1 00:00:00,000 --> 00:00:05,122 Now, at this point, you may well be thinking, well that's a lot of theory. 2 00:00:05,122 --> 00:00:09,884 I mean, yeah, here's a prediction. Our model's predict that the main 3 00:00:09,884 --> 00:00:15,440 sequence staller will evolve into a red giant, and, woo, we find red giants, but. 4 00:00:15,440 --> 00:00:20,760 That's not yet enough we want to see can I really show from observations that red 5 00:00:20,760 --> 00:00:25,820 giants evolve at after main sequence stars in other words that one turns into 6 00:00:25,820 --> 00:00:30,946 the other and we have a lab that will allow us to do this and I suggested that 7 00:00:30,946 --> 00:00:35,747 this is going to be clusters because remember, stars in a cluster whatever 8 00:00:35,747 --> 00:00:41,067 there size are all going to be formed at about the same time massive stars evolve 9 00:00:41,067 --> 00:00:44,116 faster. The later stages of evolution are rapid 10 00:00:44,116 --> 00:00:47,750 so if I let a cluster evolve for sometime and then stop. 11 00:00:47,750 --> 00:00:52,215 I should expect to find, say, lots of O and B stars but no G stars. 12 00:00:52,215 --> 00:00:57,300 That would be a reasonable thing if I find a bunch of G stars and O and B 13 00:00:57,300 --> 00:01:02,590 protostars I've done something wrong. I need to imagine that the way that the 14 00:01:02,590 --> 00:01:07,949 cluster evolves fits what model tells me. Flipping this around once I trust my 15 00:01:07,949 --> 00:01:13,514 model I can look at a cluster and figure out its age by figuring out which stars 16 00:01:13,514 --> 00:01:17,462 have evolved to what point. And moreover, it is in the context of 17 00:01:17,462 --> 00:01:20,686 clusters that spectroscopic Peralax comes in to it's own. 18 00:01:20,686 --> 00:01:24,984 When we look at a cluster, all the stars are about the same distance from us. 19 00:01:24,984 --> 00:01:29,170 Whatever inter stellar extinction and reddening is going on because of. 20 00:01:29,170 --> 00:01:33,841 Gust or clouds in between us and then, it's pretty much the same for all stars. 21 00:01:33,841 --> 00:01:38,093 So, what we can do, is for these stars, brightness, relative to each other, 22 00:01:38,093 --> 00:01:41,028 corresponds to luminosity relative to each other. 23 00:01:41,028 --> 00:01:44,262 And color corresponds to color, relative to each other. 24 00:01:44,262 --> 00:01:48,814 So, rather than taking detailed spectrum of each star, what, we have efficient 25 00:01:48,814 --> 00:01:51,390 ways of, basically, taking three filtered. 26 00:01:51,390 --> 00:01:56,937 photometric results through three filters doing what's called a color magnitude 27 00:01:56,937 --> 00:01:59,742 diagram. Which basically plots the difference 28 00:01:59,742 --> 00:02:04,293 between two colors, two filters, and the sum of the filters, or one of them. 29 00:02:04,293 --> 00:02:07,410 that's basically brightness versus temperature. 30 00:02:07,410 --> 00:02:12,041 And There are errors in the temperature, and 31 00:02:12,041 --> 00:02:15,965 there is inaccuracies because of intergalactic rendering, but they tend to 32 00:02:15,965 --> 00:02:20,049 move, inter stellar rendering, they tend to move the whole thing left to right. 33 00:02:20,049 --> 00:02:22,929 There are. Unknown extinction that moves or distance 34 00:02:22,929 --> 00:02:27,405 that moves everything up and down cause we can't determine an absolute velocity. 35 00:02:27,405 --> 00:02:31,825 But by matching onto the main sequence we can get, and large numbers of stars, a 36 00:02:31,825 --> 00:02:36,078 very accurate determination both of distance to the cluster and then of the 37 00:02:36,078 --> 00:02:38,595 absolute luminosities of all the stars there. 38 00:02:38,595 --> 00:02:42,959 So let's see a model simulation of how a cluster should evolve and then we'll 39 00:02:42,959 --> 00:02:48,840 compare that to what we actually see. The star clock 2.0 is a very fun 40 00:02:48,840 --> 00:02:55,462 Dos based simulation and what it's going to do is it's going to create for us a 41 00:02:55,462 --> 00:02:58,940 collection of stars of various masses and. 42 00:02:58,940 --> 00:03:02,960 I'm going to run the simulation, and I can stop it. 43 00:03:02,960 --> 00:03:07,625 And we see that, let's see, about 800,000 years have passed. 