1 00:00:00,000 --> 00:00:05,337 Well, I've had fun playing in our own backyard, the solar system, last week. 2 00:00:05,337 --> 00:00:10,382 we learned a lot of things. We saw ways to apply the physics we've 3 00:00:10,382 --> 00:00:13,599 learned. But we want to broaden our horizons 4 00:00:13,599 --> 00:00:17,328 beyond that. And if we look back at Athens and that 5 00:00:17,328 --> 00:00:22,154 dark field back in November. we were looking at the moon and at 6 00:00:22,154 --> 00:00:25,517 Jupiter. But most of what we saw there was the 7 00:00:25,517 --> 00:00:29,360 stars with their various colors and brightnesses. 8 00:00:29,360 --> 00:00:34,009 And we're going to spend this week indeed taking the first step beyond the solar 9 00:00:34,009 --> 00:00:37,797 system and that's going to involve trying to understand the stars. 10 00:00:37,797 --> 00:00:42,587 I think it was [INAUDIBLE] who said that we're very limited as regard to the 11 00:00:42,587 --> 00:00:44,008 stars. We can look at them. 12 00:00:44,008 --> 00:00:47,943 But we can't touch them, and we can't smell them, and we can't taste them. 13 00:00:47,943 --> 00:00:52,151 And we can't do any experiments with them, so how are we ever going to figure 14 00:00:52,151 --> 00:00:55,539 out what's going on. And what stars are made of or how they 15 00:00:55,539 --> 00:00:57,889 work. And there are two things that I would 16 00:00:57,889 --> 00:01:00,239 add. Two places where comp was wrong. 17 00:01:00,239 --> 00:01:04,338 One is, we've learned as we'll see, to look very closely and in ways that he 18 00:01:04,338 --> 00:01:07,781 couldn't have imagined. And the other thing we can do is we can 19 00:01:07,781 --> 00:01:11,770 think about stars, we can bring to bear all the physics we've learned, and 20 00:01:11,770 --> 00:01:15,447 perhaps even some new physics. And that's what we're going to spend this 21 00:01:15,447 --> 00:01:20,230 week doing, try to under, use, all of those things to see what we can say about 22 00:01:20,230 --> 00:01:24,797 stars, and so here's the plan. Of course a natural place to start, just 23 00:01:24,797 --> 00:01:29,359 as we started our discussion of planets with Earth, is to start our discussion 24 00:01:29,359 --> 00:01:32,226 by, of stars by looking at the Sun, our local star. 25 00:01:32,226 --> 00:01:36,262 We have a lot more observations of the Sun than we do of other stars. 26 00:01:36,262 --> 00:01:38,952 It's a good place to develop an understanding. 27 00:01:38,952 --> 00:01:42,170 And as we'll see, that will take us to some new physics. 28 00:01:42,170 --> 00:01:45,408 And then, once we want to look beyond the sun at other stars, 29 00:01:45,408 --> 00:01:49,204 we need to start figuring out how to measure things about the stars. 30 00:01:49,204 --> 00:01:53,335 What do you, what do we want to know? We want to know their luminosities, we 31 00:01:53,335 --> 00:01:57,187 want to know their temperatures. We want to know their sizes and masses. 32 00:01:57,187 --> 00:02:01,040 So that we can parametrize our understanding of the physics by 33 00:02:01,040 --> 00:02:04,235 matching it to the observed parameters of the models. 34 00:02:04,235 --> 00:02:08,937 And then, at the end of the week, we will take all of those things that we will 35 00:02:08,937 --> 00:02:11,469 learn. You can measure for all these stars. 36 00:02:11,469 --> 00:02:15,025 And look at the statistical distribution of such things. 37 00:02:15,025 --> 00:02:20,089 And the information from that, or review how people use the information from that, 38 00:02:20,089 --> 00:02:24,731 to, refine a set of stellar models. And give us a pretty deep understanding 39 00:02:24,731 --> 00:02:28,771 of what goes on inside stars. That's it, we move on, we start by 40 00:02:28,771 --> 00:02:32,475 talking about our sun and immediately you bump into a problem. 