Chapter 7: The Sun – Our Star

General Properties 一般性质 Average star 普通恒星 Spectral type G2 光谱类型G2 Only appears so bright because it is so close. 太近，所以显得亮 109 times Earth’s diameter 直径是地球的100倍 333,000 times Earth’s mass 质量是地球的30万倍 Consists entirely of gas (av. density = 1.4 g/cm3) 全部由气体组成，平均密度每立方厘米1.4克 Central temperature = 15 million K 中心温度1500万度 Surface temperature = 5800 K 表面温度5800度

The Photosphere 光球层 The solar corona日冕 Apparent surface layer of the sun 太阳的视表面层（肉眼看见的太阳） Depth ≈ 500 km 深度500公里 Temperature ≈ 5800 K 温度5800度 Highly opaque (H- ions) 不透明 Absorbs and re-emits radiation produced in the solar interior 吸收并再发射中心产生的辐射（光子） The solar corona日冕

Energy Transport in the Photosphere 光球层的能量传输 Energy generated in the sun’s center must be transported outward. 在太阳中心产生的能量必须象外传输 In the photosphere, this happens through 在光球层，通过 Convection 对流: Cool gas sinking down 冷气体下沉 Bubbles of hot gas rising up 热气团上升 Bubbles last for ≈ 10 – 20 min. 热气团持续10-20分钟 ≈ 1000 km

Granulation米粒 … is the visible consequence of convection …是对流效应的可视结果

The Solar Atmosphere 太阳大气 光球层 色球层 日冕层 Only visible during solar eclipses 只有日食时能看见 热流 Apparent surface of the sun 可视的太阳表面 Heat Flow Temp. incr. inward 温度向内增加 Solar interior 太阳内部

The Chromosphere色球 Region of sun’s atmosphere just above the photosphere. Visible, UV紫外, and X-ray lines from highly ionized gases Temperature increases gradually from ≈ 4500 K to ≈ 10,000 K, then jumps to ≈ 1 million K Filaments Transition region Chromospheric structures visible in Ha emission (filtergram)

The Layers of the Solar Atmosphere 太阳大气分层 Sunspot Regions 黑子区域 Ultraviolet 紫外线 Visible 可见光 Photosphere 光球层 Corona 日冕 Chromosphere色球层 Coronal activity, seen in visible light 可见光的日冕活动

Coronal activity, seen in visible light 可见光的日冕活动（视频） Coronal activity, seen in visible light 可见光的日冕活动（视频） Sun.lasc_aug_99sm.mpg 物质抛射

Approx. 10 million wave patterns! Helioseismology 日震 The solar interior is opaque (i.e. it absorbs light) out to the photosphere. 太阳内部是不透明的 Only way to investigate solar interior is through helioseismology 唯一研究太阳内部的方法是通过日震 = analysis of vibration patterns visible on the solar surface 分析表面接受到的震动的模式 Approx. 10 million wave patterns! 有1000万种模式

Cooler regions of the photosphere (T ≈ 4240 K). 光球层相对较冷的区域 Sunspots太阳黑子 Cooler regions of the photosphere (T ≈ 4240 K). 光球层相对较冷的区域 Only appear dark against the bright sun. Would still be brighter than the full moon when placed on the night sky! 背景亮，显得暗。放在夜空比月亮还亮。

Sunspots and Magnetic Fields 黑子和磁场 Magnetic North Poles 磁北极 Magnetic South Poles 磁南极 紫外照片 磁场图 Magnetic field in sunspots is about 1000 times stronger than average. 太阳黑子的磁场比平均磁场强1000倍。 In sunspots, magnetic field lines emerge out of the photosphere. 太阳黑子中，磁力线从光球层发出

Magnetic Field Lines 磁力线 Magnetic North Pole磁北极 Magnetic South Pole 磁南极 Magnetic Field Lines 磁力线

Magnetic Fields in Sunspots 测量黑子的磁场 Magnetic fields on the photosphere can be measured through the Zeeman effect 通过Zeeman效应测量磁场  Sunspots are related to magnetic activity on the photosphere

Magnetic field lines 磁力线 Magnetic Loops 磁圈 Magnetic field lines 磁力线

Sunspots 太阳黑子磁场（视频） Sun.mag_sm.mpg

Solar Activity 太阳活动 Observations at ultraviolet and X-ray wavelengths reveal that sunspots are regions of enhanced activity 黑子是太阳活动增强区.