44 00:03:07,625 --> 00:03:12,289 Stars with masses nine solar masses and above have formed. 45 00:03:12,289 --> 00:03:18,160 The ones with the higher masses are, in fact, beginning to move off the, 46 00:03:18,160 --> 00:03:21,956 Main sequence. You see them moving off to upper right. 47 00:03:21,956 --> 00:03:27,472 And later stars have not yet formed, so in 840,000 years, you get down to nine 48 00:03:27,472 --> 00:03:31,340 solar masses. Let the animation run some more, your now 49 00:03:31,340 --> 00:03:37,357 down to 1.6 million years and stars with a mass of, four solar masses have formed. 50 00:03:37,357 --> 00:03:40,724 And the heavier ones are beginning to move off. 51 00:03:40,724 --> 00:03:46,383 At this point, it pays to increase the time step because some things happen too 52 00:03:46,383 --> 00:03:50,610 slowly, but you know, let's, let's take a few more slow steps. 53 00:03:50,610 --> 00:03:55,857 Just to see what's going on, the heaviest stars will start leaving the main 54 00:03:55,857 --> 00:03:58,750 sequence before the lightest stars show up. 55 00:03:58,750 --> 00:04:02,787 Here we go, we're starting to see horizontal branch behavior. 56 00:04:02,787 --> 00:04:07,900 And the heavy stars we said, or massive stars and except their main, the main 57 00:04:07,900 --> 00:04:13,013 sequence earlier and we're seeing that before the sun varying in the sample 58 00:04:13,013 --> 00:04:15,818 comes on the Into the picture. 59 00:04:15,818 --> 00:04:21,042 Stars with masses of, say. Eight solar masses or more are already 60 00:04:21,042 --> 00:04:26,200 done with their main sequence lifetime and have disappeared and notice the 61 00:04:26,200 --> 00:04:29,640 repeditity with which horizontal helium branch. 62 00:04:29,640 --> 00:04:33,981 behavior is gone. this star is, the 25 solar mass star, is 63 00:04:33,981 --> 00:04:39,233 already done with helium burning and has probably generated a solar nebula. 64 00:04:39,233 --> 00:04:44,416 And we're down to two one-half solar masses, and we again see very rapidly 65 00:04:44,416 --> 00:04:47,637 zipping through the final stages of evolution. 66 00:04:47,637 --> 00:04:51,419 Now that we get to the lighter stars, things slow down. 67 00:04:51,419 --> 00:04:56,519 So that we indeed up my time step. And, now things will run ten times as 68 00:04:56,519 --> 00:05:01,957 fast and we finally are almost ready to have our sun come on the picture and so 69 00:05:01,957 --> 00:05:06,920 you see this is what the model predicts. These are our trajectories of, 70 00:05:06,920 --> 00:05:12,257 Stellar evolution that we should expect to see and now stop this, any time I hit 71 00:05:12,257 --> 00:05:17,261 a key and stop the simulation, this should be a possible configuration that 72 00:05:17,261 --> 00:05:22,665 we can find in actual clusters so we go out and look around at clusters and make 73 00:05:22,665 --> 00:05:28,069 these color magnitude diagrams and I will refer you to a beautiful database that 74 00:05:28,069 --> 00:05:33,220 will make them for you and let's see how they jibe with reality. 75 00:05:33,220 --> 00:05:36,805 So that was a nice simulation. We now know what theory predicts. 76 00:05:36,805 --> 00:05:40,050 Now we go out to the lab, in other words the universe, and. 77 00:05:40,050 --> 00:05:44,861 Do real clusters exhibit the properties that we saw any order of increasing 78 00:05:44,861 --> 00:05:49,489 gauges they're very, very young cluster its called these kinds are called ob 79 00:05:49,489 --> 00:05:52,230 associations they are clusters so young that. 80 00:05:52,230 --> 00:05:57,986 Hot O and B stars are still on the main sequence so you don't expect to find many 81 00:05:57,986 --> 00:06:03,391 solar mass stars on the main sequence. Here is the color magnitude diagram. 82 00:06:03,391 --> 00:06:06,971 this is to be read very much like HR diagram. 