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第三篇似乎就是红移那一篇诶
宇宙颗粒（1）
PI :现象 天文学家(observer)观测发现，宇宙中星体与星体之间并不只有气体原子(atom of gas),还有很 多颗粒 (dust grain)。颗粒(dust)比气体(atoms)稀薄(thinly)、分布不平均(without uniformity)、 分散在宇宙中 (sparse/spread/dispersed in the universe)。这些颗粒(dust grain)阻挡了远处星 球传播到地球的光/远 处星球发出的光在到达地球前必须穿过大量的颗粒(before they reach the earth)。由于这些颗粒吸收短 波/蓝波，因此使得地球上的我们观测到的那些星球比实际更暗 (dimmer)、更红(redder)/颗粒(dust)是影 响人类观察到星星亮度不同的原因。天文学家 Tumbler因此得到结论：颗粒的大小(size)比红波(red wave) 小，比蓝波(blue wave)长。 P2 :解释 远处星体发出的光是不同波长的。红色波长较长，蓝色波长较短。波长较长的光受宇宙颗粒影响 较小，如 同大海(ocean)中的波浪，如果波浪很大(wave length)而遇到很小的礁石(rock)，不会 改变原有的波动 (Q1)。由于到达地球的蓝色短波较少，可以证明宇宙中的颗粒大小与蓝光波长差 不多，因此才会阻碍大量 蓝色短波穿过宇宙(interstellar space)到达地球。
Q1 :星际间的gas相比dust grain有什么特点？ 答：气体微粒更聚集(more densely distributed)。正碌。【740 Q51 V39】【 700】 理由：文章中说的是dust比gas更散。对文中thinly那句的改写。
Q2 :科学家T怎么注意到dust的特性的？ /文章中的某某科学家是根据什么来判断dust grain 的？ 备选：通过观察和分析远处星球(certain stars)到达地球的光线(the appearance of stars from earth)。 【760】【 V40】【 700】
Q3 :提到大海波浪例子的目的。 备选：类比(analogy)说明。【740】【 730】
Q4 :主旨。 选项有： 1、 解释一个现象。【640Q50V27】 2、 解释一个发现。 3、 解释xx的原因。 4、 介绍两个假设(hypothesis)。 5、 比较两个现象。 6、 （错）不同科学家的解释。 7、 选：【700 Q50V34】 不选理由：只有一个科学家的观点。
Q5 :天文学家T研究dust时观察了什么？
备选：dust粒子会使我们在地球上观测到的星星比实际的暗而且红。
Q6 :第二段的作用。
Q7 :提到T的observation ,问以下选项中正确的是？ 备选：dust size如何如何。【700 Q50 V34】
Q8 :提到文章开头另一种物质(atom of gas),问以下选项中正确的是？
参考资料1: Robert Trumpler was measuring the distances to bright clusters of young stars in the spiral arms of our Galaxy as a way of mapping out its shape, and he used two independent methods to do so. He discovered that the more remote clusters were systematically dimmer than might be expected from their distance, showing that there was an intervening screen of material that obscured some of the light. The dust mixed in with the interstellar gas thus obscures and dims the light of distant objects, an effect we call extinction. This can confuse an observer into overestimating distances based on the luminosity alone. Within a spiral galaxy, the dust is particularly concentrated to the plane of the flat disc;indeed, 27 when such a system is viewed edge-on to our line-of-sight, the dark dust lanes can appear to divide a galaxy almost into two. The dust clouds thus complicate observations of the most active and interesting regions of massive star formation in the spiral arms. Additionally, as we ourselves live within the disc of the Milky Way, the dust in the disc completely blocks our view of the centre of the Galaxy, rendering it very difficult for study. Reddening/scattering by dust From his observations of clusters, Trumpler also realised that the intervening dust doesn’ t only extinguish the light of the distant stars, but it also affects the colour of that light. The dust particles selectively diminish the bluer wavelengths in a process known as Rayleigh scattering. Photons will bounce off particles in their path - whether atoms, molecules or small clumps of molecules - to be scattered away from their original direction. The consequence is that some of the photons that were travelling from a star towards us are then redirected away and no longer reach us. This scattering process doesn' t happen evenly to all colours, however, as light can only be scattered by particles of a similar or larger size than its wavelength. The tiny size of a typical interstellar dust grain (below half a micron in diameter) means they scatter the shorter, bluer wavelengths most efficiently. Consequently more blue light than red is removed from the star' s light as it travels through a dusty cloud, making the starlight that emerges appear redder. The more dust along the line of sight to an object, the dimmer and redder it will appear. The situation is further complicated by the way that dust is only patchily distributed across the sky, and so it is not always straightforward to determine whether extinction and reddening effects are due to a larger volume of intervening space or just a particularly dense foreground dust cloud. The reddening is a subtle effect that isn' t apparent with the unaided eye - it requires telescope images to be detected. [It should also not be confused with the Doppler effect of the reddening of a spectrum due to an emitting object receding from us at speed.] Link: http://www.gresham.ac.uk/lectures-and-events/star-dust 参考资料2: The nongaseous interstellar matter exists in the form of tiny solid particles called interstellar grains or dust. The grains are believed to be elongated in shape, and aligned with the magnetic field; they are believed to contain graphite or silicate material as wel as polycyclic aromatic hydrocarbons. The clouds obscure the view of the galaxy in certain
directions, particularly in the direction of the galactic center. They polarize and selectively scatter the starlight passing through them; blue light is scattered more than red light so that stars partially obscured by interstellar matter appear redder than their true color. Since the distances and intrinsic luminosities of many stars are estimated from analysis of their spectra, this effect, called interstellar reddening, has been responsible for errors in calculating the distances and luminosities of these stars. Dust exists in diffuse form throughout the interstellar medium. In this diffuse form each dust grain typically occupies the volume of a cube the length of a football field on each side (one million cubic meters). We detect this diffuse interstellar dust by the extinction and reddening of starlight. The dust grains block starlight, creating extinction, and they also preferentially block blue light over red light, causing reddening. Stars therefore appear redder in color than they otherwise would. This extinction and reddening is similar to the effect that makes sunsets red, especially over a smoggy city. We can see dust grains more directly in dense regions, that is, in interstellar clouds. Two types of clouds showing the effects of dust are dark clouds and reflection nebulae. We see dark clouds by their effect on background stars. They block the light from stars behind the cloud, so we see a region of the sky with very few stars. Reflection nebulae are dust clouds located near a star or stars. They shine with reflected light from the nearby stars, and are blue in color because the grains selectively reflect blue light 参考资料3: 可见光光谱(红橙黄绿蓝靛紫),从蓝光到红光,波长依次增加，波长范围约 350nm-800nm。由于作 者举了 个"大海"的例子，把光的波动性和粒子性给揉到一起了。我们可以把大海理解成"一堆波 浪"的组合,再想简 单点儿，大海就七条线儿，红橙黄绿蓝靛紫七条线儿,红线波长最大，就想象成曲线, 蓝线波长最小，直接 推到极限，想象成直线。此时，一个石头挡在大海的必经之路上，蓝线咣当一下就 被挡住了 (不论是吸收， 衍射,反射还是散射，反正是被挡住了。）红线绕过去了。于是我们看到就是 发红的"大海〃了。 |
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发表于 2018-10-15 19:57:59
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