The Solar Cycle太阳（活动）周期 11-year cycle 11年周期 => Total solar cycle = 22 years Reversal of magnetic polarity磁极反转 The Maunder butterfly diagram 蝴蝶图 Sunspot cycle starts out with spots at higher latitudes on the sun Evolve to lower latitudes (towards the equator) throughout the cycle. 太阳黑子周期从太阳高纬度向低纬度（赤道）演化。

The Sun’s Magnetic Dynamo” 太阳的磁力发动机” The sun rotates faster at the equator than near the poles. 太阳赤道比极区转动快 This differential rotation might be responsible for magnetic activity of the sun. 这种较差转动引起太阳磁场活动

The Sun’s Magnetic Cycle磁场周期 After 11 years, the magnetic field pattern becomes so complex that the field structure is re-arranged. 11年后，磁场形态太复杂，磁场结构需要重组。  New magnetic field structure is similar to the original one, but reversed! 新磁场结构和以前的类似，但极性反转。  New 11-year cycle starts with reversed magnetic-field orientation 新的11年周期始于反转的磁场方向。

Prominences 日珥 Looped prominences: gas ejected from the sun’s photosphere, flowing along magnetic loops 环形日珥：气体从光球层发出，沿磁力线运动

Eruptive Prominences 爆发日珥 Eruptive Prominences 爆发日珥 (紫外图像) Extreme events (solar flares) can significantly influence Earth’s magnetic field structure and cause northern lights (aurora borealis). 极端的事件（日珥、耀斑）可以严重影响地球的磁场结构，引起极光。

Sound waves produced by a solar flare ~ 5 minutes Solar Aurora Sound waves produced by a solar flare Coronal 日冕mass ejections

日面活动（视频）

太阳爆发会影响到人类生存空间 太阳活动影响人类活动空间

中山站─极光

中山站─极光

中山站─极光

X-ray images of the sun reveal coronal holes. X-射线图像显示日冕洞 X-ray images of the sun reveal coronal holes. X-射线图像显示日冕洞 These arise at the foot points of open field lines and are the origin of the solar wind 这些在断开的磁力线处产生，是太阳风的发源地。

Energy Production能量的产生 Energy generation in the sun (and all other stars):太阳和其他恒星能源的产生都是通过： 氢 核能量 nuclear fusion 核聚变 聚变 = fusing together 2 or more lighter nuclei to produce heavier ones. 聚合2个或更多轻的原子核为重原子核。 Nuclear fusion can produce energy up to the production of iron; 核聚变只能到铁元素 裂变 For elements heavier than iron, energy is gained by nuclear fission 重于铁的元素通过核裂变产生能量。

4 个质子比1个4He多的质量是： 0.048*10-27 kg (= 0.7 %) Energy generation in the Sun (E=mc2): The Proton-Proton Chain 太阳的能量产生：质子-质子链 Need large proton speed ( high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). 需要高速（即高温）的质子碰撞克服电磁斥力，才能起反应。 Basic reaction: 4 1H  4He + 能量 温度 T ≥ 107 K = 1000万度 4 个质子比1个4He多的质量是： 0.048*10-27 kg (= 0.7 %) 获得的能量 = m*c2 = 0.43*10-11 J （每个核反应） Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity. 太阳每秒需要1038个反应，转换500万吨物质为能量，才能克服自身引力。