2.1 电磁辐射

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  电磁辐射

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  A key  step in  considering the question of life on planets is to understand their energy sources, including energy from the host stars.  The energy from stars is powered by nucleosynthesis,  the fusion of light elements to form heavier elements (e.g., hydrogen fusion to form helium).  A by-product of fusion is the emission of electromagnetic waves, or photon packets of energy.  We  want to understand how electromagnetic  energy varies from star to star,  to ultimately understand the impact for  life on a planet. In other words, to know the planet, we need to know the star. 
  ::考虑行星生命问题的一个关键步骤是了解它们的能源来源,包括来自宿主恒星的能量。恒星的能量是由核子合成、光元素融合形成较重元素(例如氢聚变形成氦)组成的。聚变的副产品是电磁波或光子能量包的释放。我们希望理解电磁能量如何因恒星而异,最终理解地球生命的影响。换句话说,为了了解地球,我们需要了解恒星。

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  Background: the Electromagnetic Spectrum
  ::背景:电磁频谱

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  You may know  that a changing electric field will produce a magnetic field. A popular science demonstration in the elementary grades is to in a spiral fashion with a wire. When the ends of the wires are connected to the positive and negative poles of a battery, a magnetic field is created. It is critical to wind the wire because that produces a changing direction for the current. 
  ::你也许知道变换电场会产生磁场。 小学一年级的流行科学演示是用电线以螺旋式的方式进行。 当电线的末端连接到电池的正极和负极时, 磁场就会被创建。 给电线吹风至关重要, 因为电线会改变电流的方向 。

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  An electromagnetic ( EM ) wave consists of two perpendicular fields: an electric field and a magnetic field, as shown in the   below. The changing electric field  spawns  a changing magnetic field; the changing magnetic field spawns a changing electric field, and voila - the system self-propagates, sailing through space like a thought without a thinker. The electric and magnetic fields are themselves perpendicular to the direction that the wave travels. 
  ::电磁(EM)波由两个垂直场组成:电场和磁场,如下文所示。变电场产生变化的磁场;变电磁场产生变化的电场;变电磁场产生变化的电场;变电场产生变化的电场;变电磁场——系统自我推进,像没有思想的思维一样在空间中航行。电磁场本身与波所穿行的方向是垂直的。

  

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  Perpendicular electric field and magnetic fields travel as locked sine waves at the speed of light.
  ::垂直电场和磁场以光速以锁定的正弦波进行飞行。

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  The wavelength,  $\begin{align*}\lambda\end{align*}$ , of the EM wave is measured as the distance from peak to peak. Visible light is one type of electromagnetic radiation with wavelengths that range from about 400 to 700 nanometers (blue and red light respectively)  and is part of the . The energy of EM radiation scales inversely with wavelength: the shorter the wavelength, the higher the energy.  The frequency $\begin{align*}\nu,\end{align*}$ of EM radiation tells you how many waves go by a particular point each second; the longer the wavelength, the shorter the frequency.
  ::EM波的波长, , 以从峰值到峰值的距离测量。 可见光是波长大约400至700纳米( 蓝色和红色灯光)的电磁辐射的一种类型, 并且是. EM 辐射尺度的能量与波长呈反向变化: 波长越短, 能量越高。 EM 辐射的频率, 越高, 显示波的频率越短, 频率越短。 EM 辐射的频率, 告诉你波长越长, 频率越短。

  

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  This diagram of the E-M spectrum shows the wavelength ranges for different types of radiation.
  ::E-M频谱的图示显示了不同类型辐射的波长范围。

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  One constant is that  all electromagnetic  waves travel at the speed of light (c = $\begin{align*}3\times {10}^{8}\end{align*}$ m/s). All EM radiation obeys  the mathematical relationship - wavelength times frequency equals the speed of light: 
  ::一个常数是所有电磁波以光速(c = 3x108 m/s)移动。所有EM辐射都符合数学关系-波长时间频率等于光速:

