Official 19 Set 3

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Difficulties in Astronomy

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What is the lecture mainly about?
  • A. Recent advances in technology used by astronomers

  • B. How radio astronomy has led to advances in communication technology

  • C. Important discoveries made by radio astronomers

  • D. Difficulties astronomers have in observing the universe

显示答案 正确答案: D

我的笔记 编辑笔记

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    NARRATOR:Listen to part of a lecture in an astronomy class.

    MALE PROFESSOR:So, how many of you have seen the-the MilkyWay, the MilkyWay galaxy, in the sky? [to one student]You? You have?

    MALE STUDENT:Yeah, I was camping. And there was uh, no moon that night. It was super dark.MALE PROFESSOR:Uh, anybody else? Huh, not too many...Isn't that strange that the MilkyWay is the galaxy that the planet Earth is in and most of us have never seen it?Now what's the problem here?

    FEMALE STUDENT:Light pollution, right? From streetlights and stuff.

    MALE PROFESSOR:Yes. Especially unshielded streetlights, you know, ones that aren't pointed downward...Now, here's an irony: the building we're in now, the astronomy building, not far from our observatory, has unshielded lights.

    MALE STUDENT:So, the problem's pretty widespread...

    MALE PROFESSOR:It's basically beyond control, as far as expecting to view the night sky anywhere near a city... I mean, uh, I've lived around here my whole life, and I've never seen the Milky Way within the city limits- and I probably never will... there's a price for progress, huh?But let's think beyond light pollution.That's only one kind of, uh, technological advance that has interfered with astronomical research.Can anyone think of another?No? OK, let's look at it this way... we don't only get information by looking at the stars... for the past 70 years or so, we've also used radio astronomy, which lets us study radio waves from the sky...

    MALE STUDENT:How can you observe radio waves? I mean, tell anything about the stars from that?

    MALE PROFESSOR:Well, in optical astronomy-using a telescope and observing the stars that way- we rely on visible light waves.What we're seeing from Earth is actually electromagnetic radiation that's coming from stars- and just one part of it is visible light.But there are problems with that.When photons in light waves hit objects in our atmosphere- water droplets, oxygen and nitrogen molecules, dust particles, and so on- these objects are illuminated, they're lit up.And those things are also being lit by all our streetlights, by the moon, all this ambient light.And on top of that, when that visible radiation bounces off those molecules- it scatters in all directions.And, well, light from stars, even nearby in our own galaxy, doesn't stand a chance against that。Basically the light bouncing off all these objects close to Earth is brighter than what's coming from the stars.Now, radio waves are electromagnetic radiation that we can't see.Nearly all astronomical objects in space emit radio waves- whether nearby stars or objects in faraway galaxies- they all give off radio waves.And, unlike visible light waves, these radio waves can get through the various gases and dust in space, and through our own earth's atmosphere comparatively easily.

    FEMALE STUDENT:OK, then we might as well give up on optical astronomy and go with radio astronomy...

    Well, the thing is, with radio astronomy you can't just set up a telescope in your backyard and observe stars.One problem is that radio waves from these faraway objects, even though they can get through, are extremely faint.So we need to use radio telescopes specially designed to receive these waves, and then... well, we can use computers to create pictures based on the information we receive.

    MALE STUDENT:That sounds cool. So how do they do that?

    MALE PROFESSOR:Well, it's kind of like the same way a satellite dish receives its signal, if you're familiar with that.But radio telescopes are sometimes grouped together- it's the same effect as having one really big telescope-to increase radio wave gathering power.And they use electronics, quite sophisticated...[bringing the class back on topic] Yeah, it's neat how they do it, but- but for now why don't we just stick with what we can learn from it?Uh, some very important discoveries have been made by this technology, especially if you consider that some objects in space give off radio waves, but don't emit any light.We'd have trouble discovering those sorts of bodies, much less studying them, using just optical telescopes.

    FEMALE STUDENT:Well, if the radio waves are so good at getting through the universe, what's the problem?

    MALE PROFESSOR:Well, answer this: how come people have to turn off their cell phones and electronic devices when an airplane is about to take off?

    MALE STUDENT:The phones interfere with the radio communication at the airport, right?

    FEMALE STUDENT:Oh, so our radio waves here, on Earth, interfere with the waves from space?

    MALE PROFESSOR:Yes, signals from radios, cell phones, TV stations, remote controls-you name it. All these things cause interference.We don't think about that as often as we think about light pollution.But all those electrical gadgets pollute the skies, just in a different way.

