1 00:00:00,012 --> 00:00:06,199 So, here's an instance where the history is so compelling, that we have to follow 2 00:00:06,199 --> 00:00:09,595 it. So, the image here is of course the great 3 00:00:09,595 --> 00:00:13,518 spiral Andromeda. Over the course of the centuries 4 00:00:13,518 --> 00:00:19,877 astronomers had classified stars and they'd realized found planetary nebulae, 5 00:00:19,877 --> 00:00:24,377 and they knew about gaseous nebulae like the Orion Nebula. 6 00:00:24,377 --> 00:00:29,602 And then, there were these spiral nebulae of which Andromeda is a spectacular 7 00:00:29,602 --> 00:00:32,452 example. This thing is almost 3 degrees across in 8 00:00:32,452 --> 00:00:34,852 the sky. Remember, the moon is a 1/2 degree 9 00:00:34,852 --> 00:00:38,027 across, so this is huge, but very, very faint. 10 00:00:38,027 --> 00:00:41,842 You need a big telescope and to get an image like this, 11 00:00:41,842 --> 00:00:47,135 lots of light collection and processing. But, spectacular and beautiful though it 12 00:00:47,135 --> 00:00:51,786 were, though it is, it was not at all clear to anybody what it is and the, what 13 00:00:51,786 --> 00:00:56,672 this comes down to is the question that has been hounding us throughout this 14 00:00:56,672 --> 00:00:59,517 class, how far? If you know how far, you can use 15 00:00:59,517 --> 00:01:03,151 the small angle, its approximation to figure out how big. 16 00:01:03,151 --> 00:01:08,558 But how far is this thing? How do you measure the distance? And this proved to 17 00:01:08,558 --> 00:01:12,758 be a big question in early days of the twentieth century. 18 00:01:12,758 --> 00:01:17,676 So in 1920 in fact, there was a public debate conducted between two of the 19 00:01:17,676 --> 00:01:21,801 leading astronomers of the day, Harlow Shapley and Curtis. 20 00:01:21,801 --> 00:01:27,747 And Shapley made the, the, the argument was over the following question. 21 00:01:27,747 --> 00:01:34,267 These spiral nebulae, like Andromeda, are they gaseous nebulae within the Milky Way 22 00:01:34,267 --> 00:01:40,972 or are they Kant's original island universes? Kant had this reasoning that 23 00:01:40,972 --> 00:01:46,183 these spiral nebulae should be worlds, universes on their own just like the 24 00:01:46,183 --> 00:01:50,786 Milky Way, because there should be a hierarchy of structure as I understand 25 00:01:50,786 --> 00:01:53,708 it. so Shapley, in fact, argued that there is 26 00:01:53,708 --> 00:01:59,057 no way, this is a scientific argument, and Shapley was arguing that these spiral 27 00:01:59,057 --> 00:02:04,231 nebulae are gas clouds in the Milky Way. And his arguments were basically this if 28 00:02:04,231 --> 00:02:08,919 you take M31 to be a 100 kiloparsecs in radius, because that was the radius that 29 00:02:08,919 --> 00:02:13,364 Shapley had found for the radius of of the Milky Way, and he knew one galaxy he 30 00:02:13,364 --> 00:02:18,253 said, okay, if you're going to make this another galaxy, then it's probably going 31 00:02:18,253 --> 00:02:21,978 to be like the Milky Way. If you give Andromeda a radius of 100 32 00:02:21,978 --> 00:02:26,254 kiloparsecs, that puts it two mega parsecs away, six and, more than 6 33 00:02:26,254 --> 00:02:30,342 million light years away. If Andromeda were to be that far we've 34 00:02:30,342 --> 00:02:34,927 already seen that there are stars embedded in these nubulae and they had 35 00:02:34,927 --> 00:02:39,679 tracked most interestingly novae. So there were, presumably, we now know 36 00:02:39,679 --> 00:02:44,212 white dwarfs and white dwarfs with binary partners, there were novae. 