TOEFL Listening: ETS-TOEFL听力机经 - CBL23DO7RY7112XOQ$

Lecture: White dwarf star: Narrator: Listen to part of a lecture in an astronomy class. Professor: So we were talking about distant planets that might be capable of supporting life. Of course, having liquid waters is a 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 a 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 is a solitary star. It doesn't have any companions. So the planets revolving around it can maintain a stable orbit. And it's the right size. Professor: Meaning ...? Male Student: Well, if the start is 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 he said. Professor: Right. We say that the Sun's 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 to start 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. 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: Okay. 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 of 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 layers 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 a planet to support life? You know, like being in equilibrium? Professor: Actually, a white dwarf is in equilibrium. Its core is 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 would 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. 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 white dwarfs. However, about twenty years ago, planets were found orbiting a different kind of dead star, a pulsar. Now a pulsar is 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 in world somehow. And if it can happen around a pulsar, why not around the white dwarf? Male Student: So how would astronomers find these planets around white dwarfs? Professor: Possibly by looking for stars that suddenly grow dimmer for a period of time. That means that a planet is passing between us and the star. And that's the advantage of looking for planets around a 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 dwarfs 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 take a powerful orbiting telescope to detect the slight dimming as a planet passed in front of it. But you could see a planet eclipsing a white dwarf from Earth using a less powerful telescope.