1 00:00:00,680 --> 00:00:02,070 PROFESSOR: Have you ever wondered 2 00:00:02,070 --> 00:00:06,120 how all the chemical elements are made? 3 00:00:06,120 --> 00:00:08,640 Then join me as we are lifting all this data 4 00:00:08,640 --> 00:00:11,630 secrets to understand the cosmic origin of the chemical 5 00:00:11,630 --> 00:00:12,130 elements. 6 00:00:15,250 --> 00:00:18,220 Let's summarize what we've learned so far about the older 7 00:00:18,220 --> 00:00:21,760 stuff and how they can be used in our concept 8 00:00:21,760 --> 00:00:25,870 of stellar archeology to understand what happened soon 9 00:00:25,870 --> 00:00:28,750 after the big bang in terms of the chemical enrichment 10 00:00:28,750 --> 00:00:29,992 and chemical evolution. 11 00:00:36,880 --> 00:00:42,380 Still we have old stars, and we call them metal-poor. 12 00:00:46,690 --> 00:00:48,640 And they are our tool. 13 00:00:48,640 --> 00:00:53,080 They are our tool to study the early universe. 14 00:00:58,270 --> 00:01:05,580 These stars are long-lived, so they have a low mass, something 15 00:01:05,580 --> 00:01:11,140 like 0.6 to 0.8 solar masses. 16 00:01:11,140 --> 00:01:14,190 And that means they have lifetimes 17 00:01:14,190 --> 00:01:18,300 of 15 to 20 billion years. 18 00:01:21,490 --> 00:01:24,211 And that means that they are still observable. 19 00:01:27,580 --> 00:01:30,650 And that is very lucky for us. 20 00:01:30,650 --> 00:01:32,510 And they are not just observable. 21 00:01:32,510 --> 00:01:34,420 They are actually fairly easily observable, 22 00:01:34,420 --> 00:01:38,410 because they're now located in the Milky Way. 23 00:01:41,320 --> 00:01:42,990 Let's look at this again. 24 00:01:42,990 --> 00:01:50,760 So this is very quick drawing of our Milky Way. 25 00:01:50,760 --> 00:01:53,790 This is the bulge, the inner part 26 00:01:53,790 --> 00:01:57,530 of our galaxy with a supermassive black hole 27 00:01:57,530 --> 00:01:58,720 in the center. 28 00:01:58,720 --> 00:02:01,740 And this here-- actually two disks. 29 00:02:01,740 --> 00:02:04,020 So this is the disk. 30 00:02:04,020 --> 00:02:07,650 And we're about here, 2/3 on the way out. 31 00:02:07,650 --> 00:02:10,805 And the bulge contains a lot of young stars. 32 00:02:10,805 --> 00:02:12,180 There's a lot of gas, which means 33 00:02:12,180 --> 00:02:13,770 you have formed a lot of stars, which 34 00:02:13,770 --> 00:02:16,290 means you make a lot of elements and you form more stars. 35 00:02:16,290 --> 00:02:18,360 So the bulge is very metal-rich. 36 00:02:18,360 --> 00:02:23,640 And the disk here is not quite as metal-rich but still 37 00:02:23,640 --> 00:02:26,310 pretty enriched. 38 00:02:26,310 --> 00:02:29,970 Now, this is not the only part of our galaxy. 39 00:02:29,970 --> 00:02:32,850 This is just the most visible part, namely 40 00:02:32,850 --> 00:02:35,520 the Milky Way band on the night sky. 41 00:02:35,520 --> 00:02:37,200 That's when you look into the spiral 42 00:02:37,200 --> 00:02:39,420 arms that make up the disk. 43 00:02:39,420 --> 00:02:43,650 But we look in a different place for the older stars, 44 00:02:43,650 --> 00:02:46,320 because they are kind of located up 45 00:02:46,320 --> 00:02:54,420 here and below the disk in something that's 46 00:02:54,420 --> 00:02:55,800 called the halo. 47 00:02:55,800 --> 00:02:59,850 It's actually much larger than what I'm drawing right now. 48 00:02:59,850 --> 00:03:02,550 And so that's called the halo of the disk. 49 00:03:02,550 --> 00:03:07,410 It's a spherical envelope of the disk here. 50 00:03:07,410 --> 00:03:12,000 And all the old stuff is parked there. 51 00:03:12,000 --> 00:03:13,110 It's a bit of a junkyard. 52 00:03:15,780 --> 00:03:23,070 Because when a galaxy forms, you have small systems 53 00:03:23,070 --> 00:03:26,130 that actually come together and form a bigger system. 54 00:03:26,130 --> 00:03:29,280 And then here you have a bigger system too. 55 00:03:29,280 --> 00:03:32,490 And then they come together and make an even bigger one. 56 00:03:32,490 --> 00:03:36,480 So that's called the hierarchical structure 57 00:03:36,480 --> 00:03:38,250 formation paradigm. 