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18. Stellar Birth Stellar 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
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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 processes 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 into a tiny space Gravity is determined by how close atoms are to each other Pressure tends to disperse matter throughout the Universe Pressure is determined by temperature
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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
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Initiation of Star Formation
Compression of the 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 in the shell adds low-mass elements to the forming stars Usually limited to Carbon & Silicon Violent mechanisms from high-mass star death Rapidly expanding shell of gas called a “supernova remnant” Strong shock wave will initiate formation of O & B stars Gas in the shell adds high-mass elements to the forming stars May include elements as heavy as Uranium
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The Orion Nebula: A Close-Up View
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Emission, Reflection & Dark Nebulae
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A Reflection Nebula In Corona Australis
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Interstellar Reddening by Dust Grains
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Spiral Galaxies: Two Perspectives
…Face-on Edge-on…
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Protostars Form in Cold, Dark Nebulae
Basic physical processes Gravity effects must exceed pressure effects Highest probability for star formation Extremely cold temperatures minimize pressure Extremely close atoms maximize gravity Only dark nebulae have high enough density Large Barnard objects A few thousand MSun & ~ 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” By convention, all elements heavier than helium
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Bok Globules: Opaque Dust & Gas
Anglo-Australian Observatory
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Protostar Details Earliest model A protostar the mass of the Sun
Henyey & Hayashi 1950’s Stage 1 Cool nebula several times the Solar System’s 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 greater than the Sun Luminosity is ~ 100 times greater than the Sun
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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 Low- mass stars Approximately a vertical line on an H-R diagram Progression is toward the bottom Bright to dim
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Pre-Main-Sequence Evolutionary Tracks
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Progress of Star Formation
A positive feedback process Gravity & pressure increase as the nebula shrinks Pressure increases µ d Gravity increases µ d2 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 relatively narrow disk Material spins inward very rapidly Much of this material is ejected at the protostar’s poles
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Culmination of Star Formation
A negative feedback process Core pressure & temperature high enough for 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
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Protostars Become Main-Sequence Stars
Protostar temperature changes Surface Little temperature change Minimal increase for 15 MSun protostars Slight increase for 5 MSun protostars Slight decrease for 2 MSun protostars Significant decrease for 1 MSun protostars Dramatic decrease for 0.5 MSun protostars Core Dramatic temperature increase Increasing temperature ionizes the protostar’s interior Energy is transmitted outward by radiation Temperatures greater than several million kelvins initiate fusion This event marks the “birth” of a true star
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Protostar Evolution is Mass-Dependent
Very-low- mass stars M < 0.8 MSun Core temperatures are too low to ionize the interior Convection characterizes the entire interior of the star Low- mass stars 0.8 MSun < M < 4 MSun Core temperatures are high enough to ionize the interior Radiation characterizes the region surrounding the core Convection characterizes the region near the surface High- mass stars M > 4 MSun Hydrogen fusion begins very early Convection characterizes the region surrounding the core Radiation characterizes the region near the surface
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Main-Sequence Stars of Different Mass
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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 1028 kg & kg ~ 10 to 84 times the mass of Jupiter The lower mass limit is sometimes set at ~ 14 times MJup Continues to cool & contract Usually detectable only at thermal infrared wavelengths Many brown dwarfs exhibit irregular brightness changes Possible storms far more violent than on Jupiter
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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 20th 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 MSun Herbig-Haro objects Bipolar outflow compresses & heats interstellar gas May last only ~ 10,000 to 100,000 years
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Herbig-Haro Objects: Bipolar Outflow
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Clusters Reveal Formation & Evolution
Star clusters seldom 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 one electron, so the result is free protons & electrons Produce red emission nebulae Dust regions Resist dissipation by intense UV radiation from O & B stars Produce blue reflection nebulae
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A Star Cluster With An H II Region
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The H-R Diagram of a Young Star Cluster
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The Pleiades & Its H-R Diagram
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Protostars In Giant Molecular Clouds
Characteristics of molecular clouds At least 151 different molecules identified in space ~ 10,000 H2 molecules for every CO molecule Milky Way galaxy 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 MSun 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
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Carbon Monoxide Molecular Clouds
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Molecular Clouds in the Milky Way
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O & B Stars Trigger Star Formation
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Supernovae Can Trigger Star Birth
Supernova remnants are common High-mass stars exhaust their H2 supply very quickly Many old star clusters have supernova remnants Supernova remnants are violent High-mass stars die in tremendous explosions A spherical shock wave moves 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
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Supernova Remnant in the Cygnus Loop
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Important Concepts Interstellar gas & dust Stages of star formation
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 MSun Low- mass < 4 times MSun High- mass > 4 times MSun 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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