83 00:06:06,971 --> 00:06:12,306 In other words luminosity increases this way, temperature increases this way. 84 00:06:12,306 --> 00:06:16,206 And we see here. It's very hard to figure out where the 85 00:06:16,206 --> 00:06:22,114 main sequence lies but what is clear is that my guess probably somewhere around 86 00:06:22,114 --> 00:06:25,243 here. Would be the main sequence, and only the 87 00:06:25,243 --> 00:06:30,583 most massive of the stars have landed on it, and the less massive stars are a 88 00:06:30,583 --> 00:06:35,437 collection of proto-stars. The structure becomes more definite when 89 00:06:35,437 --> 00:06:38,974 you go to a young, but relatively developed cluster. 90 00:06:38,974 --> 00:06:43,759 This is the Cone Nebula. That image that I prematurely erased is a 91 00:06:43,759 --> 00:06:49,029 beautiful Hubble image which shows you, by the way, these dust cocoons that I 92 00:06:49,029 --> 00:06:53,260 spoke about in which new stars are forming, but if we look, 93 00:06:53,260 --> 00:06:59,360 At the eight, color magnitude diagram for this, we now see a well-defined, 94 00:06:59,360 --> 00:07:06,897 [SOUND] Beginning of a, main sequence and, in fact if you drew this carefully 95 00:07:06,897 --> 00:07:11,510 you would that the main sequence, let me correct myself. 96 00:07:11,510 --> 00:07:18,052 Is probably something like this, and, what we are seeing is that the, more 97 00:07:18,052 --> 00:07:24,426 massive stars have joined the main sequence and less massive stars have not 98 00:07:24,426 --> 00:07:31,571 joined it yet, and the main sequence, the sort of the lightest ma, stars Already on 99 00:07:31,571 --> 00:07:37,255 the main sequence would tell you how old the cluster is, a slightly older cluster 100 00:07:37,255 --> 00:07:43,220 again this is familiar this is a pretty image of the orion nebula and here by now 101 00:07:43,220 --> 00:07:48,904 by twelve million years of age we see a very well defined main sequence and what 102 00:07:48,904 --> 00:07:53,758 we see this is sort of a classic. A jar diagram, that would be the main 103 00:07:53,758 --> 00:07:57,051 sequence. We see that lighter stars are still on 104 00:07:57,051 --> 00:08:00,550 their way down. Remember these are the evolutionary 105 00:08:00,550 --> 00:08:06,107 trajectories down to the main sequence. And the most massive stars are already on 106 00:08:06,107 --> 00:08:09,057 their trajectories away from main sequence. 107 00:08:09,057 --> 00:08:12,007 So we have two sort of reference points here. 108 00:08:12,007 --> 00:08:17,356 We have what's called the turn off point. That is the mass of those stars that are 109 00:08:17,356 --> 00:08:19,955 just beginning to leave the main sequence. 110 00:08:19,955 --> 00:08:24,471 And then we have, if you want, the turn-on point somewhere down here, which 111 00:08:24,471 --> 00:08:28,802 is the mass of those stars that are just becoming main sequence stars. 112 00:08:28,802 --> 00:08:32,020 And both of those, when you compare, have to give you. 113 00:08:32,020 --> 00:08:35,558 Equal ages. More to the point when draws were called 114 00:08:35,558 --> 00:08:41,003 isochrones, lines along which stars would lie at the same age and matches to the 115 00:08:41,003 --> 00:08:44,542 cluster end. That's how we perfect our models, or the 116 00:08:44,542 --> 00:08:49,782 people that do that perfect their models. We can look at an older cluster, the 117 00:08:49,782 --> 00:08:53,593 Pleiades, well established cluster, 130,000,000 years old. 118 00:08:53,593 --> 00:08:58,951 And again what we can see is that the. Main sequence turning point has moved to 119 00:08:58,951 --> 00:09:02,400 the right. Note the main sequence turning point was 120 00:09:02,400 --> 00:09:08,079 well to the left of zero over here and it is a little bit to the right of zero over 121 00:09:08,079 --> 00:09:10,851 here. Older clusters have a redder, if you 122 00:09:10,851 --> 00:09:16,462 want, a less massive turnoff point from the main sequence and there are very few 123 00:09:16,462 --> 00:09:20,316 proto stars left. In fact, we're beginning to see here the 124 00:09:20,316 --> 00:09:25,358 beginning of a population of A white dwarfs were one the more massive 125 00:09:25,358 --> 00:09:31,186 stars that have already loss their shells here's an even older cluster 300,000,000 126 00:09:31,186 --> 00:09:35,610 years here I've let the software helpfully designate for us 127 00:09:35,610 --> 00:09:42,107 A few red giants and we very nicely see the main sequence turn off points here's 128 00:09:42,107 --> 00:09:43,653 the precipe. Beehive. 