58 00:03:38,250 --> 00:03:41,520 And so this is the Milky Way. 59 00:03:41,520 --> 00:03:44,040 And which means that these little guys here kind of end 60 00:03:44,040 --> 00:03:46,440 up in the outskirts, or at least a good amount 61 00:03:46,440 --> 00:03:48,450 of these little guys end up in the outskirts. 62 00:03:48,450 --> 00:03:50,160 But they are completely shredded apart. 63 00:03:50,160 --> 00:03:52,740 And what is left are all the stars 64 00:03:52,740 --> 00:03:55,140 that are being spilled into the Milky Way. 65 00:03:55,140 --> 00:03:57,240 And so this is how old stars actually 66 00:03:57,240 --> 00:04:03,570 get into the outer halo of the Milky Way. 67 00:04:03,570 --> 00:04:08,460 I should mention here that little dwarf galaxies are also 68 00:04:08,460 --> 00:04:12,423 actually in the halo of the galaxy. 69 00:04:12,423 --> 00:04:13,590 So they are also pretty old. 70 00:04:13,590 --> 00:04:17,430 So these are entire little systems here 71 00:04:17,430 --> 00:04:19,579 that are not completely shredded yet. 72 00:04:19,579 --> 00:04:22,000 So they are just in the gravitational field 73 00:04:22,000 --> 00:04:23,130 here of the Milky Way. 74 00:04:23,130 --> 00:04:25,800 And they're orbiting around the disk. 75 00:04:25,800 --> 00:04:29,700 And we also have globular clusters. 76 00:04:29,700 --> 00:04:35,460 These are clusters of stars with up to a million stars. 77 00:04:35,460 --> 00:04:38,920 They are also located here and down here. 78 00:04:38,920 --> 00:04:40,530 And they are also really old. 79 00:04:40,530 --> 00:04:43,520 We don't really know where they come from. 80 00:04:43,520 --> 00:04:45,840 So the halo contains mostly three things-- 81 00:04:45,840 --> 00:04:49,560 globular clusters, dwarf galaxy, and lots of old stars. 82 00:04:49,560 --> 00:04:54,720 And so with our telescope we can peek from here kind of here up 83 00:04:54,720 --> 00:05:00,360 into the halo and down here and observe all the old stars 84 00:05:00,360 --> 00:05:03,220 that are in this range. 85 00:05:03,220 --> 00:05:05,700 So all in all, our metal-poor stars 86 00:05:05,700 --> 00:05:08,580 are the local equivalent to what we call the high redshift 87 00:05:08,580 --> 00:05:10,140 universe. 88 00:05:10,140 --> 00:05:14,280 In a very complementary way, both metal-poor stars 89 00:05:14,280 --> 00:05:18,420 and the furthest most distant galaxies 90 00:05:18,420 --> 00:05:21,960 are used to study the early universe. 91 00:05:21,960 --> 00:05:25,860 These faraway galaxies, when their light comes to us, 92 00:05:25,860 --> 00:05:29,460 we receive it from this early time. 93 00:05:29,460 --> 00:05:34,530 And this way, we can figure out what this galaxy can tell us 94 00:05:34,530 --> 00:05:36,030 about the early universe, because it 95 00:05:36,030 --> 00:05:38,010 formed at that early time. 96 00:05:38,010 --> 00:05:39,840 Our metal-poor stars, their light 97 00:05:39,840 --> 00:05:41,850 hasn't traveled for a long time. 98 00:05:41,850 --> 00:05:45,780 It has traveled maybe just from here to us. 99 00:05:45,780 --> 00:05:47,550 That's a negligible amount of time. 100 00:05:47,550 --> 00:05:50,910 Because these stars are today located in our Milky Way. 101 00:05:50,910 --> 00:05:52,500 But they are really old. 102 00:05:52,500 --> 00:05:56,070 We see them as when they are old, 103 00:05:56,070 --> 00:05:58,890 not as when they were young, as in the case 104 00:05:58,890 --> 00:06:01,360 of these distant galaxies. 105 00:06:01,360 --> 00:06:03,900 But the fact that we see them all doesn't matter to us. 106 00:06:03,900 --> 00:06:06,030 Because these stars don't get wrinkly or anything. 107 00:06:06,030 --> 00:06:07,610 They just sit there. 108 00:06:07,610 --> 00:06:12,180 And they are just waiting for us to observe them. 109 00:06:12,180 --> 00:06:14,700 And as we will see in the following, 110 00:06:14,700 --> 00:06:16,290 these stars are really undisturbed. 111 00:06:16,290 --> 00:06:20,520 And today, they look like just what 112 00:06:20,520 --> 00:06:22,510 they did 13 billion years ago. 113 00:06:22,510 --> 00:06:24,790 So that's a huge advantage for us. 114 00:06:24,790 --> 00:06:28,730 And of course, we're going to make use of it.