TPO62听力Lecture2原文+MP3+题目+答案下载

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TPO62听力Lecture2原文+MP3+题目+答案下载

TPO62听力Lecture2原文：

Listen to part of a lecture in an astronomy class.

So we pretty much know how the biggest stars die, when a massive star reaches the end of its existence, it explodes, sending out a huge blast of energy, which then fades to invisibility within a few weeks or months.

When a giant star explodes like this, it’s called a supernova.To the naked eye, a supernova appears to be a bright new star, but it’s not a new star. It’s a dying star. However, the shock waves from a supernova excite nearby clouds of hydrogen gas, causing them to compress, which does form new stars, and that process emits a distinctive type of radiation.

Now of particular interest to us today is that the same type of radiation has been observed in places where galaxies have collided. And in those same areas, we’ve also observed a high rate of new star formation. So not surprisingly, we’ve had a theory that colliding galaxies caused the new star formation, but there was no direct evidence, that is, until recently when some European astronomers found clouds of excited hydrogen gas that could only have been caused by shock waves from a pair of colliding galaxies.

You know, we used to think of galaxies as isolated systems, like islands in the sea of space. Now we know that they are always moving, usually in clusters. I mean, think about all the gravitational forces involved. We are talking about huge systems of stars and gas and dust. And since the average separation between galaxies is only about 20 times their diameter, it isn’t surprising that they often meet, we call it an encounter, and they meet in a variety of ways.

Mergers, for example, they are the most extreme kinds of galaxy encounters. A merger is when two galaxies meet and they don’t have enough momentum to keep going on their separate ways, so they merge, becoming one galaxy. Usually a larger galaxy will absorb, sort of swallow a smaller one, this can trigger huge areas of star formation as enormous clouds of gas from the two galaxies collide and collapse into very active star-forming regions.

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