129 00:09:43,653 --> 00:09:47,956 You can see that it's an old cluster just by looking at it. 130 00:09:47,956 --> 00:09:52,259 Look at all those reddish stars and very few that are blue. 131 00:09:52,259 --> 00:09:57,468 And indeed the main sequence turning point is now, oh, at about +.3 In this, 132 00:09:57,468 --> 00:10:00,278 color scale. So moving, consistently to the right. 133 00:10:00,278 --> 00:10:03,790 Though there are a few stars, those marked over here in blue. 134 00:10:03,790 --> 00:10:07,069 That don't belong. We also see, a well defined population 135 00:10:07,069 --> 00:10:08,006 of. White dwarfs. 136 00:10:08,006 --> 00:10:10,933 But what these blue guys are, we do not understand. 137 00:10:10,933 --> 00:10:13,567 They are stars that are on the main sequence. 138 00:10:13,567 --> 00:10:16,143 Even though they are, past the turnoff point. 139 00:10:16,143 --> 00:10:20,124 And should already have been gone. We'll return to those momentarily. 140 00:10:20,124 --> 00:10:25,095 Very old, open clusters. Here's M 67 and NGC 188 it's pretty easy 141 00:10:25,095 --> 00:10:28,603 to tell that the latter is older than the former. 142 00:10:28,603 --> 00:10:33,614 And finally, become the globblier clusters, the oldest clusters we find, 143 00:10:33,614 --> 00:10:37,837 but also the richest. So they produce some very brilliantly 144 00:10:37,837 --> 00:10:40,354 rich Color magnitude diagrams. 145 00:10:40,354 --> 00:10:43,699 Here's the color magnitude diagram for M13. 146 00:10:43,699 --> 00:10:49,922 And you see that with enough statistics you can find both the main sequence and 147 00:10:49,922 --> 00:10:55,911 the turn off point and the entire red giant branch and the horizontal branch. 148 00:10:55,911 --> 00:11:01,589 But no AGB stars that goes too fast. Even an M13 we don't find in the AGB 149 00:11:01,589 --> 00:11:04,843 stars. And again you might ask, what's that guy 150 00:11:04,843 --> 00:11:08,035 doing over there? That's another one of those blue 151 00:11:08,035 --> 00:11:11,291 stragglers. they're particularly evident in this 152 00:11:11,291 --> 00:11:14,483 image. So globular clusters are the oldest star 153 00:11:14,483 --> 00:11:17,675 clusters. They're uniformly very old and indeed 154 00:11:17,675 --> 00:11:22,783 this is a picture of a very beautiful, a very beautiful Hubble image of a pretty 155 00:11:22,783 --> 00:11:26,932 globular cluster NGC6397. And you look at it and you know there's 156 00:11:26,932 --> 00:11:30,571 something wrong. I understand where the yellow stars came 157 00:11:30,571 --> 00:11:34,530 from, what is all this blue stuff? Globular clusters are old 158 00:11:34,530 --> 00:11:39,216 It turns out that this happens a lot, particularly in globular clusters. 159 00:11:39,216 --> 00:11:42,055 We have this phenomenon of blue stragglers. 160 00:11:42,055 --> 00:11:47,534 The mechanisms are still a matter of some debate but probably a combination of two 161 00:11:47,534 --> 00:11:50,702 things. These stars did not start out as massive 162 00:11:50,702 --> 00:11:56,380 stars else over the billion year or three or five or 8,000,000,000 year history of. 163 00:11:58,440 --> 00:12:02,918 These stars did not start out as high mass, the high mass stars that we find 164 00:12:02,918 --> 00:12:06,101 them to be now. Because over the twelve or thirteen or 165 00:12:06,101 --> 00:12:10,638 whatever billion year history of this globular cluster, they would long since 166 00:12:10,638 --> 00:12:14,764 have evolved off the main sequence. They're still blue and on the main 167 00:12:14,764 --> 00:12:17,946 sequence means they started out as less massive stars. 168 00:12:17,946 --> 00:12:22,602 They became so massive either by mass transfer of a close binary phenomenon 169 00:12:22,602 --> 00:12:26,845 we'll talk about later in the week. Or by the collision and merger, maybe 170 00:12:26,845 --> 00:12:31,740 they started off as two or three. Less massive stars that merged up. 171 00:12:31,740 --> 00:12:35,486 Its an, we don't see many stellar collisions out where we live in the 172 00:12:35,486 --> 00:12:38,147 spiral arms where stellar densities are very low. 173 00:12:38,147 --> 00:12:40,645 Our nearest neighbor is four light years away. 174 00:12:40,645 --> 00:12:44,174 But globular clusters are much denser, collisions are conceivable. 