slide 1 sh2 136: a spooky nebula ghoulish dust clouds: a region of star formation halloween...
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Slide 1
SH2 136: A Spooky Nebula Ghoulish dust clouds: a region of star formation
Halloween corresponds roughly to the cross-quarter day: half-way between equinox and solstice
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Your next test will be November 14
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The Formation of StarsChapter 11
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Giant Molecular Clouds: stellar nurseries
VisibleInfrared
Barnard 68
Star formation collapse of the cores of giant molecular clouds: Dark, cold, dense clouds obscuring the light of stars behind them.
(More transparent in infrared light.)
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Giant molecular clouds – stellar nurseries
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Orion Deep FieldH-alpha line 656 nm (red light)
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Great Orion Nebula
1500 light-years away
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The Horsehead Nebula
Slide 9Star Forming Region RCW 38
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Parameters of Giant Molecular Clouds
Size: r ~ 50 pcMass: > 100,000 Msun
Dense cores:
Temp.: a few 0K
R ~ 0.1 pcM ~ 1 Msun
Clouds need to contract and heat up in order to form stars.
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Coldest spots in the galaxy:T ~ 1-10 K
Composition:• Mainly molecular hydrogen• 1% dust
EGGs = Evaporating Gaseous Globulesftp://ftp.hq.nasa.gov/pub/pao/pressrel/1995/95-190.txt
Slide 14
Jeans instability:
Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments.
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Contraction of Giant Molecular Cloud Cores
• Thermal Energy (pressure)
• Magnetic Fields
• Rotation (angular momentum)
External trigger required to initiate the collapse of clouds
to form stars.
Horse Head Nebula
• Turbulence
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Shocks Triggering Star Formation
Globules = sites where stars are being born right now!
Trifid Nebula
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Sources of Shock Waves Triggering Star Formation (1)
Previous star formation can trigger further star formation through:
a) Shocks from supernovae
(explosions of massive stars):
Massive stars die young =>
Supernovae tend to happen near sites of recent star formation
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Sources of Shock Waves Triggering Star Formation (2)
Previous star formation can trigger further star formation through: b) Ionization
fronts of hot, massive O or B
stars which produce a lot of
UV radiation:
Massive stars die young => O and B stars only exist
near sites of recent star formation
Slide 19The Bubble Nebula (Cassiopeia)
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Sources of Shock Waves Triggering Star Formation (3)
Giant molecular clouds are very large and may occasionally
collide with each other
c) Collisions of giant
molecular clouds.
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Sources of Shock Waves Triggering Star Formation (4)
d) Spiral arms in galaxies like our Milky Way:
Spirals’ arms are probably
rotating shock wave patterns.
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Jeans instability:
Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments.
Slide 24
Protostars
Protostars = pre-birth state of stars:
Hydrogen to Helium fusion
not yet ignited
Still enshrouded in opaque “cocoons” of dust => barely visible in the optical, but bright in the infrared.
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Heating By Contraction
As a protostar contracts, it heats up:
Free-fall contraction→ Heating
Heating does not stop contraction because the core cools down due to radiation
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Protostars: warm clumps of gas surrounded by infalling matter
Disks: planet formation?!
Why disks?
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Role of angular momentum
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Protostellar Disks
Conservation of angular momentum leads to the formation of protostellar disks birth place of planets and moons
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2211 RMVRMV
12
1
1
2 R
R
V
V
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Protostellar Disks and Jets – Herbig Haro ObjectsDisks of matter accreted onto the protostar (“accretion
disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects
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Protostellar Disks and Jets – Herbig Haro Objects (2)
Herbig Haro Object HH34
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Protostellar Disks and Jets – Herbig Haro Objects (3)
Herbig Haro Object HH30
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From a protostar to a young star: very hot; still accreting matter
Observed in the infrared, becauseinfalling gas and dust obscure light
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• The matter stops falling on the star• Nuclear fusion starts in the core• Planets can be formed from the remaining disk
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From Protostars to Stars
Ignition of H He fusion processes
Star emerges from the enshrouding dust cocoon
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Evidence of Star Formation
Nebula around S Monocerotis:
Contains many massive, very young stars,
including T Tauri Stars: strongly variable; bright
in the infrared.
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Evidence of Star Formation (2)
The Cone Nebula
Optical Infrared
Young, very massive star
Smaller, sunlike stars,
probably formed under
the influence
of the massive
star
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Evidence of Star Formation (3)
Star Forming Region RCW 38
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Globules
~ 10 to 1000 solar masses;
Contracting to form protostars
Bok Globules:
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Globules (2)Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars
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Open Clusters of Stars
Large masses of Giant Molecular Clouds => Stars do not form isolated, but in large groups, called Open Clusters of Stars.
Open Cluster M7
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Open Clusters of Stars (2)
Large, dense cluster of (yellow and red) stars in the foreground; ~ 50 million years old
Scattered individual (bright, white) stars in the background; only ~ 4 million years old
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The Source of Stellar Energy
In the sun, this happens primarily through the proton-proton (PP) chain
Recall from our discussion of the sun:
Stars produce energy by nuclear fusion of hydrogen into helium.
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The CNO Cycle
In stars slightly more massive than the sun, a more powerful
energy generation mechanism than
the PP chain takes over:
The CNO Cycle.
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Net result is the same: four hydrogen nuclei fuse to form one helium nucleus
Why p-p and CNO cycles? Why so complicated?
Because simultaneous collision of 4 protons is too improbable
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Hydrostatic Equilibrium
Imagine a star’s interior composed of individual
shells.
Within each shell, two forces have to be in equilibrium with
each other:
Outward pressure from the interior
Gravity, i.e. the weight from all layers above
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Hydrostatic Equilibrium (2)
Outward pressure force must exactly balance the
weight of all layers above everywhere in
the star.
This condition uniquely determines the interior structure of the star.
This is why we find stable stars on such a narrow strip
(Main Sequence) in the Hertzsprung-Russell diagram.
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H-R Diagram (showing Main Sequence)
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Energy TransportEnergy generated in the star’s center must be transported to the surface.
Inner layers:
Radiative energy transport
Outer layers (including photosphere):
Convection
Bubbles of hot gas rising up
Cool gas sinking downGas particles
of solar interior-rays
Slide 52
Conduction, Convection, and Radiation
(SLIDESHOW MODE ONLY)
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Stellar Structure
Temperature, density and pressure decreasing
Energy generation via nuclear fusion
Energy transport via radiation
Energy transport via convection
Flo
w o
f en
erg
y
Basically the same structure for all stars with approx. 1 solar
mass or less.
Sun
Slide 54
Energy Transport Structure
Inner radiative, outer convective
zone
Inner convective, outer radiative
zone
CNO cycle dominant PP chain dominant
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Summary: Stellar Structure
MassSun
Radiative Core, convective envelope;
Energy generation through PP Cycle
Convective Core, radiative envelope;
Energy generation through CNO Cycle
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