1 00:00:00,012 --> 00:00:05,052 So how is it that we know so much more about the galaxy than [UNKNOWN] or 2 00:00:05,052 --> 00:00:10,892 Shapley? main reason that we have more information about the galaxy we live in 3 00:00:10,892 --> 00:00:16,602 is because they were using visible light to make optical observations, and we have 4 00:00:16,602 --> 00:00:19,977 access to the entire electro magnetic spectrum. 5 00:00:19,977 --> 00:00:23,951 And so in this clip we're going to look at the Milky Way through several 6 00:00:23,951 --> 00:00:28,121 different bands, and emphasize what it is that they teach us, each of them about 7 00:00:28,121 --> 00:00:32,125 the Milky Way, and then later we'll put together all that information and see 8 00:00:32,125 --> 00:00:35,897 what we know about the galaxy. So here's the multi-wavelength approach 9 00:00:35,897 --> 00:00:40,886 to the Milky Way, and each of these images is an image of the entire plane of 10 00:00:40,886 --> 00:00:46,580 the milky way, folded around, unfolded, so that the center is Sagittarius, the 11 00:00:46,580 --> 00:00:51,388 direction to the center of the Milky Way, is centered in the images. 12 00:00:51,388 --> 00:00:56,019 And we're seeing all of these except the bottom, are false color, 13 00:00:56,019 --> 00:01:00,982 and we're seeing images of the milky way as it appears in various 14 00:01:00,982 --> 00:01:05,322 frequency bands, and we'll start at the lowest frequency at the top. 15 00:01:05,322 --> 00:01:09,942 These are radio waves, 408Khz, the radiations that these wave lengths is 16 00:01:09,942 --> 00:01:15,147 produced, mostly, by relativistic electrons, propagating in the galactic 17 00:01:15,147 --> 00:01:18,177 magnetic field. So we see that there are a lot of 18 00:01:18,177 --> 00:01:23,197 emissions near the center of the galaxy, and we see some localized sources. The 19 00:01:23,197 --> 00:01:28,312 source on the left is Cassiopeia A, the super nova remnant, we talked about it. 20 00:01:28,312 --> 00:01:33,737 And the emission from there is in fact so brilliant that if you look carefully you 21 00:01:33,737 --> 00:01:38,912 can see a sort of triangle shaped around it, which are the diffraction images of 22 00:01:38,912 --> 00:01:43,962 the legs of the antenna, that is the radio antenna used to produce the image. 23 00:01:43,962 --> 00:01:49,900 And so this gives us a map of where high energy charged particles are found at a 24 00:01:49,900 --> 00:01:54,792 slightly higher frequency and a slightly shorter wave length. 25 00:01:54,792 --> 00:02:01,210 At 1.4GHz, we have an extremely important measurement the wave length corresponding 26 00:02:01,210 --> 00:02:06,119 to 1.4 GHz is about 21 centimeters, and this is an important frequency, 27 00:02:06,119 --> 00:02:10,215 because it allows us to see clouds of neutral atomic hydrogen. 28 00:02:10,215 --> 00:02:15,654 Now neutral atomic hydrogen is difficult to see because, if it is, unless it is 29 00:02:15,654 --> 00:02:20,874 very hot the atoms are mostly in the ground state, and, the frequency of the 30 00:02:20,874 --> 00:02:26,162 wavelength corresponding to excitation from the ground state to excited states 31 00:02:26,162 --> 00:02:30,819 is ultraviolet, so atomic hydro, cold atomic hydrogen can absord, has 32 00:02:30,819 --> 00:02:35,839 absorbtion spectrum in the ultraviolet, but there's not enough necessarily 33 00:02:35,839 --> 00:02:40,619 ultraviolet like to observe it. On the other hand, it can both absorb and 34 00:02:40,619 --> 00:02:45,641 emit, even at the relatively cool temperatures we're talking about, at this 35 00:02:45,641 --> 00:02:51,122 wavelength of 21 centimeters, this is not an atomic transition where the electron 36 00:02:51,122 --> 00:02:55,757 jumps from 1 energy state to the other. In fact, what this is, is the frequency 37 00:02:55,757 --> 00:03:00,462 with which the electron precesses. The electron carries angular momentum, 38 00:03:00,462 --> 00:03:06,087 intrinsic angular momentum in a magnetic moment, and the electron's angular 39 00:03:06,087 --> 00:03:11,669 momentum precesses around, the, direction perpendicular to its orbits. 