The Origin of Modern Astronomy

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Chapter 9
The Formation and Structure of Stars
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The space between the stars is not completely
empty, but filled with very dilute gas and dust,
producing some of the most beautiful objects in
the sky.
The Interstellar Medium (ISM)
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We are interested in the interstellar medium
because
a) Dense interstellar clouds are the birth place
of stars.
b) Dark clouds alter and absorb the light from
stars behind them.
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Compression of the ISM by Winds from Hot Stars
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The Contraction of a Protostar
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From Protostars to Stars
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Star emerges from the enshrouding dust cocoon
Ignition of H ? He fusion processes
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Factors in Nuclear Fusion

• Hydrogen atoms are ionized (bare nuclei)
• Nuclei repel each other (Coulomb barrier)
• High enough temperature means a small percentage
  will have a high enough energy to get close
  enough for strong interaction to occur (Maxwell
  distribution of velocities)
• Sufficiently high pressure ensures that enough
  reactions occur to supply energy needs of star

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Evidence of Star Formation
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Nebula around S Monocerotis
Contains many massive, very young stars,
including T Tauri Stars strongly variable
bright in the infrared.
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T Tauri Stars
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Very young stars, still in the forming
stage Typically 100,000 10 million years old
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Protostellar Disks and Jets Herbig Haro Objects
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Disks 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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Globules
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Bok Globules
10 1000 solar masses
Contracting to form protostars

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Globules
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Evaporating Gaseous Globules (EGGs) Newly
forming stars exposed by the ionizing radiation
from nearby massive stars.
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Winds from Hot Stars
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Very young, hot stars produce massive stellar
winds, blowing parts of it away into interstellar
space.
Eta Carinae

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The Orion Nebula An Active Star-Forming Region
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The Trapezium
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Only one of the trapezium stars is hot enough to
ionize hydrogen in the Orion nebula.
Infrared image 50 very young, cool, low-mass
stars
X-ray image 1000 very young, hot stars
The Orion Nebula
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Kleinmann-Low nebula (KL) Cluster of cool, young
protostars detectable only in the infrared
The Becklin-Neugebauer Object (BN) Hot star,
just reaching the main sequence
Spectral types of the trapezium stars
B3
B1
B1
O6
Visual image of the Orion Nebula
Protostars with protoplanetary disks
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The Source of Stellar Energy
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Recall 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.
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Energy Source

• 1H 1H ? 2H e ?
• 2H moving fast
• e annihilates an electron producing Gamma rays
• Neutrino escapes from sun
• 2H 1H ? 3He ?
• 3He 3He ? 4He 1H 1H

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The CNO Cycle
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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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Fusion into Heavier Elements
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Fusion into heavier elements than C, O
requires very high temperatures occurs only in
very massive stars (more than 8 solar masses)
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Hydrostatic Equilibrium
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Imagine a stars interior composed of individual
shells.
Within each shell, two forces have to be in
equilibrium with each other
Gravity, i.e. the weight from all layers above
Outward pressure from the interior
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Hydrostatic Equilibrium
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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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Regulation of Energy Production

• If the energy production were to be insufficient
  then temp of core would decrease.
• Pressure would decrease which would cause star to
  contract causing temp to increase again because
  of energy release from gravity.
• If energy production were to be too much then the
  steps would occur in reverse.

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Energy Transport
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Energy generated in the stars center must be
transported to the surface.
Inner layers of the sun Radiative energy
transport
Outer layers of the sun (including
photosphere) Convection
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Stellar Structure
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Energy transport via convection
Sun
Energy transport via radiation
Flow of energy
Energy generation via nuclear fusion
Basically the same structure for all stars with
approx. 1 solar mass or less.
Temperature, density and pressure decreasing
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Stellar Models
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The structure and evolution of a star is
determined by the laws of

• Hydrostatic equilibrium

• Energy transport

• Conservation of mass

• Conservation of energy

A stars mass (and chemical composition)
completely determines its properties.
Thats why stars initially all line up along the
main sequence.
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Interactions of Stars and their Environment
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Supernova 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.
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The Life of Main Sequence Stars
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Stars gradually exhaust their hydrogen fuel.
In this process of aging, they are gradually
becoming brighter, evolving off the zero-age main
sequence.
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The Lifetimes of Stars on the Main Sequence
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