1 00:00:00,000 --> 00:00:01,020 [MUSIC PLAYING] 2 00:00:01,020 --> 00:00:02,478 ANNA FREBEL: Have you ever wondered 3 00:00:02,478 --> 00:00:06,120 how all the chemical elements are made? 4 00:00:06,120 --> 00:00:08,640 Then, join me as we are lifting all the data 5 00:00:08,640 --> 00:00:11,630 secrets to understand the cosmic origin of the chemical 6 00:00:11,630 --> 00:00:12,130 elements. 7 00:00:15,990 --> 00:00:18,410 We're now going to look at the first chemical enrichment 8 00:00:18,410 --> 00:00:22,370 events, and how the universe recycles matter. 9 00:00:22,370 --> 00:00:25,280 [MUSIC PLAYING] 10 00:00:30,140 --> 00:00:33,890 Imagine that this is the primordial gas left over 11 00:00:33,890 --> 00:00:35,780 from after the Big Bang. 12 00:00:35,780 --> 00:00:38,720 And as we already said, the first stars 13 00:00:38,720 --> 00:00:40,520 formed from this gas. 14 00:00:40,520 --> 00:00:43,070 So here is a first star. 15 00:00:43,070 --> 00:00:46,820 And stars are not static objects. 16 00:00:46,820 --> 00:00:50,210 They actually evolve with time, which is an interesting thing, 17 00:00:50,210 --> 00:00:54,290 and we're going to look into more detail at that later. 18 00:00:54,290 --> 00:00:55,790 But for now, we're just going to say 19 00:00:55,790 --> 00:01:01,400 that they evolve, for example, into something 20 00:01:01,400 --> 00:01:04,040 that's called a red giant. 21 00:01:04,040 --> 00:01:06,196 Actually it's going to get much bigger. 22 00:01:06,196 --> 00:01:09,660 So a red giant here. 23 00:01:09,660 --> 00:01:13,670 And what happens, is already during this evolutionary phase 24 00:01:13,670 --> 00:01:19,460 here, stars have strong stellar winds 25 00:01:19,460 --> 00:01:24,380 so they can lose mass from their surface. 26 00:01:24,380 --> 00:01:27,950 Whatever is in that gas that's being lost, 27 00:01:27,950 --> 00:01:31,290 gets put back into the reservoir here. 28 00:01:31,290 --> 00:01:33,220 If this is a massive star, which is 29 00:01:33,220 --> 00:01:35,660 a given in the case of the first star, 30 00:01:35,660 --> 00:01:39,050 this star is going to keep evolving 31 00:01:39,050 --> 00:01:43,040 until it explodes as a giant supernova, 32 00:01:43,040 --> 00:01:45,140 so as an explosion of the star. 33 00:01:45,140 --> 00:01:48,350 The star gets completely disrupted. 34 00:01:48,350 --> 00:01:52,850 And naturally, everything from the outer portions as well as 35 00:01:52,850 --> 00:01:58,010 the inner portions of the star gets spilled around 36 00:01:58,010 --> 00:02:00,790 and put back into the reservoir again. 37 00:02:00,790 --> 00:02:05,270 And so, here we now have all these new elements 38 00:02:05,270 --> 00:02:07,790 from the core of the star that are 39 00:02:07,790 --> 00:02:12,070 being put into the reservoir. 40 00:02:12,070 --> 00:02:18,680 And so, some time later after the death of these four stars, 41 00:02:18,680 --> 00:02:21,740 this gas cloud is chemically enriched. 42 00:02:21,740 --> 00:02:23,840 And then, the next generation of stars 43 00:02:23,840 --> 00:02:26,680 forms from this enriched material. 44 00:02:26,680 --> 00:02:28,660 They'll evolve. 45 00:02:28,660 --> 00:02:32,510 The massive ones contribute new elements, make new elements 46 00:02:32,510 --> 00:02:34,670 and contribute them. 47 00:02:34,670 --> 00:02:37,755 Low mass stars, they don't explode in supernova. 48 00:02:37,755 --> 00:02:42,210 They just keep sitting there happily ever after, 49 00:02:42,210 --> 00:02:43,130 pretty much. 50 00:02:43,130 --> 00:02:48,290 So they do not contribute to this chemical evolution cycle, 51 00:02:48,290 --> 00:02:52,430 but all the massive stars with every new generation 52 00:02:52,430 --> 00:02:54,770 contribute to a successive build up 53 00:02:54,770 --> 00:02:57,750 of all the elements with time. 54 00:02:57,750 --> 00:03:00,500 Now an interesting consequence of that, 55 00:03:00,500 --> 00:03:08,570 is that old stars have a lower overall abundance 56 00:03:08,570 --> 00:03:11,420 of these heavy elements because they're simply 57 00:03:11,420 --> 00:03:14,420 formed at a time when the cycle here had only 58 00:03:14,420 --> 00:03:16,790 gone around a few times. 