1 00:00:00,612 --> 00:00:02,070 ANNA FREBEL: 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 --> 00:00:09,420 Then join me as we are lifting all the [INAUDIBLE] secrets 4 00:00:09,420 --> 00:00:12,105 to understand the cosmic origin of the chemical elements. 5 00:00:16,129 --> 00:00:18,140 What is the universe actually made of? 6 00:00:18,140 --> 00:00:19,120 [MUSIC PLAYING] 7 00:00:25,500 --> 00:00:29,040 Let's look at three different times in the universe 8 00:00:29,040 --> 00:00:31,500 and consider what it was made of then-- 9 00:00:31,500 --> 00:00:33,510 first, after the Big Bang. 10 00:00:38,160 --> 00:00:41,380 So if we draw a little pie chart here, 11 00:00:41,380 --> 00:00:44,370 the universe was made from just hydrogen and helium 12 00:00:44,370 --> 00:00:47,370 and tiny little bits of lithium. 13 00:00:47,370 --> 00:00:51,370 We can just cut it up like this. 14 00:00:51,370 --> 00:00:58,550 And so this is hydrogen, 75%, and helium, 25%. 15 00:00:58,550 --> 00:01:01,220 And that pretty much adds up to 100 already. 16 00:01:01,220 --> 00:01:04,709 But we'll just write it up here, so lithium is of the order 10 17 00:01:04,709 --> 00:01:10,120 to the minus 10, which is really just a tiny, tiny, tiny amount. 18 00:01:10,120 --> 00:01:13,830 And we don't really need to worry much more about lithium. 19 00:01:13,830 --> 00:01:21,000 Now at a time later, namely 4.6 billion years 20 00:01:21,000 --> 00:01:24,810 after the Big Bang, an important event 21 00:01:24,810 --> 00:01:27,060 happened-- at least for us humans. 22 00:01:27,060 --> 00:01:29,610 The sun was born! 23 00:01:29,610 --> 00:01:33,600 And from studying the chemical composition 24 00:01:33,600 --> 00:01:36,870 of the sun, as I will explain later, 25 00:01:36,870 --> 00:01:41,400 we can deduce what the universe was made of at that time. 26 00:01:41,400 --> 00:01:45,000 And as it turns out the universe looked a little bit 27 00:01:45,000 --> 00:01:48,320 different in its composition. 28 00:01:48,320 --> 00:01:51,030 It looked something like this. 29 00:01:51,030 --> 00:02:02,070 Here we have hydrogen, 71.6%, 27% helium, 30 00:02:02,070 --> 00:02:04,290 and then this one here. 31 00:02:04,290 --> 00:02:07,395 That's 1.4% of heavy elements. 32 00:02:12,150 --> 00:02:13,760 So what we can see-- 33 00:02:13,760 --> 00:02:16,820 and again, we're going to go into more detail later-- 34 00:02:16,820 --> 00:02:20,850 some of the hydrogen got converted into helium. 35 00:02:20,850 --> 00:02:23,450 So we have less hydrogen about 5 billion years 36 00:02:23,450 --> 00:02:25,670 after the Big Bang, a little bit more helium. 37 00:02:25,670 --> 00:02:28,460 And helium, through various steps, 38 00:02:28,460 --> 00:02:32,540 has been converted into heavier elements-- a whole 1.4%. 39 00:02:32,540 --> 00:02:38,210 Now if we then look at what things look like today-- 40 00:02:38,210 --> 00:02:45,650 that's 13.8 billion years after the Big Bang-- 41 00:02:45,650 --> 00:02:52,295 We see that the heavy elements make up a whole 2% elements. 42 00:02:54,662 --> 00:02:56,870 And there's a little bit more helium and a little bit 43 00:02:56,870 --> 00:03:01,270 less hydrogen. So we have a whole 2% 44 00:03:01,270 --> 00:03:05,030 of all the matter is in elements heavier 45 00:03:05,030 --> 00:03:06,440 than hydrogen and helium. 46 00:03:06,440 --> 00:03:09,320 So as you can see, the two most important elements 47 00:03:09,320 --> 00:03:11,990 in the universe clearly are hydrogen and helium. 48 00:03:11,990 --> 00:03:14,690 And really who cares about all of these other heavy elements? 