41 00:02:32,475 --> 00:02:37,315 What's the most salient thing about our sun, the most salient thing about our sun 42 00:02:37,315 --> 00:02:42,094 is that it shines it produces these four times ten to the twenty-sixth watts of 43 00:02:42,094 --> 00:02:46,934 energy it does that we know because it's a large hot object at 5800 degrees with 44 00:02:46,934 --> 00:02:50,220 that surface area, it radiates that much. So far so good. 45 00:02:50,220 --> 00:02:54,297 It's radiating out energy, it's been doing it for four and half billion years 46 00:02:54,297 --> 00:02:58,534 and the question is, why is it not cold yet, why is it still like 5800 degrees on 47 00:02:58,534 --> 00:03:02,431 the surface. And one can start assessing the ways we 48 00:03:02,431 --> 00:03:06,940 know of producing energy. And the first, the way we usually produce 49 00:03:06,940 --> 00:03:11,882 energy, is various chemical reactions. the common factor between all of these is 50 00:03:11,882 --> 00:03:16,282 that all chemical reactions involve rearranging the electrons in an atom. 51 00:03:16,282 --> 00:03:21,103 Rearranging the electrons in an atom, is essentially farming the electromagnetic 52 00:03:21,103 --> 00:03:24,930 potential energy atoms. because of the size of an atom and the 53 00:03:24,930 --> 00:03:29,751 size of the charges this produces on the order of ten to the minus nineteen joules 54 00:03:29,751 --> 00:03:32,714 per atom. And if the atom in question is a hydrogen 55 00:03:32,714 --> 00:03:37,130 atom, which is what the sun is made of. And not saying I can propose a way to 56 00:03:37,130 --> 00:03:40,034 chemically burn hydrogen in the absence of oxygen. 57 00:03:40,034 --> 00:03:45,310 But imagine if there were some a bizarre chemistry that takes place at solar 58 00:03:45,310 --> 00:03:49,418 temperatures or something. You would produce this much energy, 59 00:03:49,418 --> 00:03:55,009 figured out that there are six times ten to the 23 hydrogen atoms in a gram, and 60 00:03:55,009 --> 00:04:00,868 you find that a kilo of hydrogen would produce about 6 * 10 to the 7 joules. 61 00:04:00,868 --> 00:04:06,189 60,000,000 joules is a lot of energy, but the sun produces 10 to the 26 joules 62 00:04:06,189 --> 00:04:12,184 every second so you'd need to burn about six times 10 to the 18 kilos of, hydrogen 63 00:04:12,184 --> 00:04:15,350 per second to, power the sun by chemical forces. 64 00:04:15,350 --> 00:04:17,880 You'd run out of energy in about 10,000 years. 65 00:04:17,880 --> 00:04:22,446 it was this idea that led Lord Kelvin to the idea of Kelvin-Helmholtz heating. 66 00:04:22,446 --> 00:04:26,242 The process that we've been seeing is very important in astrophysics. 67 00:04:26,242 --> 00:04:30,643 this involves converting to heat the gravitational potential energy from the 68 00:04:30,643 --> 00:04:33,338 collapse of the solar nebula that formed the sun. 69 00:04:33,338 --> 00:04:35,704 We did an estimate of this in the homework. 70 00:04:35,704 --> 00:04:39,060 there will be solutions, at some point, if you didn't do it. 71 00:04:39,060 --> 00:04:44,230 And indeed you produced a lot more energy that way than you ever would by burning, 72 00:04:44,230 --> 00:04:49,035 hydrogen or any other chemical reaction. But the gravitational potential energy 73 00:04:49,035 --> 00:04:53,902 that the sun, liberated by condensing to form a star, would only last about 74 00:04:53,902 --> 00:04:55,787 10,000,000 years. This is great. 75 00:04:55,787 --> 00:04:59,559 The Sun has been doing this for four and a half billion years. 76 00:04:59,559 --> 00:05:03,148 And our first question is going to be a very important one. 77 00:05:03,148 --> 00:05:06,493 How does the sun shine? Where does the energy come from? 78 00:05:06,493 --> 00:05:10,995 That of course will take us to more of new physics, and, we'll take it from 79 00:05:10,995 --> 00:05:11,360 there.