the formation and structure of stars
DESCRIPTION
0. Chapter 9. The Formation and Structure of Stars. The Interstellar Medium (ISM). Gas: ~75% H, 25% He, traces of “metals” 1% “dust” (silicates, carbon, heavy elements coated with ice, About the size of the particles in smoke) 150 m average distance between dust grains. - PowerPoint PPT PresentationTRANSCRIPT
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The Formation and Structure of Stars
Chapter 9
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The Interstellar Medium (ISM)
•Gas: ~75% H, 25% He, traces of “metals”
•1% “dust” (silicates, carbon, heavy elements coated with ice, About the size of the particles in smoke)
•150 m average distance between dust grains
•“Dense” => ~10 to 1000 atoms/cm3
•“Thin” ~ 0.1 atoms/cm3
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Structure of the ISM
• HI clouds:
• Hot intercloud medium:
The ISM occurs mainly in two types of clouds:
Cold (T ~ 100 K) clouds of neutral hydrogen (HI);
moderate density (n ~ 10 – a few hundred atoms/cm3);
size: ~ 100 pc
Hot (T ~ a few 1000 K), ionized hydrogen (HII);
low density (n ~ 0.1 atom/cm3);
gas can remain ionized because of very low density.
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3 types of nebula
1. Emission
2. Reflection
3. Dark
Q: Why do emission nebula look red and reflection nebula blue?
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We see absorption in elements where the background stars are too hot to form these lines
Narrow width (low temperature; low density)
Multiple components (several clouds of ISM with different radial velocities)
=> Comes from the ISM
Evidence for the ISM
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Interstellar reddening
Q: Why do astronomers rely heavily on IR observations?
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Q: How do we know the ISM exists?
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The Various Components of the Interstellar Medium
Infrared observations reveal the presence of cool, dusty gas.
X-ray observations reveal the presence of hot gas.
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Stellar formation from the ISM:
Must be triggered by high mass stars –
• Give off intense radiation
• Explode as SNs
Collapsing cloud can form 10 to 1000 stars
• Association
• Cluster
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SN_triggered_ssc2004-04v2.wmv
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The Contraction of a Protostar
Q: Why do you think there’s a lower limit on the mass of a main-seq. star?
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The Contraction of a Protostar
Sun: ~30 million years
15 M: 160,000 years
0.2 M: 1 billion years
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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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Protostellar Disks and Jets – Herbig-Haro Objects
Herbig-Haro Object HH34
Q: What are the bipolar flows evidence of?
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Globules
Bok globules:
~ 10 – 1000 solar masses;
Contracting to form protostars
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Evaporating gaseous globules (“EGGs”): Newly forming stars
exposed by the ionizing radiation from nearby massive stars
Observations of star formation:
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200 solar mass star
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N 11B
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Trifid
V838 Mon
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Tarantula
N 49
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The Source of Stellar EnergyStars produce energy by nuclear fusion of
hydrogen into helium.
In the sun, this happens primarily through the proton-proton (P-P) chain
Q: How does the sun fuse H to He?
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The CNO Cycle
Happens in stars > 1.1 M
More efficient that the P-P chain.
Requires high T (>16 million K)
Q: Why does the CNO require a higher temp. than the P-P chain?
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Fusion into Heavier Elements
Fusion into elements heavier than C, O:
requires high temperatures (>600 million K);
occurs only in very massive stars (more than 8 solar masses).
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Stellar structure
Conservation of mass:
Weight of each shell = total weight
Conservation of energy:
E(out) = E(from within)
Hydrostatic equilibrium:
Pressure balances gravity
Energy transport:
Describes flow of energy
24dM
rdr
24dL
r edr
2
dP GM
dr r
3 2
3
16
dT L
dr ac T r
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Hydrostatic EquilibriumImagine 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 (II)
Outward pressure force must exactly balance the weight of all layers above, everywhere in the star.
This is why we find stable stars on such a narrow strip (main sequence) in the Hertzsprung-Russell diagram.
Pressure-temperature thermostat
Q: How does the P-T thermostat control the reactions in stars?
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Energy TransportEnergy generated in the star’s center must be transported to the surface.
Inner layers of the sun:
Radiative energy transport
Outer layers of the sun (including photosphere):
Convection
Basically the same structure for all stars close to 1 solar mass.
Q: Why is convection in stars important?
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Stellar ModelsThe structure and evolution of a star is determined by the laws of
• Hydrostatic equilibrium
• Energy transport
• Conservation of mass
• Conservation of energy
A star’s mass (and chemical composition) completely determines its properties.
…why stars initially all line up along the main sequence, and why there’s a mass-luminosity relation….
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The Life of Main-Sequence StarsStars gradually exhaust their hydrogen fuel.
They gradually becoming brighter, evolving off the zero-age main sequence (ZAMS).
3.5 2.5
fuel 1
rate of consumption
M
M M
Lifetime of a main-sequence star (90% of total life is on main-seq.)
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The Lifetimes of Stars on the Main Sequence
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The Orion Nebula: An Active Star-Forming Region
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The Trapezium
The Orion Nebula
Infrared image: ~ 50 very young, cool, low-
mass starsX-ray image: ~ 1000 very young, hot stars
less than 2 million years old
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The Becklin-Neugebauer object (BN): Hot star, just reaching the main
sequence
Kleinmann-Low nebula (KL): Cluster
of cool, young protostars
detectable only in the infrared
Spectral types of the trapezium
stars
Protostars with protoplanetary disksProtostars with protoplanetary disks
B3
B1
B1
O6
IR + visual
IR
Gas blown away from protostars