18. Stellar Birth. Initiation of Star Formation. The Orion Nebula: A Close-Up View. Interstellar Gas & Dust in Our Galaxy

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1 18. Stellar Birth Star observations & theories aid understanding Interstellar gas & dust in our galaxy Protostars form in cold, dark nebulae Protostars evolve into main-sequence stars Protostars both gain & lose mass Star clusters reveal formation & evolution details Protostars can form in giant molecular clouds Supernovae can trigger star birth Stellar Observations & Theories Fundamental observational difficulties Stars exist far longer than astronomers Star lifetimes range from millions to billions of years Stellar birth, life & death observed as stages Each observation is an extremely brief snapshot Fundamental observational simplicity Every star is far simpler than any living organism The materials are very simple The processes are very simple Basic physical processes Gravity tends to gather matter closer together Gravity is determined by distance between atoms Pressure tends to disperse matter farther apart Pressure is determined by temperature of atoms Interstellar Gas & Dust in Our Galaxy Emission nebulae Fluorescence similar to common light bulbs Emission lines depend on material & temperature Reflection nebulae Characteristic blue color Selective scattering of continuous spectra from stars Dust particles comparable in size to blue wavelengths Dark nebulae Characteristic blocking of background light May be partial or total blocking Thermal infrared can penetrate some dark nebulae Initiation of Star Formation Compression of interstellar medium is essential Gentle mechanisms from low-mass star death Gently expanding shell of gas called a planetary nebula Weak shock wave may initiate compression Gas adds low-mass elements to the forming stars Usually limited to Carbon & Silicon Violent mechanisms from high-mass star death Rapidly expanding gas shell is a supernova remnant Strong shock wave will initiate formation of O & B stars Gas adds high-mass elements to the forming stars May include elements as heavy as Uranium The Orion Nebula: A Close-Up View Emission, Reflection & Dark Nebulae

2 Reflection Nebula In Corona Australis Interstellar Reddening by Dust Grains Strongly scattered Weakly scattered Spiral Galaxies: Two Perspectives Face-on Edge-on Protostars Form in Cold Dark Nebulae Basic physical processes Gravity effects must exceed pressure effects Highest probability for star formation Extremely low temperatures minimize pressure Extremely close atoms maximize gravity Only dark nebulae have high enough density Large Barnard objects A few thousand M & ~ 10 pc in diameter Small Bok globules Resembles the core of a Barnard object Basic chemical composition (by mass) ~ 74% hydrogen ~ 25% helium ~ 1% metals All elements heavier than helium Bok Globules: Opaque Dust & Gas Anglo-Australian Observatory Protostar Details Earliest model Henyey & Hayashi 1950 s Stage 1 Cool nebula several times Solar System size Stage 2 Continued contraction raises the temperature Kelvin-Helmholtz contraction Stage 3 Still quite large, the cloud begins to glow Convection move heat outward Low temperature + Huge surface = Very bright A protostar the mass of the Sun After 1,000 years of contraction Surface temperature is ~ 2,000 K to 3,000 K Diameter is ~ 20 times > the Sun Luminosity is ~ 100 times > the Sun

3 Evolutionary Track of Protostars High- mass stars Approximately a horizontal line on an H-R diagram Progression is toward the left Cool to hot Solar- mass stars Approximately a V-shaped line on an H-R diagram Progression is toward the left Cool to hot Low- mass stars Approximately a vertical line on an H-R diagram Progression is toward the bottom Bright to dim Pre-Main-Sequence Evolutionary Tracks Progress of Star Formation A positive feedback process Gravity & pressure increase as the nebula shrinks Pressure increases d Gravity increases d 2 Gravity overwhelms pressure Magnetism could disrupt this in the earliest stages Additional characteristics Angular momentum is conserved The shrinking nebula spins faster & faster Original 3-D cloud deforms into a donut-like disk Material spins inward very rapidly Much of this material is ejected at the protostar s poles Culmination of Star Formation A negative feedback process High core pressure & temperature sustain H fusion A new & intense source of heat energy Core pressure rises dramatically Gravitational collapse ends Thermal & hydrostatic equilibrium established A new star stabilizes on the main sequence Protostars Become Main-Sequence Stars Protostar temperature changes Surface Little temperature change Minimal increase for 15 M protostars Slight increase for 5 M protostars Slight decrease for 2 M protostars Significant decrease for 1 M protostars Dramatic decrease for 0.5 M protostars Core Dramatic temperature increase Increasing temperature ionizes the protostar s interior Energy is transmitted outward by radiation Temperatures > several million kelvins initiate fusion This event marks the birth of a true star Protostar Evolution is Mass-Dependent Very-low- mass stars M < 0.8 M Core temperatures too low to ionize interior Convection characterizes the entire interior of the star Low- mass stars 0.8 M Sun < M < 4 M Core temperatures high enough to ionize interior Radiation characterizes the region surrounding the core Convection characterizes the region near the surface High- mass stars M > 4 M Hydrogen fusion begins very early Convection characterizes the region surrounding the core Radiation characterizes the region near the surface

