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The Formation and Structure of Stars Chapter 9 Slide 2 The Big Picture Stars exist because of gravity. Gravity causes interstellar material to collapse to form stars. Gravity determines how much energy stars generate. Gravity dictates how stars evolve and die. Mass determines gravity; Mass is the #1 property of a star. Slide 3 The space between the stars is not empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the ISM because: a) dense interstellar clouds are the birth place of stars b) dark clouds alter and absorb the light from stars behind them The Interstellar Medium (ISM) Slide 4 Various Appearances of the ISM Slide 5 Three Kinds of Nebulae Emission Nebula Reflection Nebula Dark Nebula Slide 6 The Fox Fur Nebula NGC 2246 1) Emission Nebulae (H II Regions) A very hot star illuminates a cloud of hydrogen gas; Its ultraviolet light ionizes hydrogen; Electrons recombine with protons, cascade down to the ground state, and produce emission lines, dominated by red H photons. The Keyhole Nebula Slide 7 2) Reflection Nebulae Star illuminates a nearby cloud of gas and dust; Blue light is much more likely to be scattered by dust than red light; Reflection nebula appears blue. *The same physics for the blue sky and the red sunset! Slide 8 Emission and Reflection Nebulae Slide 9 3) Dark Nebulae Barnard 86 Dense clouds of gas and dust absorb the light from the stars behind; Appear dark in front of the brighter background, which is often an emission nebula. Horsehead Nebula Slide 10 Interstellar Reddening Visible Infrared Barnard 68 Blue light is strongly scattered and absorbed by interstellar (dust) clouds. Red light can more easily penetrate the cloud, but is still absorbed to an extent (interstellar extinction). (Infrared radiation is hardly absorbed.) Interstellar clouds make background stars appear redder. The physics of reflection nebula revisited! Slide 11 Interstellar Absorption Lines The interstellar medium produces absorption lines in the spectra of stars. Distinguished from stellar absorption lines via: a) Absorption from wrong ionization states Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple lines of same transition b) Narrow (sharp) lines (temperature & density too low) c) Multiple components (several clouds of ISM with different radial velocities) Slide 12 Four Components of the ISM Component Temperature (K) Density (atoms/cm 3 ) Gas Percent of total mass Molecular clouds 20 5010 3 10 5 Molecules (H 2 and others) ~ 25% HI clouds50 1501 1000 Neutral hydrogen Other atoms ionized ~ 25% Intercloud medium 10 3 10 4 0.01Partially ionized~ 50% Coronal gas10 5 10 6 10 4 10 3 Highly ionized, from hot stars and supernovae ~ 5% Note: Emission nebulae (HII regions) occur only near very hot stars, so they comprise very small fraction of the ISM. EXTRA Slide 13 Various Views of the Interstellar Medium Infrared observations reveal the presence of cool, dusty gas. X-ray observations reveal the presence of hot gas. Slide 14 Shocks triggering star formation Henize 206 (infrared) Slide 15 The Contraction of a Protostar Slide 16 From Protostars to Stars Ignition of 4 1 H 4 He fusion processes Star emerges from the enshrouding dust cocoon (T Tauri stage) Slide 17 Evidence of Star Formation Nebula around S Monocerotis: Contains many massive and very young stars, including T Tauri stars: strongly variable and bright in the infrared. Slide 18 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 or bipolar outflows) seen as Herbig-Haro objects Slide 19 Example: Herbig-Haro Object HH34 Slide 20 Globules Bok globules: ~ 10 1000 solar masses Contracting to form protostars Slide 21 EGGs Evaporating gaseous globules (EGGs): Newly forming stars exposed by the ionizing radiation from nearby massive stars Slide 22 Energy Generation (7.2) Energy generation in the sun (and all other stars): nuclear fusion = fusing together 2 or more lighter nuclei to produce heavier ones. Nuclear fusion can generate energy up to the production of iron. For elements heavier than iron, energy is produced by nuclear fission. Binding energy due to the strong force Slide 23 Energy generation in the Sun: The Proton-Proton Chain (7.2) Basic reaction: 4 1 H 4 He + energy 4 protons have 0.048x10 -27 kg (= 0.7 %) more mass than 4 He. Energy gain = mc 2 = 0.43x10 -11 joules per reaction Need large proton speed (high temperature) to overcome Coulomb barrier (electrical repulsion between protons). Sun needs 10 38 reactions, transforming 5 million tons of mass into energy every second!. T 10 7 K = 10 million K Slide 24 The Solar Neutrino Problem The solar interior cannot be observed directly because it is highly opaque to radiation. But neutrinos can penetrate huge amounts of material without being absorbed. Davis solar neutrino experiment Early solar neutrino experiments detected a much lower flux of neutrinos than expected ( the solar neutrino problem). Recent results have proven that neutrinos change (oscillate) between different types (flavors), thus solving the solar neutrino problem. Slide 25 The Source of Stellar Energy 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 Basic reaction: 4 1 H 4 He + energy Slide 26 26 Slide 27 Fusion into Heavier Elements than C & O: Occurs only in very massive stars (more than 8 solar masses) why?. Requires very high temperatures (why?). Slide 28 Stellar Models the theory of stars Divide a stars interior into concentric shells Onion skin layer model Within each shell and between neighboring shells, require that the laws of physics are obeyed: Conservation of Mass Conservation of Energy Hydrostatic Equilibrium Energy Transport Four laws of stellar structure: Slide 29 Hydrostatic Equilibrium In each layer: This condition uniquely determines the interior structure of the star. Stable stars on a narrow strip (main sequence) in the H-R diagram. Outward force of thermal pressure Inward force of gravity (weight of all layers above) = Slide 30 Energy Transport Energy generated in the stars center must be transported to the surface. Inner layers of the Sun: Radiation Outer layers of the Sun: Convection Energy carried by photons Energy carried by convective motion of large masses Slide 31 Stellar Structure Temperature, density and pressure decreasing Energy generation via nuclear fusion Energy transport via radiation Energy transport via convection Flow of energy Stars total mass determines which part of the star has convection or radiation (cf. Ch. 10) Sun Slide 32 Calculating the stellar structure: Take the four equations representing the four laws of stellar structure and solve them simultaneously! Hydrostatic equilibrium Energy transport Conservation of mass Conservation of energy Stars mass (and chemical composition) completely determines the properties of the star. Slide 33 Interactions of Stars and their Environment 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 H II regions. Slide 34 The Life of Main-Sequence Stars As stars gradually exhaust their hydrogen fuel, they become brighter, and evolve off the zero-age main sequence. Slide 35 The Lifetimes of Stars on the Main Sequence Slide 36 The Orion Nebula: a region of active star formation Slide 37 The Trapezium The Orion Nebula The 4 trapezium stars: Brightest, young stars (< 2 million years old) in the central region of the Orion nebula X-ray image: ~ 1000 very young, hot stars Infrared image: ~ 50 very young, cool, low- mass stars Only one of the trapezium stars is hot enough to ionize hydrogen in the Orion nebula Slide 38 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 disks B3 B1 O6 Slide 39 Stellar Structure Cause and Effect Mass (M) Gravity (weight) Pressure Density Radius Temperature Fusion Rates Luminosity Available Fuel Time of Stability Main Sequence Lifetime (Life Expectancy) Hydrostatic equilibrium Pressure-Temperature Thermostat ~ M L ~ M 3.5 Lifetime = M/L ~ M 2.5 Mass-Luminosity Relation Slide 40 Red in Astronomy red emission nebulae red supergiants/giants/dwarfs red shift (in the Doppler effect) Interstellar reddening blue reflection nebulae red sunset blue sky