40 00:03:11,669 --> 00:03:16,376 So, just like the Earth's axis wobbles as it orbits around the sun due to 41 00:03:16,376 --> 00:03:21,398 gravitational tidal effects, in this case, the electron's axis wobbles as it 42 00:03:21,398 --> 00:03:26,785 orbits the proton due to electromagnetic effects, and the frequency with which it 43 00:03:26,785 --> 00:03:32,251 wobbles or precesses is 1.4 GHz. And so this is a low frequency at, 44 00:03:32,251 --> 00:03:37,909 relatively low temperatures that are, these, atoms are excited. So they emit 45 00:03:37,909 --> 00:03:41,884 these radio waves. Two great properties of this, one is it's 46 00:03:41,884 --> 00:03:47,006 emitted by neutral hydrogen, and two, because it is a radio wave, dust and gas 47 00:03:47,006 --> 00:03:51,726 clouds do not obscure it. So, when we look at 21 centimeters at the 48 00:03:51,726 --> 00:03:56,531 galaxy, we can see through the clouds and the dust, and we can see the big gas 49 00:03:56,531 --> 00:04:00,456 clouds themselves. And you see that the 21 centimeter image 50 00:04:00,456 --> 00:04:06,016 of the galaxy shows us that hydrogen is scattered throughout with of course, 51 00:04:06,016 --> 00:04:10,866 increased density near the center. Being able to observe the neutral 52 00:04:10,866 --> 00:04:15,818 hydrogen clouds in, atomic hydrogen clouds in the galaxy is going to be 53 00:04:15,818 --> 00:04:19,993 extremely important. Moving to a higher frequency, a still 54 00:04:19,993 --> 00:04:25,694 shorter wavelength, the 2.7 GHz. notice that the, the wavelength of the 21 55 00:04:25,694 --> 00:04:30,319 centimeter spin transition is rather, sharply defined. 56 00:04:30,319 --> 00:04:35,427 You change wave of frequency, you don't see the atomic hydrogen anymore. 57 00:04:35,427 --> 00:04:41,930 2.7 GHz, we're looking at, emissions by ionized gas and also fast, albeit not 58 00:04:41,930 --> 00:04:45,534 relativistic. Electrons, and again, we see Cassiopeia A 59 00:04:45,534 --> 00:04:48,861 as a source. We see the center of the galaxy as a 60 00:04:48,861 --> 00:04:52,852 source. so these again are energetic, localized 61 00:04:52,852 --> 00:04:57,517 sources producing these. And, there is also a sort of galactic 62 00:04:57,517 --> 00:05:03,274 emission, but that's been subtracted to exhibit, these point sources better in 63 00:05:03,274 --> 00:05:07,200 this picture. The middle pane, is, 115 GHz, and this is 64 00:05:07,200 --> 00:05:11,447 again an important one. In fact, this corresponds to the energy 65 00:05:11,447 --> 00:05:16,756 of a transition in carbon monoxide. Carbon monoxide? Why do, what's, there's, 66 00:05:16,756 --> 00:05:21,762 where would we find carbon monoxide? Turns out that carbon monoxide, while 67 00:05:21,762 --> 00:05:26,557 rare, is a very useful tracer molecule. It's concentrations in various regions of 68 00:05:26,557 --> 00:05:30,577 the galaxy closely trace the concentrations of molecular hydrogen. 69 00:05:30,577 --> 00:05:35,452 Where you find molecular hydrogen clouds, there are small trace concentrations of 70 00:05:35,452 --> 00:05:39,261 carbon monoxide. Following the carbon monoxide, we get a 71 00:05:39,261 --> 00:05:42,602 mapping of molecular hydrogen, and we see, as I said. 72 00:05:42,602 --> 00:05:47,966 that molecular hydrogen is constrained to the plane of the galaxy, and we also see 73 00:05:47,966 --> 00:05:52,696 that in certain places there are a broadening of sort of places where the 74 00:05:52,696 --> 00:05:57,739 hydrogen clouds spill out of the plane, and we'll see those again in a In a 75 00:05:57,739 --> 00:06:01,050 second. At a slightly higher frequency between 3 76 00:06:01,050 --> 00:06:04,198 to 25 times 103 gigahertz is how it's represented. 