37 00:02:44,212 --> 00:02:48,196 Going on in these stars and the luminosity of a nova is not really 38 00:02:48,196 --> 00:02:50,853 constant enough to make it a standard candle, 39 00:02:50,853 --> 00:02:55,324 but the maximum luminosity, the order of magnitude of luminosity of a novae is 40 00:02:55,324 --> 00:02:58,120 known. And what Shapley noted is that if you 41 00:02:58,120 --> 00:03:02,908 take M31 and put it 2 megaparsec away, then you find that its novae must be much 42 00:03:02,908 --> 00:03:05,373 more luminous than those in the Milky Way. 43 00:03:05,373 --> 00:03:10,308 This is unlikely, it's much more reasonable to put it 100 kiloparsec away 44 00:03:10,308 --> 00:03:14,887 on the other side of the Milky Way where it can safely reside. 45 00:03:14,887 --> 00:03:19,726 And he also thought that there were measurements of the rate of rotation of 46 00:03:19,726 --> 00:03:24,349 another spiral and 101. And, again, if your input, assume that in 101 has the 47 00:03:24,349 --> 00:03:29,862 size of of the Milky Way, then you find that it is rotating too fast to be bound. 48 00:03:29,862 --> 00:03:33,226 Curtis had reasonable arguments on the other side. 49 00:03:33,226 --> 00:03:38,125 His argument was that these spiral nebulae are indeed island universes far 50 00:03:38,125 --> 00:03:42,953 away and outside the Milky Way. And, again he was referring to scientific 51 00:03:42,953 --> 00:03:46,075 evidence. He was saying, let us assume that the 52 00:03:46,075 --> 00:03:49,539 novae M31 are the same novae we find in the Milky Way. 53 00:03:49,539 --> 00:03:54,782 We know the average luminosity of a nova. This puts M31 at least a 150 kiloparsec 54 00:03:54,782 --> 00:03:57,987 away. if you put M31 100 kiloparsec, 150 55 00:03:57,987 --> 00:04:03,177 kiloparsec away, notice a lot closer, almost by a factor of ten, than Shapley 56 00:04:03,177 --> 00:04:08,107 did then, its size is about 7.5 kiloparsec, which is quite reasonable. 57 00:04:08,107 --> 00:04:11,952 This is about the size of Kapteyn's measurement for the Milky Way. 58 00:04:11,952 --> 00:04:17,652 Remember Shapley's measurement of the 100 kiloparsec Milky Way had not yet been 59 00:04:17,652 --> 00:04:21,037 accepted. It has never been accepted, it was wrong 60 00:04:21,037 --> 00:04:24,193 by a factor of two. And there was this alternative small 61 00:04:24,193 --> 00:04:28,864 Milky Way, so it basically comes down to what is the size of the Milky Way will 62 00:04:28,864 --> 00:04:31,710 tell us whether Andromeda is in it or out of it. 63 00:04:31,710 --> 00:04:36,542 also, he noted, that we measure radial velocities for stars in these spirals. 64 00:04:36,542 --> 00:04:41,071 but we measure very little proper motion that suggests that they're very far away, 65 00:04:41,071 --> 00:04:44,627 unless they're, these objects are preferentially moving radially. 66 00:04:44,627 --> 00:04:48,707 Something has a large radial velocity, we expect it to have a large tangential 67 00:04:48,707 --> 00:04:53,491 velocity, remembering that the tangential velocity is related to proper motion by v 68 00:04:53,491 --> 00:04:58,056 equals v tangential is 4.74 times mu times D. 69 00:04:58,056 --> 00:05:05,022 It measure a large v and a very small mu you conclude that you have a large D. 70 00:05:05,022 --> 00:05:10,572 This is how Curtis sought to reconcile these measurements. 71 00:05:10,572 --> 00:05:15,822 Both of them had valid arguments, near carried the day. 72 00:05:15,822 --> 00:05:21,174 All they did was illuminated the issue. the issue was actually resolved 73 00:05:21,174 --> 00:05:24,613 conclusively and beautifully by Hubble in 1923. 74 00:05:24,613 --> 00:05:30,298 So we have here in fact, the plate on which Hubble made his discovery. 75 00:05:30,298 --> 00:05:34,852 So Hubble was investigating photographic plates taken of 76 00:05:34,852 --> 00:05:41,152 M31, the Andromeda spiral nebula, and in this region of M31, this is the region 77 00:05:41,152 --> 00:05:47,577 near the center is what he was taking plates of and he found a few novae. 78 00:05:47,577 --> 00:05:52,497 He was marking off novae, because measuring, as we said, the luminosity of 79 00:05:52,497 --> 00:05:55,152 novae was one way to estimate the distance. 