Chapter.9The Formation andStructure of Stars恒星的形成和结构
The Formation and Structure of Stars 恒星的形成和结构 Chapter 9
TheInterstellarMedium（ISM）星际介质Thespacebetweenthestarsisnotcompletelyempty,butfilledwithvery dilutegas anddust,producingsomeofthemostbeautifulobjectsinthesky恒星之间的空间并不是完全真空，而是有很稀薄的气体和尘埃，它们产生了天空中最漂亮的一些天体，Weareinterested intheinterstellarmediumbecause我们对星际介质感兴趣，因为a)denseinterstellarcloudsarethebirthplaceofstars致密的星际云是恒星诞生地b)darkcloudsalterandabsorbthelightfromstarsbehindthem暗云改变和吸收背后的星光
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. 恒星之间的空间并不是完全真空，而是有 很稀薄的气体和尘埃，它们产生了天空中 最漂亮的一些天体。 We are interested in the interstellar medium because 我们对星际介质感兴趣，因为 a) dense interstellar clouds are the birth place of stars 致密的星际云是恒星诞生地 b) dark clouds alter and absorb the light from stars behind them 暗云改变和吸收背后的星光 The Interstellar Medium (ISM) 星际介质
TheVariousAppearancesoftheISM星际介质的一些表象NebulaN44Roughly 40 young starsareinflatingabubbleofhotgasinsidethenebulaTrom which theyformed.Horsehead NebulaDustyforegroundgas silhouettedagainstglowinggasilluminatedbyhot,youngstars.100lyYoung starVisual-wavelengthimageburied inthenebula.5Visual-wavelengthimagee2006Brooks/Cole-Thomson
The Various Appearances of the ISM 星际介质的一些表象
Threekinds of nebulae三种星云1）EmissionNebulae（HllRegions)发射星云Hot star illuminates a gas cloud:热星照亮气体云excites and/or ionizes the gas(electronskickedintohighelenergy states)激发电子（电子跳到高能级）electronsrecombining,fallingbacktoground stateproduceemission lines电子跳回基态，产生发射线herifidNebula
Three kinds of nebulae三种星云 1) Emission Nebulae (HII Regions)发射星云 Hot star illuminates a gas cloud; 热星照亮气体云 excites and/or ionizes the gas (electrons kicked into higher energy states) 激发电子（电子跳 到高能级） electrons recombining, falling back to ground state produce emission lines 电子跳回基态，产 生发射线 The Fox Fur Nebula The Trifid Nebula NGC 2246
2）ReflectionNebulae反射星云Star illuminates gas and dust cloud（有光源）；starlight is reflected bythedust（尘埃反射）；reflectionnebulaappearsbluebecausebluelightisscatteredbylargeranglesthanredlight（反射星云偏蓝，因为蓝光散Sunlight enters Earth's atmosphere射角大）；Blue photons enteroureyesfromalldirections makingSamephenomenonmakesthedayskyappearbluetheskylookblue.(if it'snotcloudy)Biuephotonsarescattered more和蓝天成因一样easilythanlongerwavelengths
2) Reflection Nebulae 反射星云 Star illuminates gas and dust cloud （有光源）; star light is reflected by the dust （尘埃反射）; reflection nebula appears blue because blue light is scattered by larger angles than red light （反射星云偏蓝，因为蓝光散 射角大）; Same phenomenon makes the day sky appear blue (if it’s not cloudy). 和蓝天成因一样