175 00:12:44,174 --> 00:12:47,487 And in fact it's lice, likely that both mechanisms take place. 176 00:12:47,487 --> 00:12:51,939 And I spoke about the distinction between open clusters and globular clusters, and 177 00:12:51,939 --> 00:12:54,383 I said, globular clusters are much, much older. 178 00:12:54,383 --> 00:12:57,370 Astronomers had traditionally made this distinction 179 00:12:57,370 --> 00:13:01,445 By calling distinguishing population one stars from population two stars and 180 00:13:01,445 --> 00:13:05,732 initially it was a kinematic distinction, population one stars were the ones that 181 00:13:05,732 --> 00:13:09,860 were moving around roughly the way the sun is around the milky way, population 182 00:13:09,860 --> 00:13:13,882 two stars were moving in all kinds of other directions, their proper motions 183 00:13:13,882 --> 00:13:17,216 and their doppler shifts told us they had high peculiar motion. 184 00:13:17,216 --> 00:13:20,461 It turned out later that. But they what was underlying this 185 00:13:20,461 --> 00:13:25,039 extinction what is now the definition of population one stars and population two 186 00:13:25,039 --> 00:13:27,470 stars population one stars are middle rich. 187 00:13:27,470 --> 00:13:31,189 That does not mean that there are stars made of iron. 188 00:13:31,189 --> 00:13:36,733 The sun, with its trace contamination of carbon and oxygen and iron and calcium 189 00:13:36,733 --> 00:13:41,786 and so on, and silicon is a population one star, that's called metal-rich. 190 00:13:41,786 --> 00:13:46,839 The stars that we find in globular clusters are far more poor in metals, 191 00:13:46,839 --> 00:13:50,198 and. so then we call the metal poor stars 192 00:13:50,198 --> 00:13:54,508 population two. Now since we know by now that the carbon 193 00:13:54,508 --> 00:13:57,740 and the calcium and the iron as we'll see. 194 00:13:57,740 --> 00:14:01,049 That the sun incorporated into it's nebula. 195 00:14:01,049 --> 00:14:06,359 That were there when the solar nebula formed were actually formed by 196 00:14:06,359 --> 00:14:10,035 nucleosynthesis inside. A previous generation of stars. 197 00:14:10,035 --> 00:14:14,953 What we conclude is that population two stars formed in regions where they were 198 00:14:14,953 --> 00:14:19,257 essentially the first, or maybe the second generation of stars to form. 199 00:14:19,257 --> 00:14:24,298 Whereas population one stars are forming in regions where there was much previous 200 00:14:24,298 --> 00:14:27,740 star formation. Notice that its not a problem if the sun 201 00:14:27,740 --> 00:14:32,044 say 5 billion years old. Because a g-type star lives 10 billion 202 00:14:32,044 --> 00:14:36,040 years, but o and b-type stars that live say five billion years. 203 00:14:36,040 --> 00:14:41,511 Can turn over many thousands of times in the time between, say, the formation of 204 00:14:41,511 --> 00:14:47,328 the Milky Way 13 billion years ago, and the formation of the solar system 5 205 00:14:47,328 --> 00:14:50,999 billion years ago. There was lots of time for lots of 206 00:14:50,999 --> 00:14:54,670 generations of stars to have lived and, and scattered. 207 00:14:54,670 --> 00:14:58,437 Their planetary nebulae around. And in fact the conjecture, since there 208 00:14:58,437 --> 00:15:02,367 are some metals in the population two stars, the existence of a yet older 209 00:15:02,367 --> 00:15:05,866 generation called population three. We've never seen them, I think. 210 00:15:05,866 --> 00:15:09,741 But we believe they were there. These would have been the first stars in 211 00:15:09,741 --> 00:15:13,940 the universe or in any region in the universe and they would have essentially 212 00:15:13,940 --> 00:15:17,815 been pure hydrogen and helium stars. And we'll talk later about how that 213 00:15:17,815 --> 00:15:21,207 changes their dynamics and why we think they must have existed.