WEBVTT 00:00:07.121 --> 00:00:08.769 All the material objects around you 00:00:08.769 --> 00:00:10.395 are composed of submicroscopic units 00:00:10.395 --> 00:00:12.146 we call molecules. 00:00:12.146 --> 00:00:13.398 And molecules in turn are composed 00:00:13.398 --> 00:00:15.146 of individual atoms. 00:00:15.146 --> 00:00:16.980 Molecules frequently break apart 00:00:16.980 --> 00:00:19.063 and then form new molecules. 00:00:19.063 --> 00:00:20.313 On the other hand, 00:00:20.313 --> 00:00:22.248 virtually all the atoms you come in to contact with 00:00:22.248 --> 00:00:23.813 through the course of your life, 00:00:23.813 --> 00:00:25.417 the ones in the ground beneath you, 00:00:25.417 --> 00:00:27.419 the air you breath, the food you eat, 00:00:27.419 --> 00:00:29.712 those that make up every living thing, including you, 00:00:29.712 --> 00:00:31.900 have existed for billions of years 00:00:31.900 --> 00:00:34.882 and were created in places very unlike our planet. 00:00:34.882 --> 00:00:38.349 How those atoms came about is what I want to share with you. 00:00:38.349 --> 00:00:39.933 It all started 14 billion years ago 00:00:39.933 --> 00:00:41.685 with an event we call The Big Bang, 00:00:41.685 --> 00:00:44.856 which resulted in a universe consisting of gas alone. 00:00:44.856 --> 00:00:46.896 There were no stars and no planets. 00:00:46.896 --> 00:00:48.480 The gas was made up only of atoms 00:00:48.480 --> 00:00:50.479 belonging to the simplest elements. 00:00:50.479 --> 00:00:52.313 It was about 75 percent hydrogen 00:00:52.313 --> 00:00:54.396 and almost all the rest was helium. 00:00:54.396 --> 00:00:57.314 No elements like carbon, oxygen or nitrogen existed. 00:00:57.314 --> 00:00:59.396 No iron, silver or gold. 00:00:59.396 --> 00:01:03.146 In some places, the density of this gas was slightly higher than in others. 00:01:03.165 --> 00:01:05.981 Due to gravity, those places attracted even more gas, 00:01:05.981 --> 00:01:07.980 which further strengthened the pull of gravity, 00:01:07.980 --> 00:01:10.670 which then drew more gas in, and so on. 00:01:10.670 --> 00:01:13.507 Eventually, large dense gas balls formed, 00:01:13.507 --> 00:01:15.312 shrinking under their own gravity 00:01:15.312 --> 00:01:18.055 and consequently heating up on the inside. 00:01:18.055 --> 00:01:19.645 At some point, the core of such a ball 00:01:19.645 --> 00:01:22.147 gets hot enough that nuclear fusion occurs. 00:01:22.147 --> 00:01:24.896 Hydrogen atoms smash together to form helium, 00:01:24.896 --> 00:01:27.063 accompanied by a great release of energy, 00:01:27.063 --> 00:01:30.564 strong enough to counteract the shrinking force of the gravity. 00:01:30.564 --> 00:01:32.563 When the energy pushing out from the fusion reactions 00:01:32.563 --> 00:01:35.113 matches the gravity pulling all the gas inwards, 00:01:35.113 --> 00:01:37.063 an equilibrium occurs. 00:01:37.063 --> 00:01:39.369 From this a star is born. 00:01:39.369 --> 00:01:40.367 Over its lifetime, 00:01:40.367 --> 00:01:42.480 the fusion reactions in the core of a massive star 00:01:42.480 --> 00:01:44.064 will produce not only helium, 00:01:44.064 --> 00:01:46.206 but also carbon, oxygen, nitrogen 00:01:46.206 --> 00:01:50.125 and all the other elements in the periodic table up to iron. 00:01:50.125 --> 00:01:52.146 But eventually, the core's fuel runs out, 00:01:52.146 --> 00:01:54.089 leaving it to collapse completely. 00:01:54.089 --> 00:01:56.257 That causes an unbelievably powerful explosion 00:01:56.257 --> 00:01:58.480 we call a supernova. 