The formation of stars Learning Objective: How do stars form?
The Formation and Structure of Stars
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Transcript of The Formation and Structure of Stars
The Formation and Structure of Stars
Chapter 9
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
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.
3 types of nebula
1. Emission
2. Reflection
3. Dark
Q: Why do emission nebula look red and reflection nebula blue?
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
Interstellar reddening
Q: Why do astronomers rely heavily on IR observations?
Q: How do we know the ISM exists?
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.
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
SN_triggered_ssc2004-04v2.wmv
The Contraction of a Protostar
Q: Why do you think there’s a lower limit on the mass of a main-seq. star?
The Contraction of a Protostar
Sun: ~30 million years
15 M: 160,000 years
0.2 M: 1 billion years
From Protostars to Stars
Ignition of H He fusion processes
Star emerges from the
enshrouding dust cocoon
Protostellar Disks and Jets – Herbig-Haro Objects
Herbig-Haro Object HH34
Q: What are the bipolar flows evidence of?
Globules
Bok globules:
~ 10 – 1000 solar masses;
Contracting to form protostars
Evaporating gaseous globules (“EGGs”): Newly forming stars
exposed by the ionizing radiation from nearby massive stars
Observations of star formation:
200 solar mass star
N 11B
Trifid
V838 Mon
Tarantula
N 49
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?
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?
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).
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
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
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?
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?
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….
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.)
The Lifetimes of Stars on the Main Sequence
The Orion Nebula: An Active Star-Forming Region
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
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