49 00:03:14,690 --> 00:03:17,650 They make today 2% and in the early universe 50 00:03:17,650 --> 00:03:20,450 it was zero and then a little bit. 51 00:03:20,450 --> 00:03:23,210 And accordingly, astronomers already 52 00:03:23,210 --> 00:03:29,330 quite a while ago came up with the Astronomer's Periodic Table 53 00:03:29,330 --> 00:03:32,270 that is pretty simple-- actually so simple that I 54 00:03:32,270 --> 00:03:33,970 can draw it for you here. 55 00:03:33,970 --> 00:03:36,230 And it contains of three things-- 56 00:03:36,230 --> 00:03:41,200 X, Y, and Z. 57 00:03:41,200 --> 00:03:44,840 And you can guess what X is, it's hydrogen. Hydrogen is 58 00:03:44,840 --> 00:03:47,420 pretty important in the universe and it sits up there 59 00:03:47,420 --> 00:03:49,220 in the periodic table. 60 00:03:49,220 --> 00:03:53,030 Helium is also pretty important, second most common element. 61 00:03:53,030 --> 00:03:55,580 It's in the top right corner there. 62 00:03:55,580 --> 00:03:58,150 And then all the heavy elements combined, 63 00:03:58,150 --> 00:03:59,150 they were called metals. 64 00:04:01,670 --> 00:04:03,830 And they together make up Z. 65 00:04:03,830 --> 00:04:07,130 So we can simplify the universe pretty well 66 00:04:07,130 --> 00:04:11,120 to just hydrogen, helium, and metals. 67 00:04:11,120 --> 00:04:13,430 But of course we know that the devil is in the detail 68 00:04:13,430 --> 00:04:15,290 and we are really interested in these metals 69 00:04:15,290 --> 00:04:20,450 here because that is the subject of this lecture-- 70 00:04:20,450 --> 00:04:23,990 the cosmic origin of these heavy elements. 71 00:04:23,990 --> 00:04:29,120 And I should say here, of course, that metals is not-- 72 00:04:29,120 --> 00:04:31,640 in terms of the chemistry, it's not the correct description 73 00:04:31,640 --> 00:04:35,120 of all these elements that are found in the periodic table. 74 00:04:35,120 --> 00:04:37,610 But in all we are astronomers so we get away 75 00:04:37,610 --> 00:04:39,680 with calling all the elements metals 76 00:04:39,680 --> 00:04:44,510 even if in a chemical sense they are nothing but metal. 77 00:04:44,510 --> 00:04:47,600 So that's a little historical piece. 78 00:04:47,600 --> 00:04:49,880 And people still use the term metal. 79 00:04:49,880 --> 00:04:54,080 And we're going to use it for our stars as well. 80 00:04:54,080 --> 00:04:55,910 Before we move on to the next topic, 81 00:04:55,910 --> 00:05:00,050 I just want to say that you can see the universe changes 82 00:05:00,050 --> 00:05:05,150 its overall composition with time, which means that stars 83 00:05:05,150 --> 00:05:09,440 formed at different times will naturally have a slightly 84 00:05:09,440 --> 00:05:12,500 different composition as well because stars 85 00:05:12,500 --> 00:05:16,910 form from gas that is available at a given place at a given 86 00:05:16,910 --> 00:05:18,740 time in the universe. 87 00:05:18,740 --> 00:05:21,890 And the star's surface layers, they 88 00:05:21,890 --> 00:05:26,060 do reflect the composition of the birth gas cloud. 89 00:05:26,060 --> 00:05:28,160 So we are in a very lucky position 90 00:05:28,160 --> 00:05:32,070 that if we find stars born at different times 91 00:05:32,070 --> 00:05:34,190 and we study their chemical composition, 92 00:05:34,190 --> 00:05:37,730 we can reconstruct how the composition, the makeup 93 00:05:37,730 --> 00:05:39,920 of the universe, changed. 94 00:05:39,920 --> 00:05:42,730 And that's exactly what we're going to do.