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               $\begin{align*}\lambda\, \nu = c\end{align*}$     (Eqn 1)
  
  ::c(第1部分)

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  The only difference between red light and blue light is the wavelength (and consequently the energy) of the EM wave.  Our brains are tuned to distinguish different wavelengths of light as a  phenomenon that we call "color."  Likewise, the only difference between radio waves, visible light and X rays is wavelength and hence the energy of the EM wave. Given the broad wavelength range of the EM spectrum, it would seem that our eyes are not very good detectors. Through natural selection and human evolution, the wavelengths of energy that we call visible light coincide with the peak energy emitted from the surface of the Sun and the energy that passes through our atmosphere. 
  ::红光和蓝光的唯一区别是EM波的波长( 以及随之而来的能量) 。 我们的大脑被调整以区分不同波长的光作为我们称之为“彩色”的现象。 同样,无线电波、可见光和X光之间唯一的区别是波长,因此是EM波的能量。鉴于EM频谱的波长范围很广,我们的眼睛似乎不是非常好的探测器。通过自然选择和人类进化,我们称之为可见光的能量波长与太阳表面释放的峰值能量和穿过大气层的能量相吻合。

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  What is the source of energy in the Sun? Nuclear fusion reactions in the cores of stars  emit very high energy EM photons - typically gamma rays. The gamma ray photons would travel forever except that the cores of stars are very dense and  they interact with that material. Sometimes the gamma ray is scattered and sometimes it is absorbed and re-emitted.  Energy must be conserved; however, almost every interaction of EM radiation with matter (the electrons and atoms in the cores of stars) will result in a loss of energy for the incoming photon.   Some of that energy can go to accelerating electrons. Or after absorption of the photon by an atom, the higher energy photon ( $\begin{align*}{E}_{in}\end{align*}$ ) can be emitted as two lower energy photons.  Because of   the interaction of energy (high energy gamma rays from the core of the star) with matter, gamma rays ultimately emerge as  lower energy light  from the surface of the star. In the case of the Sun, the distribution of emerging EM radiation peaks at visible wavelengths. 
  ::太阳的能源来源是什么? 太阳的能量源是什么? 恒星核心中的核聚变反应会释放出非常高的能量 EM光子 -- -- 典型的伽马射线射线光子。伽马射线光子将永远移动,除非恒星核心非常稠密,它们与该材料相互作用。 有时伽马射线是分散的,有时是吸收和再排放的。 能源必须加以保护; 然而,电离线辐射与物质(恒星核心中的电子和原子)的几乎每一次相互作用都会导致射入光子的能量损失。 其中一些能量可以用于加速电子。 或者在原子吸收光子之后, 高能量光子(Ein)可以作为两个较低的能量光子。 由于能量(来自恒星核心的高能量伽马射线)与物质之间的相互作用,伽马射线最终会作为来自恒表面的低能量光线出现。 在太阳的例子中, 正在形成的电离子波长线的分布。

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  There are several of the of EM radiation and you should read through these if you need some additional background material or a refresher. NASA also maintains an  nice site  describing the . 
  ::有几种EM辐射,如果需要额外的背景材料或复习器,您应该阅读这些辐射。美国航天局还维持一个描述该辐射的不错的网站。

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  Thermodynamics and blackbody radiation 
  ::热动力学和黑体辐射

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  If an object is warmer than its surroundings, it will try to lose that extra energy and come into thermodynamic equilibrium with its surroundings. A glowing ember of coal cools by radiating energy until it reaches the same temperature as its environment. Ice melts because the energy of the warmer surrounding air  raises the temperature of the ice. The laws of thermodynamics are strictly enforced, and stars are one of nature's most powerful energy sources. An object in thermal equilibrium (by definition) is not warming up or cooling down - it maintains a constant temperature. 
  ::如果一个物体比周围更温暖, 它会试图失去额外的能量, 并进入与其周围环境的热力平衡。 燃热的煤炭会通过辐射能量冷却, 直到它达到与其环境相同的温度。 冰会融化, 因为周围温暖的空气能提高冰的温度。 热力定律得到严格执行, 恒星是自然中最强大的能源来源之一。 热平衡中的物体( 定义) 并没有升温或冷却, 它会保持恒温 。

