The Formation and Structure of Stars Chapter 9 The Formation and Structure of Stars 恒星的形成和结构

The Interstellar Medium (ISM) 星际介质 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 Various Appearances of the ISM 星际介质的一些表象

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 NGC 2246 The Trifid Nebula

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

3) Dark Nebulae 暗星云 致密气体和尘埃吸收背后星光 Barnard 86 Horsehead Nebula Dense clouds of gas and dust absorb the light from the stars behind; 致密气体和尘埃吸收背后星光 appear dark in front of the brighter background; 在明亮的背景前显得暗 Barnard 86 Horsehead Nebula

Interstellar Reddening 星际红化 Blue light is strongly scattered and absorbed by interstellar clouds 蓝光被散射和吸收更厉害 Red light can more easily penetrate the cloud, but is still absorbed to some extent 红光被吸收，但仍能穿透 Infrared radiation is hardly absorbed at all 红外光被很少吸收 Barnard 68 Interstellar clouds make background stars appear redder星际云使背景星红化（偏红） Infrared 红外 Visible可见光

Structure of the ISM 星际介质结构 The ISM occurs in two main types of clouds: 星际介质主要有两类： HI clouds 中性氢云（氢原子）: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); 冷氢云(T ~ 100 K) moderate density (n ~ 10 – a few hundred atoms/cm3); 中等密度（每立方厘米几百个原子） 尺度: ~ 100 pc Hot interstellar medium 热星际介质（氢原子核）: Hot (T ~ a few 1000 K), ionized hydrogen (HII) 热电离氢; low density (n ~ 0.1 atom/cm3); 低密度（每立方米0.1个原子）

The Orion Nebula: An Active Star-Forming Region 恒星形成区

The Trapezium The 4 trapezium stars: Brightest, very young (less than 2 million years old) stars in the central region of the Orion nebula Infrared image: ~ 50 very young, cool, low-mass stars X-ray image: ~ 1000 very young, hot stars The Orion Nebula 猎户座星云

Shocks Triggering Star Formation冲激波激发恒星形成 Henize 206 (infrared)

The Contraction of a Protostar原恒星收缩

From Protostars to Stars 原恒星到恒星（恒星的定义） Star emerges from the enshrouding dust cocoon 恒星穿出尘埃茧 Ignition of H  He fusion processes 核反应点火

Evidence of Star Formation 恒星形成证据 Nebula around S Monocerotis: 星云里 Contains many massive, very young stars, 包含很多大质量、年轻恒星

Contracting to form protostars 正在收缩形成原恒星 Globules（小球、团） Bok globules:（背景亮） ~ 10 – 1000 太阳质量; Contracting to form protostars 正在收缩形成原恒星

Evaporating gaseous globules (“EGGs”): 正在形成的恒星被显露出来。 Evaporating gaseous globules (“EGGs”): 正在形成的恒星被显露出来。

The Source of Stellar Energy 恒星能源 和太阳一样: Stars produce energy by nuclear fusion of hydrogen into helium. 氢聚变成氦产生能量。 In the sun, this happens primarily through the proton-proton (PP) chain 太阳上，主要反应是质子－质子链

The CNO Cycle 碳氮氧循环 the CNO cycle. In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over:稍大质量恒星里更 有效的核反应是碳-氮-氧循环 the CNO cycle.

Fusion into Heavier Elements Fusion into heavier elements than C, O: 聚变产生更重元素 requires very high temperatures; occurs only in very massive stars (more than 8 solar masses). 需要非常高温、只在大质量恒星产生（8个太阳质量）

Hydrostatic Equilibrium 流体静态平衡 想像恒星内部是一些壳层 在每一层，有两个力必须平衡: Gravity, i.e. the weight from all layers above 重力，即所有层的重量 Outward pressure from the interior 向外的压力

Hydrostatic Equilibrium 流体静态平衡 向外的压力必须刚好平衡所有上层的重力。 这个条件唯一决定了恒星内部的结构。 所以，在赫罗图上（H-R diagram），稳定的恒星只分布在很窄的带上（主序）。

Energy Transport 能量传输 内层：辐射传输 外层：对流 Energy generated in the star’s center must be transported to the surface. 恒星中心产生的能量必须向表面传输 Inner layers of the sun: Radiative energy transport 内层：辐射传输 Outer layers of the sun (including photosphere): Convection 外层：对流

Stellar Structure 恒星结构 Energy transport via convection 能量对流传输 Sun Energy transport via radiation 能量辐射传输 Flow of energy Energy generation via nuclear fusion 核聚变产生能量 Basically the same structure for all stars with approx. 1 solar mass or less. 一个太阳质量的恒星结构基本都这样 Temperature, density and pressure decreasing 温度、密度、压力降低

一个恒星的质量（和化学组成）完全决定了它的性质。 Stellar Models恒星模型 The structure and evolution of a star is determined by the laws of 恒星的结构和演化由下面的物理规律决定 流体静态平衡 能量传输 质量守恒 能量守恒 一个恒星的质量（和化学组成）完全决定了它的性质。 That’s why stars initially all line up along the main sequence. 所以，所有恒星最初都在主序上。

The Life of Main-Sequence Stars 主序星的生命 Stars gradually exhaust their hydrogen fuel. 核心氢燃料逐渐耗尽 In this process of aging, they are gradually becoming brighter, evolving off the zero-age main sequence. 演化过程中，逐渐变亮一点

The Lifetimes of Stars on the Main Sequence 主序星的寿命 在主序上时间（年） 光谱型 质量 光度