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Chapter 9 The Formation and Structure of Stars 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 The ISM occurs mainly in two types of clouds: • HI clouds: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm3); size: ~ 100 pc • Hot intercloud medium: 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? Evidence for the ISM 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 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 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 Star emerges from the enshrouding dust cocoon Ignition of H He fusion processes Protostellar Disks and Jets – Herbig-Haro Objects Q: What are the bipolar flows evidence of? Herbig-Haro Object HH34 Globules Bok globules: ~ 10 – 1000 solar masses; Contracting to form protostars Observations of star formation: Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars 200 solar mass star N 11B V838 Mon Trifid Tarantula N 49 The Source of Stellar Energy Stars produce energy by nuclear fusion of hydrogen into helium. Q: How does the sun fuse H to He? In the sun, this happens primarily through the proton-proton (P-P) chain 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 dM 4 r 2 dr dL 4 r 2 e dr dP GM 2 dr r dT 3 L dr 16 ac T 3 r 2 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 (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 Transport Energy 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 Models The 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 Stars Stars gradually exhaust their hydrogen fuel. They gradually becoming brighter, evolving off the zero-age main sequence (ZAMS). Lifetime of a main-sequence star (90% of total life is on main-seq.) fuel M 1 3.5 2.5 rate of consumption M M The Lifetimes of Stars on the Main Sequence The Orion Nebula: An Active Star-Forming Region The Trapezium less than 2 million years old The Orion Nebula Infrared image: ~ 50 very young, cool, lowX-raymass image: ~ 1000 stars very young, hot stars Gas blown away from protostars The BecklinNeugebauer object (BN): Hot star, just reaching the main sequence IR Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared IR + visual B3 B1 B1 O6 Spectral types of the trapezium stars Protostars with protoplanetary disks