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  Hypothetical bodies  in thermal equilibrium are historically called "black bodies" because  they do not reflect any light. However, a blackbody will emit a spectrum of light. For example, humans emit light at infrared wavelengths ("heat"). With this simplistic assumption, stars can be considered to approximate a black body: they are in thermal equilibrium, they do not reflect light; they emit a spectrum of light that is reasonably well-described by  Planck's equation below for a black body : 
  ::热平衡中的假体历来被称为“ 黑体”, 因为它们不反映任何光。 但是, 黑体会释放出光谱。 例如, 人类会以红外波长( “ 热量 ” ) 发出光。 有了这种简单化的假设, 恒星可以被视为接近黑体: 它们处于热平衡, 它们不反映光; 它们释放出一种光谱, 在普朗克的公式下面对黑体作了合理的描述 :

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                  $\begin{align*}{B}_{\lambda}=\frac{8\pi hc }{\lambda^5 ({e}^{\frac{hc}{\lambda k T}}-1)}\end{align*}$     (Eqn 2)
  ::B8hc5 (ehckT-1) (Eqn 2)

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  In equation (2): 
  ::在等式(2)中:

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  • $\begin{align*}B_\lambda\end{align*}$  is the energy of the body at various wavelengths
    ::B是身体的能量 以不同波长。
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  • $\begin{align*}\lambda\end{align*}$  is the wavelength 
    ::是波长
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  • $\begin{align*}h = 6.626 \times 10^{-34} \ {\rm m^2 kg / s}\end{align*}$  is Plancks constant
    ::h=6.626x10-34平方米/秒为普朗克常数
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  • $\begin{align*}k=1.3806 \times 10^{-23} \ {\rm m^2 kg \ s^{-2} K^{-1}}\end{align*}$  is the Boltzmann constant
    ::k=1.3806×10-23 m2kg s-2K-1是波尔兹曼常数
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  • $\begin{align*}c=2.99 \times 10^8 \ {\rm m / s}\end{align*}$  is the speed of light
    ::c=2.99×108米/秒是光速
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  • $\begin{align*}T\end{align*}$  is the temperature of the object
    ::T 是对象的温度

  

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  Three blackbody curves have been calculated using equation (2). The top curve assumes a temperature of 7000 K. The middle curve is 6000 K. The bottom curve is 5000 K.
  ::使用方程式(2)计算出三个黑体曲线。顶部曲线假定温度为7000K。中间曲线为6000K。底部曲线为5000K。

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  Using equation (2), have been calculated for three temperatures: 7000K, 6000K, and 5000K. For reference, the Sun has a temperature of about 5800 K, so these are energy distributions representative of a slightly warmer (more massive) star than the Sun and a slightly cooler (less massive) star.   There are a few things to notice about these energy distributions:
  ::使用方程式(2)计算出三种温度:7000K、6000K和5000K。例如,太阳的温度约为5800K,这些能量分布代表了比太阳略为温暖(大得多)的恒星和略为冷却(小于)的恒星。

