All the material objects around you are composed of submicroscopic units we call molecules. And molecules in turn are composed of individual atoms. Molecules frequently break apart and then form new molecules. On the other hand, virtually all the atoms you come in to contact with through the course of your life, the ones in the ground beneath you, the air you breath, the food you eat, those that make up every living thing, including you, have existed for billions of years and were created in places very unlike our planet. How those atoms came about is what I want to share with you. It all started 14 billion years ago with an event we call The Big Bang, which resulted in a universe consisting of gas alone. There were no stars and no planets. The gas was made up only of atoms belonging to the simplest elements. It was about 75 percent hydrogen and almost all the rest was helium. No elements like carbon, oxygen or nitrogen existed. No iron, silver or gold. In some places, the density of this gas was slightly higher than in others. Due to gravity, those places attracted even more gas, which further strengthened the pull of gravity, which then drew more gas in, and so on. Eventually, large dense gas balls formed, shrinking under their own gravity and consequently heating up on the inside. At some point, the core of such a ball gets hot enough that nuclear fusion occurs. Hydrogen atoms smash together to form helium, accompanied by a great release of energy, strong enough to counteract the shrinking force of the gravity. When the energy pushing out from the fusion reactions matches the gravity pulling all the gas inwards, an equilibrium occurs. From this a star is born. Over its lifetime, the fusion reactions in the core of a massive star will produce not only helium, but also carbon, oxygen, nitrogen and all the other elements in the periodic table up to iron. But eventually, the core's fuel runs out, leaving it to collapse completely. That causes an unbelievably powerful explosion we call a supernova. Now there are two things to note about how supernovas create elements. First, this explosion releases so much energy that fusion goes wild forming elements with atoms even heavier than iron like silver, gold and uranium. Second, all the elements that had been accumulating in the core of the star, like carbon, oxygen, nitrogen, iron, as well as all of those formed in the supernova explosion, are ejected in to interstellar space where they mix with the gas that's already there. History then repeats itself. Gas clouds, now containing many elements besides the original hydrogen and helium, have higher density areas that attract more matter, and so on. As before, new stars result. Our sun was born this way about 5 billion years ago. That means that the gas it arose from had itself been enriched with many elements from supernova explosions since the universe began. So that's how the sun wound up with all the elements. It's still mostly hydrogen at 71 percent, with most of the rest being helium at 27 percent. But bear in mind that while the first stars were made up of hydrogen and helium alone, the remaining elements in the periodic table make up two percent of the sun. And what about Earth? Planets form as an incidental process to star formation out of the same gas cloud as the star itself. Small planets like ours don't have enough gravity to hold on to much hydrogen or helium gas since both of those are very light. So, even though carbon, nitrogen, oxygen and so on made up only two percent of the gas cloud from which Earth was formed, these heavier elements form the bulk of our planet and everything on it. Think about this: with the exception of hydrogen and some helium, the ground you walk on, the air you breath, you, everything is made of atoms that were created inside stars. When scientists first worked this out over the first half of the 20th Century, the famous astronomer Harlow Shapley commented, "We are brothers of the boulders, cousins of the clouds."
围绕在你周围的所有物质 都是由极小的单位体构成的 我们叫它们分子 同时分子又是由 单独的原子构成 分子常常分裂 然后构成新的分子 换句话说 我们一生中接触的 所有原子 你脚下的土地 你所呼吸着的空气,你吃着的食物 包括你在内,构成所有生命体的原子 已经存在了数十亿年 并由与地球大为不同的空间组成 今天我就要与你们分享这些原子是如何产生的 这些是都从140亿年前 我们所称之为的宇宙大爆炸时开始 大爆炸形成了只有气体的宇宙 没有任何恒星和行星 而气体只由原子组成 这些原子属于最简单的元素 其中大约有75%的氢气 剩下的大约24%则都是氦气 像碳,氧,氮等元素都是不存在的 也没有铁,银或金 在某些地方,这些气体的密度会比别的地方稍高 由于重力因素,密度高地方就会吸引更多的气体 并且使重力增加 于是更多的气体被吸引,并以这种形式一直循环下去 渐渐地,形成了体积大且密度高的气体球 它们依靠自身的重力 在内部产生热量 从某种角度来看,是这种球体中心的 原子核结合产生的炙热 氢气聚集形成氦气 释放出巨大的能量 足够抵消使球体缩小的重力 从这种反应开始释放出的能量 与进入内部的气体重力相同的话 即形成了球形体 因此一个恒星也就诞生了 这颗恒星存在期间 从巨大的恒星中心引起的融合反应 不仅产生了氦气 还有碳,氧气及氮气 以及元素周期表中诸如铁等所有元素 但是随着中心燃料的逐渐减少 最后将消耗殆尽 那时,将引起难以置信的大爆炸 那就是我们所说的超新星 现在我们要了解两件关于 超新星如何制造的元素事 首先,这种爆炸会释放巨大的能量 继而形成游离的 比铁更重的元素 如银,金和铀原子等 其次,聚集在恒星中心的 所有元素 如碳,氧,氮,铁 它们不仅是由那些超新星爆炸形成的 而且包括宇宙空间喷涌而出的 这些元素与已存在的气体混合 以这种形式循环往复 气体云,现在除了原始的氢气和氦气以外 已包含着许多元素 并有更高密度的空间 去吸引更多的物质 因此,一颗新的恒星诞生了 我们的太阳就是在50亿年前以这种方式诞生的 可以说形成太阳的气体的状态 自宇宙形成以来 从超新星爆炸开始就已经形成了 所以太阳中拥有所有的元素 不过其中占最大比例的是71%的氢气 以及27%的氦气 然而要知道的是 即使最初的恒星 仅由氢气和氦气构成 元素周期表中的剩余元素 还是组成了太阳的其余2% 我们的地球又如何呢? 行星是依照恒星形成的过程 由同样的气体云聚合产生的 像地球这样的小行星由于重力不够 无法留住过多的氢气和氦气 毕竟氢气和氦气太轻了 因此,即使碳,氮气,氧气等 只组成了形成地球的气体云的2% 但这些元素却构成了地球的地面 和地上的所有东西 想一想 除了一些氢气和氦气 你脚下的土地 你呼吸的空气,以至于你本身 全都由形成恒星内形成的 原子所构成 因此当二十世纪前期 科学家们刚得出这个结论时 著名的天文学哈洛沙普利说: “我们都是岩石的兄弟,云朵的表亲。”