00:01:58.480 --> 00:01:59.595 Now there are two things to note 00:01:59.595 --> 00:02:01.563 about how supernovas create elements. 00:02:01.563 --> 00:02:04.310 First, this explosion releases so much energy 00:02:04.310 --> 00:02:05.935 that fusion goes wild 00:02:05.935 --> 00:02:08.564 forming elements with atoms even heavier than iron 00:02:08.564 --> 00:02:10.897 like silver, gold and uranium. 00:02:10.897 --> 00:02:12.730 Second, all the elements that had been accumulating 00:02:12.730 --> 00:02:14.145 in the core of the star, 00:02:14.145 --> 00:02:16.695 like carbon, oxygen, nitrogen, iron, 00:02:16.695 --> 00:02:19.561 as well as all of those formed in the supernova explosion, 00:02:19.561 --> 00:02:21.981 are ejected in to interstellar space 00:02:21.981 --> 00:02:24.397 where they mix with the gas that's already there. 00:02:24.397 --> 00:02:26.480 History then repeats itself. 00:02:26.480 --> 00:02:28.667 Gas clouds, now containing many elements 00:02:28.667 --> 00:02:31.083 besides the original hydrogen and helium, 00:02:31.083 --> 00:02:32.548 have higher density areas 00:02:32.548 --> 00:02:34.672 that attract more matter, and so on. 00:02:34.672 --> 00:02:37.548 As before, new stars result. 00:02:37.548 --> 00:02:40.302 Our sun was born this way about 5 billion years ago. 00:02:40.302 --> 00:02:42.231 That means that the gas it arose from 00:02:42.231 --> 00:02:44.266 had itself been enriched with many elements 00:02:44.266 --> 00:02:47.980 from supernova explosions since the universe began. 00:02:47.980 --> 00:02:50.646 So that's how the sun wound up with all the elements. 00:02:50.646 --> 00:02:52.862 It's still mostly hydrogen at 71 percent, 00:02:52.862 --> 00:02:55.565 with most of the rest being helium at 27 percent. 00:02:55.565 --> 00:02:56.483 But bear in mind 00:02:56.498 --> 00:02:57.813 that while the first stars were made up 00:02:57.813 --> 00:02:59.647 of hydrogen and helium alone, 00:02:59.647 --> 00:03:01.481 the remaining elements in the periodic table 00:03:01.481 --> 00:03:03.731 make up two percent of the sun. 00:03:03.731 --> 00:03:05.146 And what about Earth? 00:03:05.146 --> 00:03:07.997 Planets form as an incidental process to star formation 00:03:07.997 --> 00:03:11.126 out of the same gas cloud as the star itself. 00:03:11.126 --> 00:03:13.042 Small planets like ours don't have enough gravity 00:03:13.042 --> 00:03:15.587 to hold on to much hydrogen or helium gas 00:03:15.587 --> 00:03:17.672 since both of those are very light. 00:03:17.672 --> 00:03:20.312 So, even though carbon, nitrogen, oxygen and so on 00:03:20.312 --> 00:03:23.731 made up only two percent of the gas cloud from which Earth was formed, 00:03:23.731 --> 00:03:26.307 these heavier elements form the bulk of our planet 00:03:26.307 --> 00:03:28.266 and everything on it. 00:03:28.266 --> 00:03:29.518 Think about this: 00:03:29.518 --> 00:03:31.645 with the exception of hydrogen and some helium, 00:03:31.645 --> 00:03:33.145 the ground you walk on, 00:03:33.145 --> 00:03:35.230 the air you breath, you, 00:03:35.230 --> 00:03:36.813 everything is made of atoms 00:03:36.813 --> 00:03:38.731 that were created inside stars. 00:03:38.731 --> 00:03:40.813 When scientists first worked this out 00:03:40.813 --> 00:03:42.731 over the first half of the 20th Century, 00:03:42.731 --> 00:03:45.033 the famous astronomer Harlow Shapley commented, 00:03:45.033 --> 00:03:48.620 "We are brothers of the boulders, cousins of the clouds."