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  1.  All of these bodies emit EM radiation with a distribution of photon energies, not photons with a single energy. 
     ::所有这些机构都通过光子能量的分布发射EM辐射,而不是单一能量的光子。
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  2. The hottest object (blue curve) has the most intense signal and the peak of that curve is shifted to slightly shorter wavelengths.
     ::最热的物体(蓝色曲线)有最强烈的信号,该曲线的峰值被移动到略微短的波长。
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  3. The other curves are all nested under the blue curve.  That is, the intensity of energy (flux is the luminosity per unit area) emitted by the 6000 K object (gold curve) is less than the intensity of the 7000 K object and the intensity of the 5000 K object (red curve) is lower than the two hotter objects. This calculation  implicitly assumes that all 3 objects  are  the same size. However, if the cooler  object is an evolved red giant star, it will have an expanded radius - in this case, the shape of the curve will be the same as indicated by Eqn (2), but the intensity will  scale up  and could be greater than the flux of the hotter stars because the size of the star is larger. 
     ::其它曲线都在蓝色曲线下嵌套。 也就是说, 由 6000 K 对象( 黄金曲线) 排放的能量强度( 流量是单位面积的光度) 低于 7000 K 对象的强度, 而 5000 K 对象( 红曲线) 的强度低于 两个热点对象 。 这个计算暗含地假设所有 3 个对象的大小相同 。 但是, 如果冷却对象是一个进化的红巨星, 它的半径将会扩大 - 在这种情况下, 曲线的形状将与 Eqn (2) 所显示的相同, 但是由于恒星的大小较大, 其强度将会扩大, 并且可能大于热点恒星的流量 。
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  4. The numbers at the peak of each curve indicate the wavelength where the peak flux occurs. The hotter star  emits  the maximum amount of  energy at bluer wavelengths than the cooler stars. 
     ::每个曲线峰值的数值表示峰值通量发生的波长。热点恒星以比冷点恒星更低的波长释放出最大能量。

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  All objects with thermal energy radiate a spectrum of energy with different intensity (strength) at different wavelengths given by equation (2). The is determined by the temperature of the blackbody.  
  ::所有具有热能的物体都以方程(2)给出的不同波长以不同强度(强度)以不同强度(强度)向能量频谱辐射。由黑体的温度决定。

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  Wien's Law
  ::《维恩法》

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  Wien's law relates the  characteristic temperature of a blackbody to the wavelength   where the peak energy is emitted: 
  ::Wien的法律将黑体的典型温度与排放峰值能量的波长联系起来:

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  $\begin{align*}\lambda_{max} = \frac{2.898 \times 10^6 [K \cdot nm]}{T [K]}\end{align*}$      (Eqn 3)
  ::* max= 2. 898×106 [Knm] T[K] (Eqn 3)

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  E quation (3) says that if you know the temperature of an object, you can predict the peak wavelength (or color) of the object. Conversely, if you measure the peak wavelength, you can derive the blackbody temperature of the object with Wien's law. 
  ::方程式(3) 表示, 如果您知道对象的温度, 您可以预测对象的峰值波长( 或颜色) 。 相反, 如果您测量了峰值波长, 您可以用 Wien 的定律得出对象的黑体温度 。

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  Flux and Luminosity
  ::奢侈和奢华

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  Luminosity is the total amount of energy that an object (like a star) puts out each second. It has dimensional units of energy per second.  In the same way that a 100 W bulb will always put out 100 Watts whether we are standing close or farther away, the luminosity of a star does not depend on our distance from it.
  ::光度是物体(如恒星)每秒释放的能量总量。 它每秒有维维能量单位。 同样, 100 W 灯泡总是会释放100瓦, 不论我们距离是否近, 恒星的光度并不取决于我们距离它远。

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  However, astronomers do not measure luminosity; they measure flux: the luminosity that is intercepted by a detector such as a photographic plate or a digital camera. If you imagine a spherical surface - a bubble - around a star, then the luminosity is the integrated (total) light from the surface of that bubble. The flux, which is the luminosity per unit area, decreases as the surface area of the spherical volume increases. This is the same phenomenon that happens with expanding balloons. The balloon has a certain amount of material, usually latex or rubber. Analogous to luminosity, that amount of material is constant, no matter how much air is in the balloon. However, as the balloon expands, that constant amount of material is stretched over a larger surface area. The walls of the balloon get thinner and the amount of material per unit area decreases. The luminosity of a star is constant. The flux that we measure depends on whether we are "up close" or far away from the star. 
  ::然而,天文学家并不测量光度;它们测量通量:光度被摄影板或数字相机等探测器截获的光度。如果你想象一颗恒星周围的球面表面——一个气泡,那么光度就是该气泡表面的集成(总)光。通量,即每个单位面积的光度,随着球体体体积面积的增加而下降。这是气球膨胀时发生的同样现象。气球有一定数量的材料,通常是乳胶或橡胶。对光度的比较,这种材料的数量是恒定的,无论气球中有多少空气。然而,随着气球的膨胀,这种恒定量的量会伸展到更大的表面面积。气球的壁会变薄,每单位面积的物质数量会下降。恒星的光度是恒定的。我们测量的通量取决于我们是否“接近”还是远离恒星。

