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貌似是14年7月的寂静，如果有乱码可以去论坛2003至今的寂静总结帖
P1 一个 observer 发现宇宙中星体与星体之间除了气体原子(atom of gas)组成的星云之外,更多的是颗粒(dust grain),dust 比 atoms 更 thinly,它们分散在宇宙里,比起相对集中的星云更分散,withoutuniformity。dust
比 atoms 更 thinlydispersed in the universe。有些星球发出的光必须穿过大量的这些陨石 before theyreach theearth,而 dust 才是影响星系亮度的原因。这种粒子会使我们在地球上观测到的星星比实际的暗而且红。因为 这些 dust 会影响星星发出来的光,它会使星星的光的波长移动。然后说 T 天文学家,他研究确定了 dust 的大小。 比如,红光的波动被 shift,那 dust 的大小就和红光波长差不多,如果是蓝光的波长被 shift,那大小就和蓝光的 波长差不多。(大概是说科学家观测到星星通过某太空某处 dust 以后发生红移,就推断说那个地方 dust 的 size 应 该在红色波和蓝色波之间等等。)
P2 解释为什么,发现 dust 容易吸收 blue 光不吸红光,而 red 光则到达了地球。科学家说远处星体发出的光是有 不同波长的,红色光的波长较长,蓝色光的波长较短,更容易穿过 interstellar space 来到地球。波长较长的光 不受宇宙中颗粒的影响,就如同大海中的波浪,如果海浪的 wavelength 大,遇到很小的礁石,并不会改变其原来 的波动。而由于到达地球的蓝色光(短波)较少,所以可以证明宇宙中的颗粒大小与蓝色光的波长差不多,因为这 样才会阻挡大量的蓝色光到达地球。
有一段帮助理解文章的寂静保留了下来 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。
题目:
1. 对比题,问 atoms(原子)的特性。选那个原子更加聚集,并不像灰尘一样是散布的。/ 我选了他们和 dust 相 比可能要 dense 一些。
2.主旨题。
3. 那个天文学家研究 Dust 时观察了什么?答案这种粒子会使我们在地球上观测到的星星比实际的暗而且红。
4. 科学家怎么注意到这个的/科学家如何知道石头星云的特性?选的是通过观察星星/答案是通过在地球上对光线 的观察和分析 /选项是比较 certainstars 的实际(还是原状什么的)【原文表述】这个人经过比较,发现从星星的 光线穿过 dust,到达地球时候,这时候看到的该星星显得亮度减弱,而且显得偏红。 5、举大海波浪的例子是什么目的
【参考答案】做一个类比 analogy 说明 dust 的大小影响光线的吸收
【原文表述】ocean wave 遇到比自己小的石头,就不会被干扰,所以红色波长遇到比自己小的 dust,也会不受干

扰的过去 6、问第二段的作用
(感谢   同学找到一篇十分相似的文章,值得看一下。)
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 effects of dust
Extinction by dust
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, 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.] |
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发表于 2016-1-20 18:54:24
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