1 00:00:00,012 --> 00:00:05,045 So we're going to start with the disk and the disk contains as we said, 60 billion 2 00:00:05,045 --> 00:00:09,912 solar, masses of stars, orbiting, in a particular direction. 3 00:00:09,912 --> 00:00:14,995 They are of course, in Keppler orbits. Along with that is some gas and dust. We 4 00:00:14,995 --> 00:00:19,862 saw the, evidence of this, Within the disk there is a very exciting 5 00:00:19,862 --> 00:00:25,255 and interesting sub structure, which is difficult to tease out, and it 6 00:00:25,255 --> 00:00:31,134 turns out that the stars in the disc orbit in all poss, in many orbits, some 7 00:00:31,134 --> 00:00:37,675 of them more eccentric and some circular. But if you look for the concentrations of 8 00:00:37,675 --> 00:00:44,305 particular kinds of stars, blue stars, OB stars, the attendant ionized hydrogen, H2 9 00:00:44,305 --> 00:00:49,742 regions that come indicates star formation. and to some extent the 10 00:00:49,742 --> 00:00:56,302 molecular hydrogen clouds, those are not scattered uniformly over the disk, but in 11 00:00:56,302 --> 00:01:02,772 fact are concentrated in these spiral arms, these line, linear structures that 12 00:01:02,772 --> 00:01:07,089 wind through the disk. Now, how are you going to find that, if 13 00:01:07,089 --> 00:01:12,288 you're looking length wise through the disc, how are you going to recognize that 14 00:01:12,288 --> 00:01:17,487 there are discreet structures? Well, the, this is where 21 centimeter hydrogen 15 00:01:17,487 --> 00:01:22,645 line, gave us, great benefit. So, what happens is, that imagine if you 16 00:01:22,645 --> 00:01:28,210 will, that here we have our disc, and here is the center, and here about 1/3 of 17 00:01:28,210 --> 00:01:34,443 the way out is the sun, and imagine that the direction of the angular momentum is 18 00:01:34,443 --> 00:01:40,613 such that this is the direction of rotation, and what this, graph shows you 19 00:01:40,613 --> 00:01:47,610 is the observations that you would get by observing 21 centimeter emissions from, 20 00:01:47,610 --> 00:01:54,371 coming at the Earth from this direction. And what will happen is, I told you the 21 00:01:54,371 --> 00:02:01,033 21 centimeter line is a very sharp line and so, of course, because, relative to 22 00:02:01,033 --> 00:02:07,687 the sun's motion in it's Kepler orbit. objects, at different parts of the disc 23 00:02:07,687 --> 00:02:11,237 are moving. There will be a Doppler shift associated 24 00:02:11,237 --> 00:02:16,837 to the relative motion of, whatever is emitting the radiation, relative to the 25 00:02:16,837 --> 00:02:20,062 sun. We can, measure that Doppler shift, and 26 00:02:20,062 --> 00:02:26,376 what we find here is a spectrum of the observed 21 cm emission in a particular, 27 00:02:26,376 --> 00:02:31,792 from a particular direction. And, what we see is that this would be 28 00:02:31,792 --> 00:02:38,427 the correspond to the wave length, 21 cm. We see that there is somewhat red shifted 29 00:02:38,427 --> 00:02:43,062 light coming in, and then a lit, a light with a slight 30 00:02:43,062 --> 00:02:49,217 blue shift and then a whole lot of light received or radio waves received that 31 00:02:49,217 --> 00:02:55,082 have been blue shifted by various amounts and typically, the way we draw this, or 32 00:02:55,082 --> 00:03:01,554 we are shown this when we are students is a graph like this with 3 nice peaks in 33 00:03:01,554 --> 00:03:08,417 it, and we are given to understand, aha, the way this works is that there are 3 34 00:03:08,417 --> 00:03:15,702 clouds along this direction, each of them moving with the characteristic, Keplerian 35 00:03:15,702 --> 00:03:20,838 velocity, in it's orbit, and, these lead to three different Doppler shifts. 36 00:03:20,838 --> 00:03:25,727 And, by noticing the three discrete Doppler shifts, we can realize that, 37 00:03:25,727 --> 00:03:30,617 there were three clouds in the way And by reconstructing sort of doing a tumblr 38 00:03:30,617 --> 00:03:34,867 graphic analysis as we sweep the direction in which we are looking we can 39 00:03:34,867 --> 00:03:38,587 follow these clouds and reconstruct the structure of the disk. 40 00:03:38,587 --> 00:03:43,247 what's nice about this image is that it shows us more what real life is. 41 00:03:43,247 --> 00:03:47,677 These three peaks are in fact to be interpreted by, as a result of emissions 42 00:03:47,677 --> 00:03:52,082 by 20 discreet objects because we know something about the line shape. 