80 00:05:55,152 --> 00:05:59,667 How do you know that a star is a nova? Well, it's a nova if it appears as a star 81 00:05:59,667 --> 00:06:03,972 in today's plate but was not there on the plate you took a week ago. 82 00:06:03,972 --> 00:06:09,677 So you compare new plates to old plates, and the new stars that have popped up are 83 00:06:09,677 --> 00:06:13,875 novae and there is a date on when he, he took the measurement in October of 1923. 84 00:06:13,875 --> 00:06:18,765 And then he went back and he realized that one of the novae he had marked, the 85 00:06:18,765 --> 00:06:23,909 one in the upper right-hand corner, actually was present in older plates and 86 00:06:23,909 --> 00:06:28,608 then was absent in yet older plates, but was present and he had found a variable 87 00:06:28,608 --> 00:06:31,379 star. And, this was a very exciting discovery, 88 00:06:31,379 --> 00:06:35,391 because unlike novae for which you only know the maximum luminosity. 89 00:06:35,391 --> 00:06:39,990 For variable stars, we have a period luminosity relationship, we can compute 90 00:06:39,990 --> 00:06:43,988 the luminosity of this star. Since we can measure its brightness, we 91 00:06:43,988 --> 00:06:49,102 can use that to find the distance and Hubble was clearly excited. You can see 92 00:06:49,102 --> 00:06:54,200 that he vigorously crossed out the n and with a big exclamation mark, marked on 93 00:06:54,200 --> 00:06:57,565 the plate, this is a variable, that's the VAR up there. 94 00:06:57,565 --> 00:07:02,914 He knew he was about to measure the size to Andromeda, he made the calculation and 95 00:07:02,914 --> 00:07:08,369 the period luminosity relations showed it that Andromeda was 285 kiloparsec away. 96 00:07:08,369 --> 00:07:13,734 That is too big to fit, even in Shapley's large hundred kiloparsec galaxy. 97 00:07:13,734 --> 00:07:18,796 Andromeda is a separate galaxy. Now, the distance measurement was wrong. 98 00:07:18,796 --> 00:07:23,604 We'll discuss that in a second, but you have this pause for a minute and 99 00:07:23,604 --> 00:07:28,590 appreciate the sort of Copernican magnitude of that varied why it was that 100 00:07:28,590 --> 00:07:32,413 Hubble was so excited. He had just realized that the universe is 101 00:07:32,413 --> 00:07:36,971 not one galaxy but two, and of course, once we have two, presumably millions, 102 00:07:36,971 --> 00:07:40,125 and as we know today, hundreds of billions of galaxies. 103 00:07:40,125 --> 00:07:45,198 And so he had increase the size of the universe exponentially with this one 104 00:07:45,198 --> 00:07:48,252 variable star. A very exciting discovery following its 105 00:07:48,252 --> 00:07:52,604 trail always gives me goosebumps. Just looking at that plate, I think. 106 00:07:52,604 --> 00:07:56,887 Imagine Hubble's excitement. So, I said his distance was wrong and 107 00:07:56,887 --> 00:08:02,591 this is a good exercise in following how creative and how exacting the astronomers 108 00:08:02,591 --> 00:08:07,308 had to be to figure out this three-dimensional structure that we take 109 00:08:07,308 --> 00:08:12,715 for granted because we read it in books. So remember, that the period-luminosity 110 00:08:12,715 --> 00:08:17,949 relation was discovered by Henrietta Leavitt, in Large Magellanic Cloud, 111 00:08:17,949 --> 00:08:21,032 Cepheids. there are Cepheid, these are Population I 112 00:08:21,032 --> 00:08:23,640 classical Cepheids in the Large Magellanic Cloud. 113 00:08:23,640 --> 00:08:27,761 Now, she did not know the distance of a Large Magellanic Cloud exactly, what she 114 00:08:27,761 --> 00:08:30,120 discovered was a period brightness relation. 115 00:08:30,120 --> 00:08:34,017 and she knew they were all about the same distance, because they're all in the 116 00:08:34,017 --> 00:08:37,092 Magellanic Cloud. So she knew she found a period luminosity 117 00:08:37,092 --> 00:08:39,780 relation, but she couldn't calibrate it. She 118 00:08:39,780 --> 00:08:44,345 couldn't know exactly what luminosity corresponded to what period only that 119 00:08:44,345 --> 00:08:48,382 twice the period corresponded to some factor times the luminosity. 