6河外星系-3

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Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:
1.0 x 1011
MSun
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Orbital Velocity Law
rv Mr G

2
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
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How do stars orbit in our galaxy?


Stars in the disk orbit in circles going in the same direction with a little up-and-down motion Orbits of halo and bulge stars have random orientations
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Thought Question
Why do orbits of bulge stars bob up and down(忽沉忽浮 )?
A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk
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Multiple supernovae create huge hot bubbles that can blow out of disk
Gas clouds cooling in the halo can rain back down on disk



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
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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
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银河系的银盘
构成：星族I恒星、气体和尘埃 直径：D ~ 30 kpc 厚度：h ~70-300 pc
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银河系的旋臂结构
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人马臂 猎户臂 盾牌－南十字臂 矩尺臂

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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 halo
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Orbits of stars in the bulge and halo have random orientations
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How do stars orbit in our galaxy?
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Stars in the disk all orbit in the same direction with a little up-and-down motion
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Star-gas-star cycle : recycles gas from old stars into new star systems
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How is gas recycled in our galaxy?
第六章 河外星系
/~xkong/introast 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
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LEARNING 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 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.
High-mass stars have strong stellar winds that blow bubbles of hot gas
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Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae
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We see our galaxy edge-on Primary features: disk, bulge, halo, globular clusters
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If we could view the Milky Way from above the disk, we wouldxkong@ see its spiral arms 2013-7-26 10
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The center of the Galaxy in radio waves
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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 （V~100 kms-1）， 离银心最远距离达100 kpc
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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
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The Milky Way 银河系
There’s no place like home！
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Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules
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Radio emission in supernova remnants is from particles accelerated to near light speed
Cosmic rays probably come from supernovae
英仙臂
天鹅臂
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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 6×4 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

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Molecular clouds in Orion
Composition: • Mostly H2 • About 28% He • About 1% CO • Many other molecules
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Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle Radiation from newly formed stars is eroding these starforming clouds
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Thought Question
Why do orbits of bulge stars bob up and down(忽沉忽浮 )?
A. They’re stuck to interstellar medium B. Gravity of disk stars pulls toward disk C. Halo stars knock them back into disk
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What does our galaxy look like?
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The Milky Way galaxy appears in our sky as a faint band of light
银河系结构

～2×1011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 (bulge) 、 银晕 (halo) 、银冕 (corona)
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Dusty gas clouds obscure our view because they absorb visible light
This is the
interstellar medium that
makes new star systems
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X-rays from hot gas in supernova remnants reveal newlymade heavy elements
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Supernova remnant cools and begins to emit visible light as it expands New elements made by supernova mix into interstellar medium