Download The Formation and Structure of Stars

Foundations of Astronomy | 13e
Seeds
Chapter 11
The Formation and Structure of
Stars
© Cengage Learning 2016
Guidepost
• In this chapter, you will consider how the
interstellar medium condenses into stars
and what the conditions inside stars must
be like
– How do stars form?
– What is the evidence that stars are forming
now?
– How do stars maintain their stability?
– How do stars make energy?
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11-1 Making Stars from the Interstellar
Medium
• Stars are being born, live a finite lifetime,
and die. Star death can ignite star birth.
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The Formation of Stars
• Stars are formed during the
collapse of the cores of
giant molecular clouds
– Clouds must contract
and heat up to ignite
thermonuclear
processes--1 million K!
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Contraction of Giant Molecular Cloud
Cores
• An external trigger is required to initiate
the collapse of clouds
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Shocks Triggering Star Formation
• Shock wave moves towards interstellar
gas cloud
– Passes through and compresses gas cloud
• Motion of particles in the cloud continue
post-shock wave
• Densest part of the cloud becomes
gravitationally unstable
• Stars are born within the contracting
regions of the gas
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Sources of Shock Waves
• Massive stars die young → supernovae
tend to happen near sites of recent star
formation
• Ionization fronts of hot, massive O or B
stars producing UV radiation
• Collisions of giant molecular clouds
• Spiral arms in galaxies like our Milky Way
are probably rotating shock-wave patterns
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Shocks Triggering Star Formation (cont’d.)
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Bow Shock
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Sources of Shock Waves
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Black Widow Pulsar
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Protostars
• Pre-birth state of stars
– H → He fusion not yet ignited
•Protostars:a forming star compressed
enough to be opaque at all wavelengths,
but not hot enough to generate fusion
–“cocoon nebulae” hide the forming star
•Fusion is the birth of a star!
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Heating by Contraction
• As a protostar contracts, it heats up
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From Protostars to Stars
• Higher-mass stars
evolve more
rapidly than less
massive stars
• Birthline: where
stars are
detectable at
visible
wavelengths
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From Protostars to Stars (cont’d.)
• The birth line: star emerges from the
enshrouding dust cocoon
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11-2 The Orion Nebula: Evidence of Star
Formation
• The visible nebula is only a small part of a
vast, dusty molecular cloud
• single O type star produces UV, causes glow
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The Orion Nebula (cont’d.)
• Infrared observations reveal clear
evidence of active star formation deeper in
the molecular cloud behind the visible
nebula
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The Orion Nebula (cont’d.)
• Many of the young stars in the Orion
Nebula are surrounded by disks of gas
and dust
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The Trapezium in the Orion Nebula
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Open Clusters of Stars
• Large masses of giant molecular clouds
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Young Star Clusters
• Ultraviolet radiation and strong stellar
winds from young, hot, massive stars in
open star clusters compress the
surrounding gas
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11-3 Young Stellar Objects and
Protostellar Disks
• Conservation of
angular momentum
leads to the formation
of protostellar disks
→ birth place of
planets and moons
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Protostellar Disks and Jets – Herbig-Haro
Objects
• Accretion disks that often lead to the
formation of jets (directed outflows, bipolar
outflows)
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Protostellar Disks and Jets – Herbig-Haro
Objects (cont’d.)
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Herbig-Haro Object HH30
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11-4 Stellar Structures
• Basically the same structure
for all stars with approx. 1
solar mass or less
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Hydrostatic Equilibrium
• Imagine a star’s interior composed of
individual shells
– Within each shell, two forces must be in
equilibrium with each other
– Outward pressure force must exactly balance
the weight of all layers above everywhere in
the star
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Hydrostatic Equilibrium (cont’d.)
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Energy Transport
• Energy generated in the star’s center must
be transported to the surface is one of
three ways
• However, in stars only two energy
transport mechanisms play a role
– Inner layers: radiative energy transport
– Outer layers (incl. photosphere): convection
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Energy Transport (cont’d.)
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Other examples
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11-5 The Source of Stellar Energy
• Stars produce energy by nuclear fusion of
hydrogen into helium
– In the sun, this happens primarily through the
proton-proton (PP) chain
• In stars slightly more massive than the
sun, a more powerful energy generation
mechanism than the PP chain takes over
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The CNO Cycle
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Energy Transport Structure
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Discussion Questions
• What are three different ways a giant
molecular cloud can be triggered to
contract?
• How does the energy transport differ from
a high-mass star to that of a low-mass star
like the Sun?
– Hint: see Figure 11-14
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Discussion Questions (cont’d.)
• If we could see in infrared light, what
would a clear night sky look like? Are we
missing out by being able to see only in
visible light?
– Hint : Think about views in and near the Milky
Way versus far away from the Milky Way
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