77 00:06:04,198 --> 00:06:09,146 These are infrared emissions and these far infrared emissions in fact are given 78 00:06:09,146 --> 00:06:11,974 off. We see that there are not that many point 79 00:06:11,974 --> 00:06:16,255 sources, but sort of a. continuous emission from the entire 80 00:06:16,255 --> 00:06:21,789 galaxy, furthermore, we see that these emissions seem to track the H2 emissions 81 00:06:21,789 --> 00:06:27,144 and where there is added H2 there are molecular hydrogen clouds, there's this 82 00:06:27,144 --> 00:06:31,500 infrared emission indeed. At this wave length we are basically 83 00:06:31,500 --> 00:06:35,727 observing the dust. It is extremely important that we can see 84 00:06:35,727 --> 00:06:41,038 the dust in the Milky Way Because then we know how, what we need to subtract, 85 00:06:41,038 --> 00:06:46,037 because we can use this tracing of the dust and of the molecular hydrogen, to 86 00:06:46,037 --> 00:06:51,424 figure out what the extinction is and try to reconstruct the correct luminosity of 87 00:06:51,424 --> 00:06:56,521 objects as seen through the dust, and notice as I said that the dust tracks the 88 00:06:56,521 --> 00:07:02,241 molecular hydrogen rather well The image below that is a higher frequency, shorter 89 00:07:02,241 --> 00:07:07,559 wave length yet. This is the near infrared, and in the near infrared dust 90 00:07:07,559 --> 00:07:11,924 is not hot enough to glow. What we're mostly seeing is the emission 91 00:07:11,924 --> 00:07:14,802 of stars. Which stars emit a lot of infrared 92 00:07:14,802 --> 00:07:16,350 radiation? Red stars. 93 00:07:16,350 --> 00:07:20,770 Which stars emit most of the infrared radiation? Giant red stars, red giants. 94 00:07:20,770 --> 00:07:25,550 And so we're looking at cool giants here. We see in fact that we're seeing point 95 00:07:25,550 --> 00:07:28,614 sources, and only towards the center of the galaxy 96 00:07:28,614 --> 00:07:32,830 we're seeing sort of the far side of the galaxy in the center. 97 00:07:32,830 --> 00:07:38,929 In that directions we see distant stars melded together into this great big glow. 98 00:07:38,929 --> 00:07:44,143 But on the periphery, we're actually able to resolve individual sources. 99 00:07:44,143 --> 00:07:49,292 The important thing is that we can see these bright stars in the infrared, 100 00:07:49,292 --> 00:07:53,492 through the dust, because remember that scattering and absorption and dust is 101 00:07:53,492 --> 00:07:57,717 highly frequency dependent. The high frequencies scatter more, and it turns 102 00:07:57,717 --> 00:08:02,102 out are absorbed more, and so looking in the infrared, we can see through the dust 103 00:08:02,102 --> 00:08:04,607 and distinguish the stars that are behind it. 104 00:08:04,607 --> 00:08:08,617 Stars that produce lots of infrared will of course be more visible in this 105 00:08:08,617 --> 00:08:11,302 wavelength, but we can see through the dust. 106 00:08:11,302 --> 00:08:14,477 Dust. These are all the things that Kapteyn and 107 00:08:14,477 --> 00:08:19,677 Shapley did not have at their disposal. At the bottom we finally see a visible 108 00:08:19,677 --> 00:08:23,377 light image of the Milky Way, this is a familiar image. 109 00:08:23,377 --> 00:08:28,727 We see the dark dust lanes which traverse the Milky Way, where there are clouds of 110 00:08:28,727 --> 00:08:34,084 dust and, molecular hydrogen absorbing the light and we cannot see stars there. 111 00:08:34,084 --> 00:08:39,269 of course in the visible light it's the blue luminous stars that dominate what we 112 00:08:39,269 --> 00:08:41,466 see. But we cannot see them, and it's 113 00:08:41,466 --> 00:08:46,422 interesting to compare the lowest image with say the middle image, and see how 114 00:08:46,422 --> 00:08:50,912 closely the molecular hydrogen clouds track the gaps in visible light. 115 00:08:50,912 --> 00:08:56,763 the gaps in the Milky Way as we see it, are precisely at the positions where 116 00:08:56,763 --> 00:09:03,068 molecular hydrogen and the attendant, clouds of dust are obscuring the light. 117 00:09:03,068 --> 00:09:08,179 This is the, technology from space observatories and X-ray's and gamma rays 118 00:09:08,179 --> 00:09:12,625 that I haven't even included here, 2 radio telescopes that were not available 119 00:09:12,625 --> 00:09:17,566 to [UNKNOWN], and it's thanks to that we have such a, so much better of picture of 120 00:09:17,566 --> 00:09:21,513 how the Milky Way is constructed. Let's see what it has all taught us.