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Emission Nebulae * Ultraviolet. photons from hot. stars are absorbed ... Rosette Nebula. Great Orion Nebula. North America Nebula. Trifid Nebula. Hot. Stars. Earth. H ...

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WHERE

STARS

ARE

BORN


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The Interstellar Medium (ISM)

The space between the stars looks empty . . .

But it’s not!


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Gas

Dust

~ 0.0001 cm

* 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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Rosette Nebula

Emission Nebulae

* Ultraviolet

photons from hot

stars are absorbed

by gas atoms –

cause gas to glow.



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Trifid Nebula

North America Nebula


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Hot

Stars

H atoms

UV

Earth


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UV

Hydrogen atom

Electron ejected

Electron recaptured

Photon emitted


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Red Balmer

emission Line


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Molecules – mainly hydrogen (H2).

Carbon monoxide (CO) in Orion



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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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Molecules

abundant

here

Dark Nebulae


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Emission nebula

‘Horsehead’ Nebula

Reflection

nebula



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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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Molecular

Cloud

Shrink & heat

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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Forming star?

Star birth in the

Trifid Nebula


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* Accretion disk may form around young stars . . .

Bipolar Flow


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Bipolar

Flow

Disk

p. 273



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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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Protostar mass < 0.08 M: No hydrogen ignition:

‘Brown Dwarf’


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The ‘Pistol’ Star

M > 100 M

Star disrupted

by the pressure

of photons.


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