59 00:03:16,790 --> 00:03:28,210 So all stars contain little of the heavy elements 60 00:03:28,210 --> 00:03:29,980 heavier than hydrogen and helium. 61 00:03:33,910 --> 00:03:40,010 And consequently, younger stars, starting 62 00:03:40,010 --> 00:03:42,560 with the Sun, and even younger than that, 63 00:03:42,560 --> 00:03:46,240 they contain a relatively larger amount. 64 00:03:46,240 --> 00:03:47,775 So they are more enriched. 65 00:03:50,420 --> 00:03:55,550 And we already had it, the Sun, has 1.4% 66 00:03:55,550 --> 00:03:57,470 of all these heavy elements. 67 00:03:57,470 --> 00:04:02,330 And a star that would be born today would have 2%. 68 00:04:02,330 --> 00:04:06,560 These old stars here however, compared to the sun, 69 00:04:06,560 --> 00:04:12,170 contain only a millionth of what the sun contains. 70 00:04:12,170 --> 00:04:13,920 So a millionth of 1%. 71 00:04:13,920 --> 00:04:16,250 That's a really, really small number. 72 00:04:16,250 --> 00:04:20,959 So that really makes old stars stand out. 73 00:04:20,959 --> 00:04:24,380 The issue for us is that we need to figure out a way how 74 00:04:24,380 --> 00:04:28,910 to measure the element composition of our stars 75 00:04:28,910 --> 00:04:33,140 so that we can figure out are there older or younger, 76 00:04:33,140 --> 00:04:35,030 which really means have they formed 77 00:04:35,030 --> 00:04:38,930 early on in this cycle here or much later. 78 00:04:38,930 --> 00:04:42,320 We equate that to old age or younger age, 79 00:04:42,320 --> 00:04:45,050 but we do so without an actual age measurement. 80 00:04:45,050 --> 00:04:48,990 So it's an inferred quantity for now, 81 00:04:48,990 --> 00:04:52,760 but various independent tests have 82 00:04:52,760 --> 00:04:55,620 shown that this is a pretty good assumption, 83 00:04:55,620 --> 00:04:59,570 and that stars with very little of all the elements 84 00:04:59,570 --> 00:05:03,470 really are old and formed as some stars in these very 85 00:05:03,470 --> 00:05:05,840 early generations. 86 00:05:05,840 --> 00:05:07,580 Now in terms of the nomenclature, 87 00:05:07,580 --> 00:05:11,330 we have to introduce one important term, namely 88 00:05:11,330 --> 00:05:12,470 old stars. 89 00:05:12,470 --> 00:05:15,110 Well, as I just said, we don't really have an age measurement. 90 00:05:15,110 --> 00:05:19,430 We just infer that it formed soon after the Big Bang. 91 00:05:19,430 --> 00:05:23,360 And so, what astronomers use is the term metal poor, 92 00:05:23,360 --> 00:05:28,940 because that actually describes what the star's composition is. 93 00:05:28,940 --> 00:05:34,120 It is poor in heavy elements, metals as astronomers say, 94 00:05:34,120 --> 00:05:36,620 and it is poor compared to the Sun. 95 00:05:36,620 --> 00:05:38,730 The Sun is our reference star. 96 00:05:38,730 --> 00:05:41,600 The Sun has 1.4% of metals. 97 00:05:41,600 --> 00:05:44,570 And our old stars from the early universe 98 00:05:44,570 --> 00:05:48,020 contain only a tiny, tiny fraction of this here. 99 00:05:48,020 --> 00:05:50,970 And so, we call them metal poor. 100 00:05:50,970 --> 00:05:52,940 And when you look for the older stars, 101 00:05:52,940 --> 00:05:55,520 or want to look for the older stars, what you actually 102 00:05:55,520 --> 00:05:57,170 have to do, is you have to search 103 00:05:57,170 --> 00:05:59,210 for the most metal poor stars. 104 00:05:59,210 --> 00:06:02,260 [MUSIC PLAYING]