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1、2018/9/25,,1,第六章 河外星系,研究历史 星系分类 星系测量 特殊星系：银河系 星 系 团 星系演化,http:/ GOALS,Describe the overall structure of the Milky Way Galaxy and specify how the various regions differ from one another. Describe the orbital paths of stars in different regions of the Galaxy and explain how these motions are accounted
2、 for by our understanding of how the Galaxy formed. Discuss some possible explanations for the existence of the spiral arms observed in our own and many other galaxies. Describe some of the phenomena observed at the center of our Galaxy.,2018/9/25,,3,本章内容,银河系外貌银河系结构银河系物质银河系形成银河系中心,2018/9/25,,4,The M
3、ilky Way 银河系,Theres no place like home！,2018/9/25,,5,2018/9/25,,6,What does our galaxy look like?,2018/9/25,,7,The Milky Way galaxy appears in our sky as a faint band of light,2018/9/25,,8,Dusty gas clouds obscure our view because they absorb visible lightThis is the interstellar medium that makes n
4、ew star systems,2018/9/25,,9,We see our galaxy edge-onPrimary features: disk, bulge, halo, globular clusters,2018/9/25,,10,If we could view the Milky Way from above the disk, we would see its spiral arms,2018/9/25,,11,银河系结构,21011恒星、质量 1012M，直径30 kpc 具有旋涡结构的盘状星系。 主要成分：银盘 (disk) (旋臂spiral arm)、 核球 (bu
5、lge) 、 银晕 (halo) 、银冕 (corona),2018/9/25,,12,2018/9/25,,13,构成：星族I恒星、气体和尘埃 直径：D 30 kpc 厚度：h 70-300 pc,银河系的银盘,2018/9/25,,14,银河系的旋臂结构,2018/9/25,,15,天鹅臂,英仙臂,猎户臂,人马臂,盾牌南十字臂,矩尺臂,2018/9/25,,16,核球 (bulge)与银心 (Galactic center),银心在人马座方向核球呈椭球形，大小 约 64 kpc，恒星分布十分密集，数密度 1,600 ly-3, 是银河系平均恒星密度的105倍,2018/9/25,,17,T
6、he center of the Galaxy in radio waves,2018/9/25,,18,银晕 (Galactic halo),球状星团 年老的星族II恒星、以银心为中心球状分布， 在椭圆轨道上绕银心旋转（V100 kms-1）， 离银心最远距离达100 kpc热气体弥漫的X射线辐射表明在银晕中存在大量的热气体暗物质 :银晕中的不可见物质质量远远超过银河系可见物质质量,2018/9/25,,19,银盘为pop.I恒星：年轻的、富金属恒星 (疏散星团) 银晕和核球中为星族II恒星：年老的、贫金属恒星,2018/9/25,,20,How do stars orbit in our
7、galaxy?,2018/9/25,,21,Stars in the disk all orbit in the same direction with a little up-and-down motion,2018/9/25,,22,Orbits of stars in the bulge and halo have random orientations,2018/9/25,,23,2018/9/25,,24,2018/9/25,,25,Thought Question,Why do orbits of bulge stars bob up and down(忽沉忽浮 )?A. They
8、re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk,2018/9/25,,26,Thought Question,Why do orbits of bulge stars bob up and down(忽沉忽浮 )?A. Theyre stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock
9、them back into disk,2018/9/25,,27,Suns orbital motion (radius and velocity) tells us mass within Suns orbit:1.0 x 1011 MSun,2018/9/25,,28,Orbital Velocity Law,The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass (Mr) within that orbit,2018/9/25,,2
10、9,What have we learned?,What does our galaxy look like? Our galaxy consists of a disk of stars and gas, with a bulge of stars at the center of the disk, surrounded by a large spherical haloHow do stars orbit in our galaxy? Stars in the disk orbit in circles going in the same direction with a little
11、up-and-down motion Orbits of halo and bulge stars have random orientations,2018/9/25,,30,How is gas recycled in our galaxy?,Star-gas-star cycle : recycles gas from old stars into new star systems,2018/9/25,,31,High-mass stars have strong stellar winds that blow bubbles of hot gas,2018/9/25,,32,Lower
12、 mass stars return gas to interstellar space through stellar winds and planetary nebulae,2018/9/25,,33,X-rays from hot gas in supernova remnants reveal newly-made heavy elements,2018/9/25,,34,Supernova remnant cools and begins to emit visible light as it expandsNew elements made by supernova mix int
13、o interstellar medium,2018/9/25,,35,Radio emission in supernova remnants is from particles accelerated to near light speedCosmic rays probably come from supernovae,2018/9/25,,36,Multiple supernovae create huge hot bubbles that can blow out of diskGas clouds cooling in the halo can rain back down on
14、disk,2018/9/25,,37,Molecular clouds in OrionComposition:Mostly H2About 28% HeAbout 1% COMany other molecules,Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protonsMolecular clouds form next, after gas cools enough to allow to atoms to combine into molecules,2018/9/25,,38,Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycleRadiation from newly formed stars is eroding these star-forming clouds,
