The Interstellar Medium ( 星際物質 、星際介質 )

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0. The Interstellar Medium ( ). Chapter 10. 0. Guidepost. - PowerPoint PPT Presentation

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<ul><li><p>The Interstellar Medium( )Chapter 100</p></li><li><p>You have begun your study of the sun and other stars, but now it is time to study something that is nearly invisible. The thin gas and dust that drifts through space between the stars is produced in part by dying stars and can give birth to new stars. This chapter will show you how important spectroscopic analysis is in astronomy and will help you answer three essential questions: How do visual-wavelength observations show that space isnt empty? How are observations at non-visible wavelength evidence of invisible matter between the stars? How does the matter between the stars interact with the stars? Guidepost0</p></li><li><p>Studying something that is nearly invisible takes careful thought and scientific ingenuity, and that will introduce you to an important question about science: How can you distinguish between a fact and a theory?The matter between the stars is the starting point for the life story of the stars. The next four chapters will trace the birth, life, and death of stars.Guidepost (continued)0</p></li><li><p>I. Visible-Wavelength ObservationsA. NebulaeB. Extinction () and Reddening ()C. Interstellar Absorption LinesD. Clouds and Whats in Between</p><p>II. Long- and Short-Wavelength ObservationsA. 21-cm ObservationsB. Molecules in SpaceC. Infrared Radiation from DustD. X Rays From the Interstellar MediumE. Ultraviolet Observations of the Interstellar Medium</p><p>III. A Model of the Interstellar MediumA. Four Components of the Interstellar MediumB. The Interstellar CycleOutline0</p></li><li><p>A World of Interstellar MediumWe are interested in the interstellar medium becausea) Dense interstellar clouds are the birth place of starsb) Dying stars will release matter back into space The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky.0</p></li><li><p>Bare-Eye Nebula: OrionOne example of an interstellar gas cloud (nebula) is visible to the bare eye: the Orion nebula0</p></li><li><p>Bare-Eye Nebula: OrionOne example of an interstellar gas cloud (nebula) is visible to the bare eye: the Orion nebula0</p></li><li><p>Three Kinds of Nebulae (1)1) Emission NebulaeHot star illuminates a gas cloud;excites and/or ionizes the gas (electrons kicked into higher energy states);electrons recombining, falling back to ground state produce emission lines.The Fox Fur NebulaNGC 2246The Trifid Nebula0</p></li><li><p>Three Kinds of Nebulae (2)Star illuminates a gas and dust cloud;star light is reflected by the dust;reflection nebulae appear blue because blue light is scattered by larger angles than red light;the same phenomenon makes the day sky appear blue (if its not cloudy).2) Reflection Nebulae0</p></li><li><p>Three Kinds of Nebulae (3)Dense clouds of gas and dust absorb the light from the stars behind; Barnard 86Horsehead Nebulaappear dark in front of the brighter background; 3) Dark Nebulae0</p></li><li><p>Red light can more easily penetrate the cloud, but is still absorbed to some extent.Interstellar ReddeningBlue light is strongly scattered and absorbed by interstellar clouds.Infrared radiation is hardly absorbed at all.Interstellar clouds make background stars appear redder.VisibleBarnard 680Infrared</p></li><li><p>Interstellar Reddening (2)The Interstellar Medium absorbs light more strongly at shorter wavelengths.0</p></li><li><p>Interstellar Absorption LinesThe interstellar medium produces absorption lines in the spectra of stars. These can be distinguished from stellar absorption lines through:a) Absorption from wrong ionization statesNarrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple componentsb) Small line width (too low temperature; too low density)c) Multiple components (several clouds of ISM with different radial velocities)0</p></li><li><p>Structure of the ISMHI clouds:Hot intercloud medium:The ISM occurs in two main types of clouds:Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 a few hundred atoms/cm3); size: ~ 100 pc Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3); gas can remain ionized because of very low density.0</p></li><li><p>Observing Neutral Hydrogen:The 21-cm (radio) line (I)Electrons in the ground state of neutral hydrogen have slightly different energies, depending on their spin orientation.Magnetic field due to proton spinMagnetic field due to electron spinOpposite magnetic fields attract =&gt; Lower energyEqual magnetic fields repel =&gt; Higher energy21 cm line0</p></li><li><p>The 21-cm Line of Neutral Hydrogen (II)Transitions from the higher-energy to the lower-energy spin state produce a characteristic 21-cm radio emission line.=&gt; Neutral hydrogen (HI) can be traced by observing this radio emission. 0</p></li><li><p>Observations of the 21-cm Line (1)All-sky map of emission in the 21-cm lineG a l a c t i c p l a n e0</p></li><li><p>Observations of the 21-cm Line (2)HI clouds moving towards Earth(from redshift/blueshift of line)HI clouds moving away from EarthIndividual HI clouds with different radial velocities resolved0</p></li><li><p>Molecules in SpaceIn addition to atoms and ions, the interstellar medium also contains molecules. Molecules also store specific energies in theira) rotationb) vibrationTransitions between different rotational / vibrational energy levels lead to emission typically at radio wavelengths.0</p></li><li><p>The Most Easily Observed Molecules in SpaceCO = Carbon Monoxide Radio emissionOH = Hydroxyl Radio emission.The Most Common Molecule in Space:Difficult to observe! Use CO as a tracer for H2 in the ISM!H2 = Molecular Hydrogen Ultraviolet absorption and emission:But: Where theres H2, theres also CO.0</p></li><li><p>Molecular CloudsMolecules are easily destroyed (dissociated) by ultraviolet photons from hot stars. They can only survive within dense, dusty clouds, where UV radiation is completely absorbed.Molecular Clouds:Largest molecular clouds are called Giant Molecular Clouds: Diameter 15 60 pcTemperature 10 KTotal mass 100 1 million solar massesCold, dense molecular cloud coreHI CloudUV emission from nearby stars destroys molecules in the outer parts of the cloud; is absorbed there.Molecules survive0</p></li><li><p>Interstellar DustProbably formed in the atmospheres of cool starsMostly observable through infrared emissionIRAS (infrared) image of infrared cirrus of interstellar dustInfrared and radio emissions from molecules and dust are efficiently cooling gas in molecular clouds.0</p></li><li><p>The Coronal GasAdditional component of very hot, low-density gas in the ISM:n ~ 0.001 particles/cm3Observable in X-raysCalled Coronal gas because of its properties similar to the solar corona (but completely different origin!)Probably originates in supernova explosions and winds from hot starsX-ray image of the Cygnus regionOur sun is located within (near the edge of) a coronal gas bubble.T ~ 1 million K0</p></li><li><p>The Four Components of the Interstellar Medium0</p><p>ComponentTemperature [K]Density [atoms/cm3]Main ConstituentsHI Clouds50 1501 1000Neutral hydrogen; other atoms ionizedIntercloud Medium (HII)103 - 1040.01Partially ionized H; other atoms fully ionizedCoronal Gas105 - 10610-4 10-3All atoms highly ionized HMolecular Clouds20 - 50103 - 105Neutral gas; dust and molecules</p></li><li><p>The Interstellar CycleStars, gas, and dust are in constant interaction with each other.Stars are formed from dense molecular cloud cores.Hot stars ionize gas, producing HII regions.Young star clusters illuminate the remnants of their mother clouds, producing reflection nebulae.Young star clusters leave trails of rarefied ISM behind.Supernovae of massive stars produce coronal gas and enrich the ISM with heavier elements.Supernovae trigger shock waves in the ISM that lead to the compression of dense clouds and new star formation.0</p><p>*************************</p></li></ul>

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