120 00:08:48,382 --> 00:08:52,956 Now, Hertzsprung and Shapley both sequentially calibrated Leavitt's 121 00:08:52,956 --> 00:08:57,546 relationship using Cepheids in the solar neighborhood to which they knew the 122 00:08:57,546 --> 00:09:02,581 distance from parallax measurements. What they neglected was that these were 123 00:09:02,581 --> 00:09:07,351 Cepheids in the galactic plane and extinction had made them appear dimmer by 124 00:09:07,351 --> 00:09:11,380 a factor of four, so there was a 75% extinction to these Cepheids. 125 00:09:11,380 --> 00:09:16,769 They thought they were four times dimmer than they really were therefore, their 126 00:09:16,769 --> 00:09:19,666 period-luminosity relation was completely off. 127 00:09:19,666 --> 00:09:24,781 In, now, Hubble was applying this measurement to a Cepheid in Andromeda, 128 00:09:24,781 --> 00:09:28,611 where he's looking outside of the plane of the galaxy. 129 00:09:28,611 --> 00:09:33,618 His luminosity is off by a factor of four, because, you remember that our 130 00:09:33,618 --> 00:09:39,548 calculation of distance is proportional to the square root of the luminosity, 131 00:09:39,548 --> 00:09:43,097 because brightness decays looks like L / D^2. 132 00:09:43,097 --> 00:09:47,942 The fact that he was fourth, a factor of four off in the luminosity of this 133 00:09:47,942 --> 00:09:51,410 Cepheid, meant he was approximately a factor a two off. 134 00:09:51,410 --> 00:09:55,194 In, the distance in fact, the factor he was off by, was 2.7. 135 00:09:55,194 --> 00:09:59,690 Andromeda is farther then he measured by a factor of about 2.7. 136 00:09:59,690 --> 00:10:04,304 It's not it's, it's about 2.5 million light years away from here. 137 00:10:04,304 --> 00:10:09,731 Shapley on the other hand, had been measuring the size of the Milky Way, by 138 00:10:09,731 --> 00:10:15,910 measuring distances to globular cluster variables, but globular cluster variables 139 00:10:15,910 --> 00:10:20,592 are a population too. Those are W Virginis stars not Cepheids. 140 00:10:20,592 --> 00:10:25,024 And, at a given period, a W Virginis star is about four times less luminous than 141 00:10:25,024 --> 00:10:29,303 the Cepheid with the same period. Remember, we showed those two graphs of 142 00:10:29,303 --> 00:10:34,414 Population I and Population II Cepheids. And so Shapley, unknowingly, was actually 143 00:10:34,414 --> 00:10:37,242 using the right period luminosity relation, 144 00:10:37,242 --> 00:10:41,593 but, remember, he was missing extinction and so he overestimated the distances. 145 00:10:41,593 --> 00:10:45,678 And, this lead for awhile to a great puzzle, now that we had the distance to 146 00:10:45,678 --> 00:10:49,785 Andromeda, and we could therefore estimate its size by making a small angle 147 00:10:49,785 --> 00:10:52,866 approximation. You now know the distance, you have the 148 00:10:52,866 --> 00:10:56,987 angle, three degrees, figure out the size of Andromeda and figure out soon the 149 00:10:56,987 --> 00:11:01,205 sizes of other galaxies, the Milky Way was larger than all the galaxies they 150 00:11:01,205 --> 00:11:03,460 saw. This didn't seem reasonable. 151 00:11:03,460 --> 00:11:06,688 And the diffuse extinction that Trumpler 152 00:11:06,688 --> 00:11:12,709 discovered in 30s helped allay some of Shapley's correct, some of Shapley's 153 00:11:12,709 --> 00:11:17,835 errors, and the distinction between Population I and Population II Cepheids 154 00:11:17,835 --> 00:11:22,675 was not clear until 1952 when Bodey realized that there two populations of 155 00:11:22,675 --> 00:11:26,109 Cepheids and two different period luminosity relations. 