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Lecture_2Beta Pictoris The next topic in our syllabus is the formation of planets. So, I think it would be a good time to talk about one place—the only place so far—where we can actually see this process going on. Who’s heard of Beta Pictoris? Okay, well, in that case,...

Lecture_2
Beta Pictoris The next topic in our syllabus is the formation of planets. So, I think it would be a good time to talk about one place—the only place so far—where we can actually see this process going on. Who’s heard of Beta Pictoris? Okay, well, in that case, maybe I’d better give you a little background on it first. Beta Pictoris is a bright star that, unfortunately for us here, can only be seen from the southern hemisphere. It’s also a fairly close star; if anything that’s 63 light years away can be called “close”. At any rate, it’s close enough that some of our larger and more advanced telescopes can make very detailed observations of it. Beta Pictoris first caught the attention of astronomers’ way back in 1983. That’s when they detected exceptionally strong infrared energy coming from that direction using instruments on a satellite orbiting the earth. I’m sure I don’t need to spend much time going over this, but remember that infrared energy is energy with a very long wavelength—much longer than that of visible light. This type of energy is usually seen coming from very young stars. These are often stars that are surrounded by huge disks of orbiting dust. We see these stars in infrared, not because they produce more of that type of energy, but because the material in the disks stops energy of shorter wavelengths from reaching us. And it these dusty disks; you know, the disks that block the energy; it is these disks from which planets are thought to form. Eventually, using new techniques and better equipment, astronomers were able to observe the disk around Beta Pictoris directly. They no longer had to infer its existence; they could actually see it. Having formed only about 20 million years ago, Beta Pictoris is at just about the age when we would expect it to have a disk, and so far it is the only star around which one has been observed. We only expect to see these disks around relatively young stars. If the star were much older, the disk would have already disappeared; because it would already have coalesced into planets. (Student 1) Excuse me, professor. Are you saying that a star 20 million years old is young? Yes, that’s exactly what I’m saying. You see, it’s important for us to keep things in perspective here. Twenty million years may not seem very young, but a star that old is really still just in its infancy. That might be a little hard to take in, I understand, but remember that our own star, the sun, is at least four and a half billion years older. (Student 2) Can we see any planets, or any signs that planets have begun to form. You mean around Beta Pictoris? That’s a good question and that’s where I was headed next, because there are signs that something is going on. Just recently we discovered that the dust in the disk around the star isn’t distributed uniformly. What I mean by that is it isn’t smooth. It has kind of like waves or ripples in it. Interestingly enough, these features are found at distances from the star that are very similar to the distances of the giant planets in our own solar system. From this, even though we can’t actually see them yet, many astronomers are concluding that Beta Pictoris is well along the way to developing its own family of orbiting bodies. Our best guess is that the ripples are probably caused by the gravity of two large planets that have already formed. Because of these ripples, some astronomers think that the Beta Pictoris system is probably in an intermediate phase of development, uh, that is they think it’s probably a link between even younger stars with smooth dusty disks and much more evolved systems; systems in which most of the disk has already formed into planets. Beta Pictoris is probably the only place that’s close enough for us to observe the stages of planetary formation and the other processes that take place in the first 100 million years of a star’s life. Why did astronomers first start paying attention to Beta Pictoris? A It was discovered to have planets. B It is close to earth. C A dusty disk was seen around it. D It was giving off an unusual amount of radiation. According to the lecture, which of the following statements are true about Beta Pictoris? 2 answer. A It is a very large star. B It is relatively easy to observe. C It is nearly as old as our Sun. 我怎么会选这个,还是听的不仔细。 D It is the only star known to have a dusty disk. According to the lecture, what do the ripples in the disk around BP tell astronomers? A Planets have already formed. B There isn’t much material in the disk. C The disk is cooling rapidly. D The disk is very young. According to the lecture, what is the value of BP to astronomers? A It is the first star around which planets have been discovered. B It is the closest star to our solar system. C Its radiation is easily detected. D It can show us how planetary systems evolve. What can be inferred about the professor when she says this …? A She thinks the students should already know about BP. B She sees that the students are unfamiliar with BP. C She doesn’t think BP is difficult to understand. D She has talked to the class about BP before. Why does the professor say this…? A The concept should be reviewed. B The concept is not important. C The concept is simple. D The concept is new.
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