托福听力 ：“度年如日”的宜居行星在哪里？
很多同学听到天文学讲座就会瑟瑟发抖，觉得天文学是超级可怕的存在。“只要不出天文学，我们就还是好朋友”。
其实，我们托福讲座都是各个学科的知识，听懂之后可以很大程度上扩充大家的知识面。天文学也不例外，只要我们稍微了解一点天文学的背景知识，就会发现其实托福天文听力也没有那么难。今天我们就来借一篇天文学真题讲座来说说吧。
habitable zone 宜居带
在天文学中，恒星周围宜居带是指在给定的大气压下，行星表面能够支持液态水的围绕恒星运行的轨道范围。宜居带的边界是基于地球在太阳系中的位置和它从太阳吸收到的辐射能的数量。由于液态水对地球生物圈的重要性，宜居带的性质及其内的物体可能有助于确定类地生命和智慧的范围和分布。通常研究方向是宜居带的位置，热量来源和稳定性。
宜居带
有了以上背景知识，我们来听这段音频：
听力原文：
(male professor) So we were talking about distant planets that might be capable of supporting life, of course having liquid water is the key. The planet has to be close enough to its star so all the water isn’t frozen and far enough away that it doesn’t vaporize. And that comfortable area for water around star is called, anyone?
(female student) The habitable zone
(professor) Right, but in addition to the planet’s location, we need to consider the type of star that could support such a planet. As I mentioned previously, our Sun has some special characteristics that help make Earth habitable.
(male student) Oh, right, the Sun’s a solitary star.It doesn’t have any companions, so the planets revolving around can maintain a stable orbit.
equilibrium
我们来听这段音频
听力原文：
And it’s the right size.
(professor) Meaning?
(male student) Well, if the star’s too big, it’ll burn out before life has time to develop.
(female student) And the star ’ s internal stability is important, too. It has to achieve, uh, equilibrium, I think you said.
(professor) Right, we say that the Sun is at equilibrium. That simply means that there’s a balance between the pressure pushing outward and the pressure pushing inward. The outward pressure, that’s the light and heat coming from the hydrogen burning at its core; the pressure pushing inward is gravity. This equilibrium allows the star to burn constantly for billions of years. So, those are generally the types of stars we look for in searching for habitable planets. And after looking at thousands of stars, we ’ ve identified about thirty that seem to be suitable candidates.
red giant 红巨星
红巨星是恒星燃烧到后期所经历的一个较短的不稳定阶段，当恒星度过它漫长的青壮年期——主序星阶段，步入老年期将会进入。红巨星时期的恒星表面温度相对很低，但极为明亮，因为它们的体积非常巨大。之所以被称为红巨星是因为看起来的颜色是红的，体积又很巨大的缘故。
white dwarf 白矮星
当恒星的不稳定状态达到极限后，红巨星会进行爆发，把核心以外的物质都抛离恒星本体，物质向外扩散成为星云，残留下来的内核就是我们能看到的白矮星。
白矮星体积小，亮度低，但是密度高，质量大，高问题，可能有几十亿年寿命。在地球上通过望远镜可以清晰观测到，因为其体积小，我们会观测到它有周期性的变暗现象，进而推测出其他行星的存在。
有了以上背景知识，我们来听这段音频
听力原文：
But, recently, an astronomer asked a totally unexpected question: “Is it possible that a habitable planet could be orbiting a white dwarf star?”
(female student) Um, a white dwarf?
(professor) OK, simply put, a white dwarf is the end result when a low mass star, less than half the mass of our Sun, when that star has died, burned all its fuel. Once that happens, the star’s core first contracts, then because of the heat generated by this contraction, its outer shell expands, so the star becomes huge, what’s called a red giant.
(male student) But that would destroy the planets around it, wouldn’t it?
(professor) Well, certainly the ones closest to it. But the outer planets might remain. Anyway, eventually, the red giant sheds its outer layer of gas and the hot dense core that’s left is called a white dwarf.
(female student) But you said white dwarfs are dead. So how can they have any of the characteristics needed for planet to support life, you know, like being in equilibrium?
(professor) Actually, a white dwarf is in equilibrium. Its core’s so dense it’ll generate heat and light for billions of years, long enough for life to develop. So, it’s really not an implausible notion at all. Still, the conditions wouldn’t exactly be earth-like. I mean, there’d be much less heat than the Sun generates. So any habitable zone would have to be much closer in. So close, in fact that a year on the planet would be about as long as one day on Earth. And there wouldn’t be any seasons because the gravitational pull from the star would be so strong that the planet’s axis wouldn’t be tilted.
pulsar 脉冲星
中子星（neutron star）是除黑洞外密度最大的星体，恒星演化到末期，经由重力崩溃发生超新星爆炸之后，可能成为的少数终点之一，质量没有达到可以形成黑洞的恒星在寿命终结时塌缩形成的一种介于白矮星和黑洞之间的星体，其密度比地球上任何物质密度大相当多倍。
脉冲星（Pulsar），就是旋转的中子星，因不断地发出电磁脉冲信号而得名。脉冲星是在1967年首次被发现的。当时，还是一名女研究生的贝尔，发现狐狸星座有一颗星会发出一种周期性的电波。经过仔细分析，科学家认为这是一种未知的天体。因为这种星体不断地发出电磁脉冲信号，就把它命名为脉冲星。
solar eclipse 日食
日食是月球运动到太阳和地球中间，如果三者正好处在一条直线时，月球就会挡住太阳射向地球的光，月球身后的黑影正好落到地球上，这时发生日食现象。
有了以上背景知识，我们来听这段音频：
听力原文：
(male student) But didn’t you say the innermost planets would have been destroyed?
(professor) Yes, and that’s why no one’s really been looking for planets around the white dwarfs. However, about twenty years ago, planets were found orbiting a different kind of dead star, a pulsar. Now, a pulsar’s the remnant of an extremely massive star that exploded. But new planets could have formed from the gas and debris ejected in this explosion or maybe the remaining outer planets could have been kicked inward somehow, and if it can happen around the pulsar,why not around the white dwarf?
(male student) So, how would astronomers find these planets around the white dwarfs?
(professor) Possibly by looking for stars that suddenly grow dimmer for a period of time that means a planet is passing between us and a star. And that’s the advantage of looking for planets around the white dwarf. White dwarfs are so small that it would be much more obvious when a planet passes in front of one, because it would block so much of the white dwarf’s light. It’d be like a solar eclipse, um, when the Moon passes between Earth and the Sun.
If white dwarfs were as large as other stars, it’d take a powerful orbiting telescope to detect the slight dimming as a planet pass in front of it. But you could see a planet eclipsing a white dwarf from Earth using a less powerful telescope.
到这里，与这篇讲座相关的背景知识就给大家讲完了，希望大家已经能将这篇讲座轻松听懂啦。
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