4 Main-Sequence Stars of Different Mass Brown Dwarfs: Failed Stars A minimum mass is required for fusion Pressure & temperature cannot get high enough Minor lithium fusion can occur Surface temperature may reach ~ 2,000 K Brown dwarf characteristics Mass between kg & kg ~ 10 to 84 times the mass of Jupiter The lower mass limit is sometimes set at ~ 14 times M Jup Continues to cool & contract Detectable only at thermal infrared wavelengths Many brown dwarfs exhibit irregular brightness changes Possible storms far more violent than on Jupiter Protostars Both Gain & Lose Mass Protostar formation is extremely dynamic Matter is drawn inward along an accretion disk Matter is hurled outward perpendicular to this disk T Tauri stars 20 th brightest star in the constellation Taurus Exhibit both emission & absorption spectral lines Surrounded by hot low-density gas Doppler shift indicates a velocity of 80 km. sec -1 Luminosity varies irregularly over several days Mass ~ 3 M Herbig-Haro objects Bipolar outflow compresses & heats interstellar gas May last only ~ 10,000 to 100,000 years Herbig-Haro Objects: Bipolar Outflow Clusters Reveal Formation & Evolution Star clusters never have stars of uniform mass High-mass stars evolve very quickly O & B spectral class stars emit abundant UV radiation Low-mass stars evolve very slowly K & M spectral class stars emit abundant IR radiation The destiny of excess gas & dust H II regions H I regions are neutral (non-ionized) hydrogen H II regions are singly-ionized hydrogen Hydrogen has only 1 electron Result is free protons & electrons Produce red emission nebulae Dust regions Resist dissipation by strong UV radiation from O & B stars Produce blue reflection nebulae A Star Cluster With An H II Region

5 H-R Diagram of a Young Star Cluster The Pleiades & Its H-R Diagram Protostars In Giant Molecular Clouds Characteristics of molecular clouds 195 different molecules identified in space ~ 10,000 H 2 molecules for every CO molecule The Milky Way contains ~ 5,000 molecular clouds These include several star-forming regions 17 molecular clouds outline the local arm of our galaxy Orion nebula s parent cloud contains ~ 500,000 M Spectral emission lines Cold dark interstellar hydrogen clouds Emission in the UV, visible & IR regions of the spectrum Molecular interstellar gas clouds Emission in the microwave region of the spectrum Carbon Monoxide Molecular Clouds Molecular Clouds in the Milky Way O & B Stars Trigger Star Formation

6 Supernovae Can Trigger Star Birth Supernova remnants are common High-mass stars exhaust their H 2 supply very quickly Many old star clusters have supernova remnants Supernova remnants are violent High-mass stars die in tremendous explosions Spherical shock wave goes outward at supersonic speeds This compresses interstellar gas & dust clouds Often results in associations rather than clusters New stars are moving too fast to stay gravitationally bound New stars quickly disperse in various directions Probably the situation when our Sun formed Supernova Remnant in the Cygnus Loop Important Concepts Interstellar gas & dust Emission, reflection & dark nebulae Potential birthplace of stars Stages of star formation Initiation Coldest & densest regions are ideal Contest between gravity & pressure Compression mechanism required Progress Positive feedback: Gravity > Pressure Collapse accelerates until fusion Culmination Heat from fusion increases pressure Equilibrium is established Protostar evolution depends on mass Very-low- mass < 0.8 times M Sun Low- mass < 4 times M Sun High- mass > 4 times M Sun Mass gain & loss in protostars Circumstellar accretion disk inflows Bipolar outflows T Tauri [variable] stars Herbig-Haro objects Star clusters give evolution details Few clusters have same-age stars Luminosity & color on H-R diagram Stellar models fit observations well Star formation in molecular clouds ~ 5,000 in the Milky Way galaxy 17 define our galactic spiral arm Compression mechanisms UV emissions from OB associations Supernova explosions

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