43 00:03:52,082 --> 00:03:56,283 And you can sort of do a fitting. This gives you a more realistic image of 44 00:03:56,283 --> 00:04:00,370 what astronomers are up against. With all that they have come up with a 45 00:04:00,370 --> 00:04:05,083 pretty good picture of what the structure is in the milky way's disk and here is an 46 00:04:05,083 --> 00:04:07,715 artist's conception of what this looks like. 47 00:04:07,715 --> 00:04:12,142 this is an artist conception from circa 2008 and in the past decade. 48 00:04:12,142 --> 00:04:18,042 Our understanding of this has changed the milky way was traditionally fought to 49 00:04:18,042 --> 00:04:22,767 have 4 large spiral arms, it turns out that now 2 of them, the norma and 50 00:04:22,767 --> 00:04:28,915 Sagittarius arms have been demoted to sort of secondary sub arms, and the Milky 51 00:04:28,915 --> 00:04:35,550 Way really, is thought to only have two large arms, the Scuttum Centauris arm and 52 00:04:35,550 --> 00:04:41,506 the Perseus arm that come, emerge from the edges of the, as I said, newly 53 00:04:41,506 --> 00:04:46,851 discovered, galactic bar. And You can ask where the sun is in this 54 00:04:46,851 --> 00:04:52,804 picture, that would be a good question. So the sun in this image sits about here, 55 00:04:52,804 --> 00:04:58,906 which interestingly is not on either the Scutum Centaurus arm or the Perseus arm. 56 00:04:58,906 --> 00:05:04,452 In fact it lives on some sort of branched structure over here that is called to 57 00:05:04,452 --> 00:05:09,515 Orion Spur. And, in addition, there's this recent 58 00:05:09,515 --> 00:05:18,365 discoveries, include these 2 3-kiloparsec arms, that are very closely wound near 59 00:05:18,365 --> 00:05:25,129 the galactic, bulge. So we've learned how to map the structure 60 00:05:25,129 --> 00:05:31,358 of the disk. It's important to remember, again, that what is concentrated along 61 00:05:31,358 --> 00:05:35,859 the disk are blue stars and, ionized hydrogen clouds. 62 00:05:35,859 --> 00:05:40,397 There are stars and certainly gas elsewhere in the galaxy. 63 00:05:40,397 --> 00:05:47,287 how do we know this? Well, it's easiest to look at a galaxy we can view this way. 64 00:05:47,287 --> 00:05:53,007 So let me cheat history for a while and ignore the fact that we're in the 20s. 65 00:05:53,007 --> 00:05:58,762 This is a beautiful spiral galaxy, M83. We can see the concentration of blue 66 00:05:58,762 --> 00:06:04,587 light indicating new hot stars, and red regions indicating ionized hydrogen 67 00:06:04,587 --> 00:06:08,634 emissions in the spiral arms of the galaxy, but when we look at the same 68 00:06:08,634 --> 00:06:12,869 galaxy on the right in the infared we see that the contrast between the arms and 69 00:06:12,869 --> 00:06:16,952 the rest of the disk is far reduced. There are stars, there is luminosity in 70 00:06:16,952 --> 00:06:20,740 the rest of the disk. It's just less of it is new blue stars and ionized 71 00:06:20,740 --> 00:06:23,266 hydrogen. Those are concentrated in the arms. 72 00:06:23,266 --> 00:06:27,358 It's very important to realize the space between the arms is far from being a 73 00:06:27,358 --> 00:06:31,418 vacuum. So we have the stars in the disc. Other 74 00:06:31,418 --> 00:06:38,228 than that, the disc of course is the location of most of the gas and dust in 75 00:06:38,228 --> 00:06:43,560 the inner galaxy. As we said, the gas is only 5 billion 76 00:06:43,560 --> 00:06:50,297 solar masses, so something like 7% or 8% of the mass of stars in the disk exists 77 00:06:50,297 --> 00:06:54,241 in the form of gas. This is mostly atomic hydrogren, some 78 00:06:54,241 --> 00:06:58,828 molecular hydrogen, some ionized hydrogen, and of course traces of 79 00:06:58,828 --> 00:07:02,516 everything else. the dust as I said is tracked well by 80 00:07:02,516 --> 00:07:08,100 the, the gases I said is tracked well by dust or the molecular hydrogen is tracked 81 00:07:08,100 --> 00:07:11,666 well by dust. in terms of mass, dust contributes very 82 00:07:11,666 --> 00:07:16,109 little to the mass of the galaxy. what is important about it, of course, is 83 00:07:16,109 --> 00:07:19,623 it's optical properties, it does contribute to extinction. 84 00:07:19,623 --> 00:07:24,984 the dust in the, gas, tend to be confined to the galactic plane, in general more 85 00:07:24,984 --> 00:07:29,060 tightly than the stars, though we saw that there are excursions, 86 00:07:29,060 --> 00:07:33,513 and much, of the dust and gas, is confined to within, what we call the 87 00:07:33,513 --> 00:07:36,895 solar circle. A circle of radius 8 kilo parsecs around 88 00:07:36,895 --> 00:07:40,034 the center of the galaxy, so about where the sun is. 89 00:07:40,034 --> 00:07:44,454 Past where the sun is, the dust and gas density drops off rather quickly. 90 00:07:44,454 --> 00:07:47,250 So that's the structure of the disk, what else is there?