  • 旁白:听下面一段的天文学课程。

    教授:嗯,你们中有多少人注意过银河系,观测过它的天象?你,你看过?

    相似:是的,教授。我曾露营的时候观测过,当时天上没有月亮,四周很暗。教授:还有别的同学吗?嗯,没有多少同学看过。银河系是我们地球安身的地方,但大多数同学们却从来没有看到过,这难道不奇怪吗?嗯,既然如此,问题在哪儿呢?

    学生:光污染,是吗?比如街道上的路灯等…

    教授:是的,特别是没有加灯罩的路灯,它们的光是漫射的。现在,我给大家讲一件比较讽刺的事情。和观测楼隔得不远的、我们现在所在的这栋天文中心,就安装了无罩灯。

    学生:所以这个问题其实特别普遍。

    教授:它基本上是无法控制的,期待在城市附近的任何地方看到夜空。。。我的意思是,我这一辈子都住在这个地方,在市区里我从来没有看到过银河系的模样,而且可能我永远也看不到。所以,发展、进步是要付出代价的,对吧?那么,我们再想想,除了光污染这只是一种科技进步导致的天文观测不便。有同学能想到别的吗?没有吗?好吧,我来给大家一些提示吧!过去 70 年里,我们不只是靠肉眼观测天空来获得天文信息,我们还用无线电技术,他使得我们可以研究天空中的各种波段。

    学生:你怎么观测波段呢?我的意思是,怎样利用波段来解读行星相关的信息?

    教授:嗯,在光电天文学领域中,我们的观测工具是望远镜,通过可见光波来研究星体。我们在地球上接受到的图像,实际上是来自其他星球的电磁波,其中一部分是可见光。而且,这种方法有一些问题。当光波中的光子击打在我们大气层的水滴、氧气和氮气分子上时,它们就被点亮了。而这些东西同时也可能会被我们的街灯、月光等不明光源点亮。最重要的是,当可见光波从这些分子上反射出去的时候,它就向四周分散了。这样一来,从其他星球发射出来的光—即使他们的距离和地球很近—根本不可能能够经受起这样的能量消耗。基本上,在靠近地球的所有物体上反射的光比来自星体的光要亮。而无线电波则是我们看不到的电磁波。几乎空间中的所有天体都会发射电磁波,无论是近星域还是遥远星域的物体,它们随时随地都在发射电磁波。而且这些波与可见光不同之处在于,它们可以相对容易地穿透各种气障和尘障,穿过我们的大气层。

    学生:这样的话,我们会放弃光电望远镜而只使用无线电波吗?

    嗯,情况是,我们当然不能只在后院立一个望远镜然后就依靠它观察星空。这样做的一个问题是,从遥远星体上发射过来的电波,即便它们能够穿透大气层,依然非常的微弱。所以我们还需要建立射电望远镜;它们的原理是接收电波,并且通过电波信息建立电脑模拟图像。

    学生:这听上去很不错!但到底是怎么做到的呢?

    教授:如果你了解的话,卫星接收盘接收信号的原理和射电望远镜的工作原理差不多。但射电望远镜的接收器常常建立在一起的,从而组成一个更大功率的信号接收盘。并且它们使用大功率电源,而且工艺要求非常高。嗯,射电望远镜的原理差不多是这样的,但我们还是接着跟进我们课堂上的内容吧!这项创新使得我们获得了许多重大的发现,特别是关于那些在太空中只发射无线电波而不发射光的物体的。我们曾经在研究这一类天体时很不顺利,因为光学镜不能进行太多有效的研究。

    学生:嗯,如果无线电波真的能够穿透宇宙空间的话,它有什么问题呢?

    教授:它的答案与这个问题是一样的:为什么人们在飞机起飞的时候要关闭手机等电子设备呢?

    学生:因为电话干扰了飞机和机场的无线电通信,是吗?

    学生:这样说来,地球上的无线电信号干扰了空间传来的电波?

    教授:对的,电台、手机、电视台、无限遥控等,你们所知的无线电设备都能干扰空间电波。我们对此的考虑不如光污染多。但这些电子仪器同样污染了天空,只是方式不一样罢了。

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    选项分析:通听全文,教授在整个讲座中讲了两个观察天文的方法1. Radio Astronomy 2. Optical Astronomy,并讲了两种方法在观察过程中会出现的缺陷。


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