156 00:11:26,109 --> 00:11:31,898 Now, today in the 90s we manage to calibrate Cepheid periods directly from 157 00:11:31,898 --> 00:11:37,198 parallax measurements to distant Cepheids parallax measurements give us a correct 158 00:11:37,198 --> 00:11:40,547 distance. we know enough about extinction in the 159 00:11:40,547 --> 00:11:44,916 Milky Way to correct them. And you can use Cepheid variables to 160 00:11:44,916 --> 00:11:48,090 measure distances out to as large as 30 million, megaparsec. 161 00:11:48,090 --> 00:11:53,114 So that's 10 million light years sorry, that's almost a 100 million light years 162 00:11:53,114 --> 00:11:58,095 away that we can use Cepheids as distance markers, because we can still measure 163 00:11:58,095 --> 00:12:00,687 their periods, that's the distance record. 164 00:12:00,687 --> 00:12:05,252 Of course it's still you, that gives you a luminosity to compare to the brightness 165 00:12:05,252 --> 00:12:07,842 you see. You have to figure out something about 166 00:12:07,842 --> 00:12:12,142 extinction in the 100 million light years between you and the star, but we have a 167 00:12:12,142 --> 00:12:16,612 better understanding of extinction at least in the Milky Way then we had 168 00:12:16,612 --> 00:12:18,867 before. And so we have Cepheids more variable, 169 00:12:18,867 --> 00:12:22,638 relevant, we have a more reliable distance scale. 170 00:12:22,638 --> 00:12:27,908 However, this gives you again an appreciation for how difficult it is to 171 00:12:27,908 --> 00:12:33,014 actually build a three-dimensional universe out of that dark sky above us. 172 00:12:33,014 --> 00:12:36,387 Now, once Hubble discovers one galaxy is outside, 173 00:12:36,387 --> 00:12:41,427 he immediately goes on and measures the distances to many others, because you can 174 00:12:41,427 --> 00:12:46,612 find Cepheid variables now that you know what you're looking for and now you can 175 00:12:46,612 --> 00:12:50,857 just look at the nebulae. And Hubble comes up with a classification 176 00:12:50,857 --> 00:12:53,983 scheme, another contribution of, of Hubble's is 177 00:12:53,983 --> 00:12:59,324 the classification scheme for galaxies by their apparent shape and it has this 178 00:12:59,324 --> 00:13:03,247 tuning fork shape. So, to the left are elliptical galaxies 179 00:13:03,247 --> 00:13:08,183 in order of increasing electricity, so some galaxies are ellipsoids and 180 00:13:08,183 --> 00:13:12,411 Hubble is measuring how eccentric the ellipsoid looks to us. 181 00:13:12,411 --> 00:13:15,883 Of course, this is a crazy classification, because it tells you 182 00:13:15,883 --> 00:13:20,054 nothing about a galaxy because if you take a football and look at it head on, 183 00:13:20,054 --> 00:13:25,032 you can't distinguish it from a sphere. And so, this the E kind of classification 184 00:13:25,032 --> 00:13:30,494 tells you about the shape of the galaxy as it appears to us, but not much about 185 00:13:30,494 --> 00:13:35,550 the actual shape of galaxy. And then, there's this branching off into 186 00:13:35,550 --> 00:13:42,192 spirals with sort of increasingly intricate spiral arm structure on the one 187 00:13:42,192 --> 00:13:46,028 hand, and barred spirals on the other hand. And 188 00:13:46,028 --> 00:13:53,363 so, the Milky Way we now know recently is a barred spiral, the Milky Way is an SBb 189 00:13:53,363 --> 00:13:58,735 type barred spiral. Andromeda, which has no bar, is an Sb 190 00:13:58,735 --> 00:14:04,010 spiral galaxy, later other kinds of sort of less 191 00:14:04,010 --> 00:14:10,487 organized spirals were added by others and a class for the degree of coherence 192 00:14:10,487 --> 00:14:15,397 of the spiral arms was introduced, so, and give it, it can give a more precise 193 00:14:15,397 --> 00:14:18,912 classification. And we'll talk about the properties of 194 00:14:18,912 --> 00:14:22,985 various kinds of galaxies as we go on now that we know they exist.