lecture notes 14

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Published on November 2, 2007

Author: Callia

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Slide1:  SH2 136: A Spooky Nebula Ghoulish dust clouds: a region of star formation Halloween corresponds roughly to the cross-quarter day: half-way between equinox and solstice Your next test will be November 14:  Your next test will be November 14 The Formation of Stars:  The Formation of Stars Chapter 11 Giant Molecular Clouds: stellar nurseries:  Giant Molecular Clouds: stellar nurseries Visible Infrared Barnard 68 Star formation collapse of the cores of giant molecular clouds: Dark, cold, dense clouds obscuring the light of stars behind them. (More transparent in infrared light.)  Slide5:  Giant molecular clouds – stellar nurseries Slide6:  Orion Deep Field H-alpha line 656 nm (red light) Slide7:  Great Orion Nebula 1500 light-years away Slide8:  The Horsehead Nebula Slide9:  Star Forming Region RCW 38 Parameters of Giant Molecular Clouds:  Parameters of Giant Molecular Clouds Size: r ~ 50 pc Mass: > 100,000 Msun Dense cores: Temp.: a few 0K R ~ 0.1 pc M ~ 1 Msun Clouds need to contract and heat up in order to form stars. Slide13:  Coldest spots in the galaxy: T ~ 1-10 K Composition: Mainly molecular hydrogen 1% dust EGGs = Evaporating Gaseous Globules ftp://ftp.hq.nasa.gov/pub/pao/pressrel/1995/95-190.txt Slide14:  Jeans instability: Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments. Contraction of Giant Molecular Cloud Cores:  Contraction of Giant Molecular Cloud Cores Thermal Energy (pressure) Magnetic Fields Rotation (angular momentum)  External trigger required to initiate the collapse of clouds to form stars. Horse Head Nebula Turbulence Shocks Triggering Star Formation:  Shocks Triggering Star Formation Globules = sites where stars are being born right now! Trifid Nebula Sources of Shock Waves Triggering Star Formation (1):  Sources of Shock Waves Triggering Star Formation (1) Previous star formation can trigger further star formation through: a) Shocks from supernovae (explosions of massive stars): Massive stars die young => Supernovae tend to happen near sites of recent star formation Sources of Shock Waves Triggering Star Formation (2):  Sources of Shock Waves Triggering Star Formation (2) Previous star formation can trigger further star formation through: b) Ionization fronts of hot, massive O or B stars which produce a lot of UV radiation: Massive stars die young => O and B stars only exist near sites of recent star formation Slide19:  The Bubble Nebula (Cassiopeia) Sources of Shock Waves Triggering Star Formation (3):  Sources of Shock Waves Triggering Star Formation (3) Giant molecular clouds are very large and may occasionally collide with each other c) Collisions of giant molecular clouds. Sources of Shock Waves Triggering Star Formation (4):  Sources of Shock Waves Triggering Star Formation (4) d) Spiral arms in galaxies like our Milky Way: Spirals’ arms are probably rotating shock wave patterns. Slide23:  Jeans instability: Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments. Protostars:  Protostars Protostars = pre-birth state of stars: Hydrogen to Helium fusion not yet ignited Still enshrouded in opaque “cocoons” of dust => barely visible in the optical, but bright in the infrared. Heating By Contraction:  Heating By Contraction As a protostar contracts, it heats up: Free-fall contraction → Heating Heating does not stop contraction because the core cools down due to radiation Slide26:  Protostars: warm clumps of gas surrounded by infalling matter Disks: planet formation?! Why disks? Slide27:  Role of angular momentum Protostellar Disks:  Protostellar Disks Conservation of angular momentum leads to the formation of protostellar disks  birth place of planets and moons Protostellar Disks and Jets – Herbig Haro Objects:  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; bipolar outflows): Herbig Haro Objects Protostellar Disks and Jets – Herbig Haro Objects (2):  Protostellar Disks and Jets – Herbig Haro Objects (2) Herbig Haro Object HH34 Protostellar Disks and Jets – Herbig Haro Objects (3):  Protostellar Disks and Jets – Herbig Haro Objects (3) Herbig Haro Object HH30 Slide33:  From a protostar to a young star: very hot; still accreting matter Observed in the infrared, because infalling gas and dust obscure light Slide35:  The matter stops falling on the star Nuclear fusion starts in the core Planets can be formed from the remaining disk From Protostars to Stars:  From Protostars to Stars Ignition of H  He fusion processes Star emerges from the enshrouding dust cocoon Evidence of Star Formation:  Evidence of Star Formation Nebula around S Monocerotis: Contains many massive, very young stars, including T Tauri Stars: strongly variable; bright in the infrared. Evidence of Star Formation (2):  Evidence of Star Formation (2) The Cone Nebula Optical Infrared Young, very massive star Smaller, sunlike stars, probably formed under the influence of the massive star Evidence of Star Formation (3):  Evidence of Star Formation (3) Star Forming Region RCW 38 Globules:  Globules ~ 10 to 1000 solar masses; Contracting to form protostars Bok Globules: Globules (2):  Globules (2) Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars Open Clusters of Stars:  Open Clusters of Stars Large masses of Giant Molecular Clouds => Stars do not form isolated, but in large groups, called Open Clusters of Stars. Open Cluster M7 Open Clusters of Stars (2):  Open Clusters of Stars (2) Large, dense cluster of (yellow and red) stars in the foreground; ~ 50 million years old Scattered individual (bright, white) stars in the background; only ~ 4 million years old The Source of Stellar Energy:  The Source of Stellar Energy In the sun, this happens primarily through the proton-proton (PP) chain Recall from our discussion of the sun: Stars produce energy by nuclear fusion of hydrogen into helium. The CNO Cycle:  The CNO Cycle In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over: The CNO Cycle. Slide47:  Net result is the same: four hydrogen nuclei fuse to form one helium nucleus Why p-p and CNO cycles? Why so complicated? Because simultaneous collision of 4 protons is too improbable Hydrostatic Equilibrium:  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 (2):  Hydrostatic Equilibrium (2) Outward pressure force must exactly balance the weight of all layers above everywhere in the star. This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram. H-R Diagram (showing Main Sequence):  H-R Diagram (showing Main Sequence) Energy Transport:  Energy Transport Energy generated in the star’s center must be transported to the surface. Inner layers: Radiative energy transport Outer layers (including photosphere): Convection Bubbles of hot gas rising up Cool gas sinking down Gas particles of solar interior g-rays Conduction, Convection, and Radiation:  Conduction, Convection, and Radiation (SLIDESHOW MODE ONLY) Stellar Structure:  Stellar Structure Temperature, density and pressure decreasing Energy generation via nuclear fusion Energy transport via radiation Energy transport via convection Flow of energy Basically the same structure for all stars with approx. 1 solar mass or less. Sun Energy Transport Structure:  Energy Transport Structure Inner radiative, outer convective zone Inner convective, outer radiative zone CNO cycle dominant PP chain dominant Summary: Stellar Structure:  Summary: Stellar Structure Mass Sun Radiative Core, convective envelope; Energy generation through PP Cycle Convective Core, radiative envelope; Energy generation through CNO Cycle

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