the formation and structure of stars chapter 9. the big picture stars exist because of gravity....
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The Formation and Structure of Stars
Chapter 9
The Big Picture
• Stars exist because of gravity.
• Gravity causes interstellar material to collapse to form stars.
• Gravity determines how much energy stars generate.
• Gravity dictates how stars evolve and die.
• Mass determines gravity;Mass is the #1 property of a star.
The space between the stars is not empty, but filled with very dilute gas and
dust, producing some of the most beautiful objects in the sky.
We are interested in the ISM because:
a) dense interstellar clouds are the birth place of stars
b) dark clouds alter and absorb the light from stars behind them
The Interstellar Medium (ISM)
Various Appearances of the ISM
Three Kinds of Nebulae
• Emission Nebula• Reflection Nebula• Dark Nebula
The Fox Fur Nebula
NGC 2246
1) Emission Nebulae (HII Regions)
A very hot star illuminates a cloud of hydrogen gas;
Its ultraviolet light ionizes hydrogen;
Electrons recombine with protons, cascade down to the ground state, and produce emission lines, dominated byred Hα photons.
The Keyhole Nebula
2) Reflection Nebulae
Star illuminates a nearby cloud of gas and dust;
Blue light is much more likely to be scattered by dust than red light;
Reflection nebula appears blue.
*The same physics for the blue sky and the red sunset!
Emission and Reflection Nebulae
3) Dark Nebulae
Barnard 86
Dense clouds of gas and dust absorb the light from the stars behind;
Appear dark in front of the brighter background, which is often an emission nebula.
Horsehead Nebula
Interstellar Reddening
Visible Infrared
Barnard 68
Blue light is strongly scattered and absorbed by interstellar (dust) clouds.
Red light can more easily penetrate the cloud, but is still absorbed to an extent (“interstellar extinction”).
(Infrared radiation is hardly absorbed.)
Interstellar clouds make background stars appear
redder.
The physics of reflection nebula revisited!
Interstellar Absorption LinesThe interstellar medium produces
absorption lines in the spectra of stars.
Distinguished from stellar absorption lines via:
a) Absorption from wrong ionization states
Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in
the background; multiple lines of same transitionb) Narrow (sharp) lines (temperature & density too low)
c) Multiple components (several clouds of ISM
with different radial velocities)
Four Components of the ISM
ComponentTemperature
(K)Density
(atoms/cm3)Gas
Percent of total mass
Molecular clouds 20 – 50 103 – 105
Molecules(H2 and others) ~ 25%
HI clouds 50 – 150 1 – 1000 Neutral hydrogen Other atoms ionized
~ 25%
Intercloud medium 103 – 104 0.01 Partially ionized ~ 50%
Coronal gas 105 – 106 10–4 – 10–3
Highly ionized,from hot stars and supernovae
~ 5%
Note: Emission nebulae (HII regions) occur only near very hot stars, so they comprise very small fraction of the ISM.
EXTRA
Various Views of the Interstellar Medium
Infrared observations reveal the presence of cool, dusty gas.
X-ray observations reveal the presence of hot gas.
Shocks triggering star formation
Henize 206 (infrared)
The Contraction of a Protostar
From Protostars to Stars
Ignition of41H → 4He
fusion processes
Star emerges from the enshrouding dust cocoon(T Tauri stage)
Evidence of Star Formation
Nebula around S Monocerotis:
Contains many massive and very young stars,
including T Tauri stars: strongly variable and bright in the infrared.
Protostellar Disks and Jets– Herbig-Haro Objects
Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows or bipolar outflows) seen as Herbig-Haro objects
Example:
Herbig-Haro Object HH34
Globules
Bok globules:
~ 10 – 1000 solar masses
Contracting to form protostars
EGGsEvaporating gaseous globules (“EGGs”): Newly forming stars
exposed by the ionizing radiation from nearby massive stars
Energy Generation (§7.2)Energy generation in the sun (and all other stars):
nuclear fusion= fusing together 2 or more lighter nuclei to produce heavier ones.
Nuclear fusion can generate energy up to the production of iron.
