The Formation of StarsChapter 11
The last chapter introduced you to the gas and dust between the stars. Here you will begin putting together observations and theories to understand how nature makes stars. That will answer four essential questions:
• How are stars born?
• How do stars make energy?
• How do stars maintain their stability?
• What evidence do astronomers have that theories of star formation are correct?
Guidepost
Astronomers have developed a number of theories that explain the birth of stars. Are they true? That raises one of the most important questions you will meet concerning science:
• How certain can a theory be?
As you learn how nature makes new stars you will see science in action as evidence and theory combine to produce real understanding.
Guidepost
I. Making Stars from the Interstellar MediumA. Star Birth in Giant Molecular CloudsB. Heating By ContractionC. Protostars (原恆星 )D. Evidence of Star Formation
II. The Source of Stellar EnergyA. A Review of the Proton-Proton ChainB. The CNO Cycle
III. Stellar StructureA. Energy TransportB. What Supports the Sun?C. Inside StarsD. The Pressure-Temperature Thermostat
Outline
IV. The Orion NebulaA. Evidence of Young Stars
Outline (continued)
The Life Cycle of StarsDense, dark clouds, possibly forming stars in the future
Young stars, still in their birth
nebulae
Aging supergiant
Credit: You-Hua Chu (UIUC)
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分子雲- 孕育恆星的搖籃
Giant Molecular Clouds
• Size ~ 50-200 pc• M~104-106M⊙
• H2 ~ 300 #/cm3
– Most of the molecular gas are in modest extinction regions
• T ~ 8-50K( peak at mm, submm, infrared)
Molecular Cores
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)
Parameters of Molecular Cores
Temp.: a few K
R ~ 0.1 pc
M ~ 1 Msun
Much too cold and too low density to ignite thermonuclear processes
Clouds need to contract and heat up in order to form stars.
Contraction of Molecular Cores
• Thermal Energy (pressure)
• Magnetic Fields
• Rotation (angular momentum)
External trigger can help to initiate the collapse of clouds
to form stars. Horse Head Nebula
• Turbulence
Factors resisting the collapse of a gas cloud:
Shocks Triggering Star Formation
Globules = sites where stars will form or are being born right now!
Trifid Nebula
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
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
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
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.
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
Heating By ContractionAs a protostar contracts, it heats up:
Free-fall contraction→ Heating
From Protostars to StarsHigher-mass stars evolve more rapidly from protostars to stars
Protostellar Disks
Conservation of angular momentum leads to the formation of protostellar disks birth place of planets and moons
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; bipolar outflows): Herbig-Haro Objects
Protostellar Disks and Jets – Herbig-Haro Objects (2)
Herbig-Haro Object HH34
Protostellar Disks and Jets – Herbig-Haro Objects (3)
Herbig-Haro Object HH30
From Protostars to Stars
The Birth Line:
Star emerges from the enshrouding dust cocoon
Evidence of Star Formation
Nebula around S Monocerotis:
Contains many massive, very young stars,
including T Tauri Stars: strongly variable; bright
in the infrared.
Evidence of Star Formation (2)
The Cone Nebula
Optical Infrared
Young, very massive star
Smaller, sun-like stars,
probably formed under
the influence
of the massive
star.
Evidence of Star Formation (3)
Star Forming Region RCW 38
Globules
~ 10 to 1000 solar
masses;
Contracting to form
protostars
Bok Globules:
Globules (2)Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars
Open Clusters of StarsLarge masses of Giant Molecular Clouds => Stars do not form isolated, but in large groups, called Open Clusters of Stars.
Open Cluster M7
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.
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.
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
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
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
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.
H-R Diagram (showing Main Sequence)
Energy Transport Structure
Inner radiative, outer convective
zone
Inner convective, outer radiative
zone
CNO cycle dominant PP chain dominant
Summary: Stellar Structure
MassSun
Radiative Core, convective envelope;
Energy generation through PP Cycle
Convective Core, radiative envelope;
Energy generation through CNO Cycle
The Orion Nebula: An Active Star-Forming Region
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
Visual image of the Orion Nebula
Protostars with protoplanetary disks
B3
B1B1
O6
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Star formation standard model (star formation paradigm)
Shu et al. 1987Shu et al. 1987
Standard Evolutionary Scenario
Cloud collapse Protostar with disk
infall
outflow
Formation planets Solar system
Factor 1000 smaller
t=0 t=105 yr
t=106-107 yr t>108 yr
Single isolated low-mass star
n~104-105 cm-3
T~10 K
n~105-108 cm-3
T~10-300 K
Stages
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The Standard model (current version)