The Formation and Structure of Stars Chapter 9 The Formation and Structure of Stars

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.

The Interstellar Medium (ISM) 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

Various Appearances of the ISM

Three Kinds of Nebulae Emission Nebula Reflection Nebula Dark Nebula

1) Emission Nebulae (HII 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 Hα photons. The Keyhole Nebula NGC 2246 The Fox Fur Nebula

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!

Emission and Reflection Nebulae

3) Dark Nebulae 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. Barnard 86 Horsehead Nebula

Interstellar Reddening The physics of reflection nebula revisited! 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. Barnard 68 Infrared Visible

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)

Four Components of the ISM EXTRA Component Temperature (K) Density (atoms/cm3) Gas Percent of total mass Molecular clouds 20 – 50 103 – 105 Molecules (H2 and others) ~ 25% HI clouds 50 – 150 1 – 1000 Neutral hydrogen Other atoms ionized Intercloud medium 103 – 104 0.01 Partially ionized ~ 50% Coronal gas 105 – 106 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.

Various Views of the Interstellar Medium Infrared observations reveal the presence of cool, dusty gas. X-ray observations reveal the presence of hot gas.

Shocks triggering star formation Henize 206 (infrared)

The Contraction of a Protostar

Ignition of 41H → 4He fusion processes From Protostars to Stars Star emerges from the enshrouding dust cocoon (T Tauri stage) Ignition of 41H → 4He fusion processes

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.

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

Example: Herbig-Haro Object HH34

Globules Contracting to form protostars Bok globules: ~ 10 – 1000 solar masses Contracting to form protostars

EGGs Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

Energy Generation (§7.2) nuclear fusion Energy generation in the sun (and all other stars): nuclear fusion Binding energy due to the strong force = 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.

Energy generation in the Sun: The Proton-Proton Chain (§7.2) Need large proton speed (high temperature) to overcome Coulomb barrier (electrical repulsion between protons). Basic reaction: 41H → 4He + energy 4 protons have 0.048x10-27 kg (= 0.7 %) more mass than 4He. T ≥ 107 K = 10 million K Energy gain = mc2 = 0.43x10-11 joules per reaction Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second!.

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. 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. Davis solar neutrino experiment

The Source of Stellar Energy Recall from our discussion of the Sun: Stars produce energy by nuclear fusion of hydrogen into helium. Basic reaction: 41H → 4He + energy In the Sun, this happens primarily through the proton-proton (PP) chain

Fusion into Heavier Elements than C & O: Requires very high temperatures (why?). Occurs only in very massive stars (more than 8 solar masses) —why?.

Stellar Models—the theory of stars Divide a star’s interior into concentric shells — “Onion skin layer model” Within each shell and between neighboring shells, require that the laws of physics are obeyed: Four laws of stellar structure: • Conservation of Mass • Conservation of Energy • Hydrostatic Equilibrium • Energy Transport

Hydrostatic Equilibrium In each layer: Inward force of gravity (weight of all layers above) Outward force of thermal pressure = This condition uniquely determines the interior structure of the star. Stable stars on a narrow strip (main sequence) in the H-R diagram.

Energy Transport Radiation Convection Energy generated in the star’s 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

Stellar Structure Sun Flow of energy Energy transport via convection Energy transport via radiation Flow of energy Energy generation via nuclear fusion Star’s total mass determines which part of the star has convection or radiation (cf. Ch. 10) Temperature, density and pressure decreasing

Calculating the stellar structure: Take the four equations representing the four laws of stellar structure and solve them simultaneously! Conservation of mass Conservation of energy Star’s mass (and chemical composition) completely determines the properties of the star. Energy transport Hydrostatic equilibrium

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 HII regions.

The Life of Main-Sequence Stars As stars gradually exhaust their hydrogen fuel, they become brighter, and evolve off the zero-age main sequence.

The Lifetimes of Stars on the Main Sequence

The Orion Nebula: a region of active star formation

The Trapezium The 4 trapezium stars: Brightest, young stars (< 2 million years old) in the central region of the Orion nebula Only one of the trapezium stars is hot enough to ionize hydrogen in the Orion nebula Infrared image: ~ 50 very young, cool, low- mass stars X-ray image: ~ 1000 very young, hot stars The Orion Nebula

Spectral types of the trapezium stars Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared The Becklin- Neugebauer object (BN): Hot star, just reaching the main sequence Spectral types of the trapezium stars B3 B1 B1 O6 Protostars with protoplanetary disks

Stellar Structure— Cause and Effect Mass (M) Available Fuel → ~ M Gravity (weight) → Hydrostatic equilibrium Pressure Temperature Pressure-Temperature Thermostat → → Density Fusion Rates → → Radius Luminosity → Mass-Luminosity Relation L ~ M 3.5 Time of Stability Main Sequence Lifetime (“Life Expectancy”) Lifetime = M/L ~ M –2.5

“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