托福听力考试涉及的一个话题——天文

讲了这么多期天文学的背景知识，相信大家已经对于天文学领域的一些常见概念有了比较清晰的了解。我们今天来讲一个非常有意思的内容，也是很多天文学讲座中非常喜欢涉及的一个内容——宇宙中是否还存在其他的生命呢？或者其他的生命形式？今天就来说一说这个话题的内容。

我们知道宇宙非常大，大到我们根本没办法想象的程度。就目前我们所有观察到的，就有不计其数的星系，每个星系还包含有不计其数的星体。同时，科学家也在不断地发现星体。就银河系本身来说，或许也会有很多大小合适、温度合适的星球，这些星球具备存在生命的条件，但是现在科学家又没有足够的证据，去进行证明，现在，除了地球是有生命存在的，目前还没有发现任何其他的星球也存在生命。

但是努力探索，是科学家们宝贵的精神，很多科学家也做出过对于一些星体上可能存在生命物质的判断，还有一些科学家也尝试过去探索类地行星，致力于去寻找这么一个和地球很相像的可居住星球。而这个内容，是很多天文学讲座喜欢去触及的。下面，我截取出来一部分天文学讲座的内容，给大家感受一下，关于这个主题，文章的内容部分一般怎么展开以及部分题目会如何考察，比如下面这篇文章：

(male professor) So, we’ve been searching for stars similar to our Sun, what we might call sun-like stars, since presumably planets orbiting such stars are the most likely to have liquid water, a requirement for life as we know it. And what mostly determines whether liquid water can exist, of course, is the planet’s temperature. Too hot or too cold and any water would either vaporize or freeze. OK, but this assumption that stars with habitable planets must be sun-like, radiating huge amounts of thermal nuclear energy, this might not be actually necessary.

Yes, Stephanie?

(female student) Where would the planet get its heat then?

(professor) One possibility is that it could orbit a different kind of star, a white dwarf. Do you remember what a white dwarf is?

(female student) It’s a dead star, like the ember that’s left after a star has burned up all its fuel but it’s cooler and dimmer than sun-like stars, right? And much smaller.

(professor) True. But there are still enough energy in their core to generate some heat. Now, to get enough heat for liquid water, a planet would need to orbit its white dwarf very tightly.

(female student) But before becoming a white dwarf, doesn’t a star first expand and then collapse? And wouldn’t the expansion phase destroy any planets closest to it?

(professor) Yes, but, some outer planets could survive and move closer in. Or new planets could form after the star collapses. White dwarfs exist for many billions of years. So there’s plenty of time for such changes to occur.

(female student) There aren’t that many white dwarfs in the universe, though. Are there?

(professor) Actually, we’re finding that they are plentiful in the universe. We detect them in an indirect way. When a planet passes in front of its host star, this transit eclipses some of the star’s light. By measuring this periodic dimming, we infer the existence of a planet. Since white dwarfs are only slightly bigger than Earth, a planet might block out 50 percent of the white dwarf’s light, which is observable through a ground-based telescope. The transit of a planet orbiting a sun-like star produces just a miniscule change in luminosity.

......（未完待续）

2. Why does the student talk about how a white dwarf forms?

A) To demonstrate her familiarity with the life cycle of stars

B) To speculate about the ultimate fate of the Sun

C) To suggest a topic to explore in a future class discussion

D) To question a hypothesis described by the professor

3. What factor makes it relatively easy to detect planets orbiting a white dwarf?

A) The planets typically reflect a significant amount of light.

B) The planets block a significant amount of the star’s light during transit.

C) The planets have a higher mass than the white dwarfs that they orbit.

D) Planets around white dwarfs have very slow orbits.

这篇天文学讲座，通过文章的开头部分，教授给出了一些背景信息，通过背景信息部分的描述，我们可以看出文章内容是打算围绕星球的可居住条件展开。符合可居住星球的条件未必是像太阳一样的。还有可能是其他类型的星球支持可居住星球，比如：白矮星。接下来，文章下面的部分主要就是围绕白矮星，进行定义、解释以及白矮星具备什么样可以支持可居住星球的条件进行展开。

针对这个部分内容，这里给出大家两个问题，可以尝试去进行一下作答。天文学的讲座很多同学比较排斥、恐惧，事实上，文章内容有时候感觉比较抽象晦涩，但是实际上的考点是比较容易的，都是比较浅显、直白的细节题目，基本上就是原文的某处细节的简单改写。上面2个题目答案分别是：D B。你们都做对了么？

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