0:00:07.121,0:00:08.769 All the material objects around you 0:00:08.769,0:00:10.395 are composed of submicroscopic units 0:00:10.395,0:00:12.146 we call molecules. 0:00:12.146,0:00:13.398 And molecules in turn are composed 0:00:13.398,0:00:15.146 of individual atoms. 0:00:15.146,0:00:16.980 Molecules frequently break apart 0:00:16.980,0:00:19.063 and then form new molecules. 0:00:19.063,0:00:20.313 On the other hand, 0:00:20.313,0:00:22.248 virtually all the atoms you come in to contact with 0:00:22.248,0:00:23.813 through the course of your life, 0:00:23.813,0:00:25.417 the ones in the ground beneath you, 0:00:25.417,0:00:27.419 the air you breath, the food you eat, 0:00:27.419,0:00:29.712 those that make up every living thing, including you, 0:00:29.712,0:00:31.900 have existed for billions of years 0:00:31.900,0:00:34.882 and were created in places very unlike our planet. 0:00:34.882,0:00:38.349 How those atoms came about is what I want to share with you. 0:00:38.349,0:00:39.933 It all started 14 billion years ago 0:00:39.933,0:00:41.685 with an event we call The Big Bang, 0:00:41.685,0:00:44.856 which resulted in a universe consisting of gas alone. 0:00:44.856,0:00:46.896 There were no stars and no planets. 0:00:46.896,0:00:48.480 The gas was made up only of atoms 0:00:48.480,0:00:50.479 belonging to the simplest elements. 0:00:50.479,0:00:52.313 It was about 75 percent hydrogen 0:00:52.313,0:00:54.396 and almost all the rest was helium. 0:00:54.396,0:00:57.314 No elements like carbon, oxygen or nitrogen existed. 0:00:57.314,0:00:59.396 No iron, silver or gold. 0:00:59.396,0:01:03.146 In some places, the density of this gas was slightly higher than in others. 0:01:03.165,0:01:05.981 Due to gravity, those places attracted even more gas, 0:01:05.981,0:01:07.980 which further strengthened the pull of gravity, 0:01:07.980,0:01:10.670 which then drew more gas in, and so on. 0:01:10.670,0:01:13.507 Eventually, large dense gas balls formed, 0:01:13.507,0:01:15.312 shrinking under their own gravity 0:01:15.312,0:01:18.055 and consequently heating up on the inside. 0:01:18.055,0:01:19.645 At some point, the core of such a ball 0:01:19.645,0:01:22.147 gets hot enough that nuclear fusion occurs. 0:01:22.147,0:01:24.896 Hydrogen atoms smash together to form helium, 0:01:24.896,0:01:27.063 accompanied by a great release of energy, 0:01:27.063,0:01:30.564 strong enough to counteract the shrinking force of the gravity. 0:01:30.564,0:01:32.563 When the energy pushing out from the fusion reactions 0:01:32.563,0:01:35.113 matches the gravity pulling all the gas inwards, 0:01:35.113,0:01:37.063 an equilibrium occurs. 0:01:37.063,0:01:39.369 From this a star is born. 0:01:39.369,0:01:40.367 Over its lifetime, 0:01:40.367,0:01:42.480 the fusion reactions in the core of a massive star 0:01:42.480,0:01:44.064 will produce not only helium, 0:01:44.064,0:01:46.206 but also carbon, oxygen, nitrogen 0:01:46.206,0:01:50.125 and all the other elements in the periodic table up to iron. 0:01:50.125,0:01:52.146 But eventually, the core's fuel runs out, 0:01:52.146,0:01:54.089 leaving it to collapse completely. 0:01:54.089,0:01:56.257 That causes an unbelievably powerful explosion 0:01:56.257,0:01:58.480 we call a supernova. 0:01:58.480,0:01:59.595 Now there are two things to note 0:01:59.595,0:02:01.563 about how supernovas create elements. 0:02:01.563,0:02:04.310 First, this explosion releases so much energy 0:02:04.310,0:02:05.935 that fusion goes wild 0:02:05.935,0:02:08.564 forming elements with atoms even heavier than iron 0:02:08.564,0:02:10.897 like silver, gold and uranium. 0:02:10.897,0:02:12.730 Second, all the elements that had been accumulating 0:02:12.730,0:02:14.145 in the core of the star, 0:02:14.145,0:02:16.695 like carbon, oxygen, nitrogen, iron, 0:02:16.695,0:02:19.561 as well as all of those formed in the supernova explosion, 0:02:19.561,0:02:21.981 are ejected in to interstellar space 0:02:21.981,0:02:24.397 where they mix with the gas that's already there. 0:02:24.397,0:02:26.480 History then repeats itself. 0:02:26.480,0:02:28.667 Gas clouds, now containing many elements 0:02:28.667,0:02:31.083 besides the original hydrogen and helium, 0:02:31.083,0:02:32.548 have higher density areas 0:02:32.548,0:02:34.672 that attract more matter, and so on. 0:02:34.672,0:02:37.548 As before, new stars result. 0:02:37.548,0:02:40.302 Our sun was born this way about 5 billion years ago. 0:02:40.302,0:02:42.231 That means that the gas it arose from 0:02:42.231,0:02:44.266 had itself been enriched with many elements 0:02:44.266,0:02:47.980 from supernova explosions since the universe began. 0:02:47.980,0:02:50.646 So that's how the sun wound up with all the elements. 0:02:50.646,0:02:52.862 It's still mostly hydrogen at 71 percent, 0:02:52.862,0:02:55.565 with most of the rest being helium at 27 percent. 0:02:55.565,0:02:56.483 But bear in mind 0:02:56.498,0:02:57.813 that while the first stars were made up 0:02:57.813,0:02:59.647 of hydrogen and helium alone, 0:02:59.647,0:03:01.481 the remaining elements in the periodic table 0:03:01.481,0:03:03.731 make up two percent of the sun. 0:03:03.731,0:03:05.146 And what about Earth? 0:03:05.146,0:03:07.997 Planets form as an incidental process to star formation 0:03:07.997,0:03:11.126 out of the same gas cloud as the star itself. 0:03:11.126,0:03:13.042 Small planets like ours don't have enough gravity 0:03:13.042,0:03:15.587 to hold on to much hydrogen or helium gas 0:03:15.587,0:03:17.672 since both of those are very light. 0:03:17.672,0:03:20.312 So, even though carbon, nitrogen, oxygen and so on 0:03:20.312,0:03:23.731 made up only two percent of the gas cloud from which Earth was formed, 0:03:23.731,0:03:26.307 these heavier elements form the bulk of our planet 0:03:26.307,0:03:28.266 and everything on it. 0:03:28.266,0:03:29.518 Think about this: 0:03:29.518,0:03:31.645 with the exception of hydrogen and some helium, 0:03:31.645,0:03:33.145 the ground you walk on, 0:03:33.145,0:03:35.230 the air you breath, you, 0:03:35.230,0:03:36.813 everything is made of atoms 0:03:36.813,0:03:38.731 that were created inside stars. 0:03:38.731,0:03:40.813 When scientists first worked this out 0:03:40.813,0:03:42.731 over the first half of the 20th Century, 0:03:42.731,0:03:45.033 the famous astronomer Harlow Shapley commented, 0:03:45.033,0:03:48.620 "We are brothers of the boulders, cousins of the clouds."