  

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  The luminosity of a star is constant. However the intercepted flux depends on our distance from the source.
  ::恒星的光度是恒定的。 但是被截取的通量取决于我们与源的距离。

   

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  Stefan-Boltzmann Law
  ::Stefan-Boltzmann 法律

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  An object in thermal equilibrium maintains a constant temperature - it neither warms up nor cools down. The Stefan-Boltzmann law  relates the  flux that is radiated by a blackbody to the equilibrium temperature of that body. 
  ::热平衡中的物体保持恒定温度 - 它既不暖和也不冷却。 Stefan-Boltzmann 法律将黑体辐射的通量与身体的平衡温度联系起来。

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  $\begin{align*}F=\sigma \ T^4 \ {\rm J \ m^{-2} s^{-1}}\end{align*}$    (Eqn 4) 
  ::T4 J m-2s-1 (Eqn 4)

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  In this equation: 
  ::在这一方程式中:

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  • $\begin{align*}F\end{align*}$  is the radiated energy per unit area per unit time (the flux)
    ::F是每单位时间(通量)单位面积的辐射能。
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  • $\begin{align*}\sigma = 5.67 \times 10^{-8} \ {\rm J \ s^{-1} m^{-2} \ K^{-4}}\end{align*}$  is the Stefan-Boltzmann constant
    ::5.67×10-8J s-1m-2K-4是Stefan-Boltzmann常数
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  • $\begin{align*}T\end{align*}$  is the equilibrium temperature of the object in degrees Kelvin; 
    ::T 是对象在开尔文度的平衡温度;

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  W e can relate the luminosity of a star to the flux and therefore to the temperature of the star at a particular distance, r  : 
  ::我们可以将恒星的光度与通量联系起来,因此与恒星在特定距离的温度联系起来,r:

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    $\begin{align*}L=F 4 \pi r^2=\sigma T^4 4 \pi r^2\end{align*}$       (Eqn 5)
  ::L=F4r2T44r2(Eqn 5)

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  We get to choose "r" for the particular situation we are investigating.  We might want "r" to be the radius of the star - in this case we would be defining the temperature at the surface of the star. Or we might want "r" to be the orbit of the Earth - in this case, we would be solving for the temperature from solar radiation at the distance of the Earth. In both of those cases ( r = the radius of the star;  r = the orbit of the Earth), if we integrate (or "add up") the flux on the surface of a sphere at that distance, the total equals the luminosity of the star. 
  ::对于我们所调查的特定情况,我们可以选择“r”作为“r”作为恒星的半径。我们也许希望“r”作为恒星的半径——在这种情况下,我们将确定恒星表面的温度。或者我们可能希望“r”作为地球的轨道——在这种情况下,我们将解决地球距离的太阳辐射的温度问题。在这两种情况下(r =恒星的半径;r =地球的轨道),如果我们将(或“加载”)星体表面的通量整合到(或“加载”该距离的球体表面,那么总和等于恒星的光度。

   

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     Luminosity, flux, temperature and radius
  ::亮度、通量、温度和半径

  Equation (5) tells us that if the temperature of the star doubles, the luminosity increases by a factor of 16 (2*2*2*2) . If the radius of a star doubles but the temperature stays the same, the luminosity increases by a factor of four.  If the radius increases by a factor of two and the temperature decreases by a factor of two, how does the luminosity of the star change according to equation 5?