For elements heavier than iron, energy is produced by nuclear fission.
Binding energy due to the strong force
Energy generation in the Sun:The Proton-Proton Chain (§7.2)
Basic reaction:
41H → 4He + energy
4 protons have 0.048x10-27 kg (= 0.7 %) more mass than 4He.
⇒ Energy gain = mc2
= 0.43x10-11 joules per reaction
Need large proton speed (high temperature) to overcome
Coulomb barrier (electrical repulsion between protons).
Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second!.
T ≥ 107 K = 10 million K
The Solar Neutrino Problem• The solar interior cannot be observed directly because it is highly opaque to radiation.• But neutrinos can penetrate huge amounts of material without being absorbed.
Davis solar neutrino experiment
• Early solar neutrino experiments detected a much lower flux of neutrinos than expected (→ the “solar neutrino problem”).
• Recent results have proven that neutrinos change (“oscillate”) between different types (“flavors”), thus solving the solar neutrino problem.
The Source of Stellar EnergyRecall from our discussion of the Sun:
Stars produce energy by nuclear fusion of hydrogen into helium.
In the Sun, this happens primarily
through the proton-proton
(PP) chain
Basic reaction:
41H → 4He + energy
26
Fusion into Heavier Elements than C & O:
Occurs only in very massive stars (more than 8 solar masses)
—why?.
Requires very high temperatures (why?).
Stellar Models—the theory of stars
Divide a star’s interior into concentric shells — “Onion skin layer model”
Within each shell and between neighboring shells, require that the laws of physics are obeyed:
• Conservation of Mass
• Conservation of Energy
• Hydrostatic Equilibrium
• Energy Transport
Four laws of stellar structure:
Hydrostatic EquilibriumIn each layer:
This condition uniquely determines the interior structure of the star.
Stable stars on a narrow strip (main sequence) in the H-R
diagram.
Outward force of thermal
pressure
Inward force of gravity
(weight of all layers above)
=
Energy TransportEnergy generated in the star’s center must be transported to the surface.
Inner layers of the Sun:
Radiation
Outer layers of the Sun:
ConvectionEnergy carried by photons Energy carried by convective
motion of large masses
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
Star’s total mass determines which part of
the star has convection or radiation (cf. Ch. 10)
Sun
Calculating the stellar structure:Take the four equations representing the four laws of stellar structure and solve them simultaneously!
Hydrostatic equilibrium
Energy transport
Conservation of mass
Conservation of energy
Star’s mass (and chemical composition) completely
determines the properties of the star.
Interactions of Stars and their EnvironmentSupernova explosions of
the most massive stars inflate and blow away remaining gas of star
forming regions.
Young, massive stars excite the remaining gas of their star forming regions, forming HII regions.
The Life of Main-Sequence Stars
As stars gradually exhaust their hydrogen fuel,
they become brighter, and evolve off the zero-age main sequence.
The Lifetimes of Stars on the Main Sequence
TheOrion Nebula:
a region of active star formation
The Trapezium
The Orion Nebula
The 4 trapezium stars: Brightest, young stars
(< 2 million years old) in the central region of the
Orion nebula
X-ray image: ~ 1000 very young, hot starsInfrared image: ~ 50 very young, cool, low-
mass stars
Only one of the trapezium stars is hot
enough to ionize hydrogen in the Orion
nebula
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 disks
B3
B1
B1
O6
Stellar Structure— Cause and Effect
→
Mass (M)
→Gravity (weight)
→Pressure
→Density
Radius
Temperature→Fusion Rates
→→Luminosity
Available Fuel
Time of StabilityMain Sequence Lifetime (“Life Expectancy”)
Hydrostatic equilibrium
Pressure-Temperature Thermostat
~ M
L ~ M 3.5
Lifetime = M/L ~ M –2.5
Mass-Luminosity Relation
“Red” in Astronomy
• red emission nebulae• red supergiants/giants/dwarfs• red shift (in the Doppler effect)• Interstellar reddening
• blue reflection nebulae• red sunset• blue sky
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