbed by gas atoms cause gas to glow.
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Great Orion Nebula
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North America Nebula
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Hydrogen atom
Electron ejected
Electron recaptured
Photon emitted
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Molecules mainly hydrogen (H2).
Carbon monoxide (CO) in Orion
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Looking for Dust
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Reflection Nebulae
Pleiades Star Cluster
Glows due to scattered (reflected) starlight.
Dust scatters blue light more efficiently than
red light.
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Dark Nebulae
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Emission nebula
Horsehead Nebula
Reflection nebula
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Star Formation
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Results from collapse of a molecular cloud.

• Cloud collisions
• Supernova blast wave
• Expanding emission nebula
• Galactic density wave

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p. 271
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Central temp ? 10 million K Hydrogen fusion
ignites
protostars
(on main sequence)
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Star birth in the Eagle Nebula
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Star birth in the Trifid Nebula
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Accretion disk may form around young stars . .
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Bipolar Flow
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p. 273
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Protostellar Disks
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Evolutionary tracks on the HR diagram
Time required for contraction to main
sequence depends on mass.
pgs. 274-5
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Limits to Star Formation
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Protostar mass lt 0.08 M? No hydrogen
ignition Brown Dwarf
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M gt 100 M? Star disrupted by the pressure of
photons.