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."
你身邊一切事物 都是由極微細的單位組成 這些單位稱為分子 而分子則由一粒一粒原子組成 分子不斷分裂 然後又組成新分子 另一方面 你一生中所接觸到的原子 大地、空氣、食物 組成你和一切生物的原子 已存在數以億年 發源地與地球非常不同 以下說的就是原子如何誔生 一切源於一百四十億年前 發生了宇宙大爆炸 產生了只有氣體的宇宙 無恆星亦無行星 氣體只由原子組成 那些原子屬於最簡單的元素 當中75%為氫 其餘多為氦 仍未有碳、氧或氮等元素 亦無鐵、銀或金 氣體密度有些地方稍高 因萬有引力而吸引更多氣體 引力因而更強 再吸引更多氣體,循環不息 最後形成大氣團 氣團因自身引力而萎縮 結果內部發熱 氣團核心熱到若干程度 就發生核融合 氫原子融合為氦 同時釋放巨大能量 大得可對抗向內拉扯的引力 融合所生向外推的能量 足以對抗向內拉扯的引力時 就會達平衡狀態 恆星就自此而生 巨大恆星一生之中 核心的融合反應不只產生氦 亦產生週期表中各元素 自碳、氧、氮至鐵止 但核心最終將耗盡燃料 完全崩潰 崩潰造成巨大爆炸 這爆炸稱為超新星爆發 對超新星如何產生元素 有兩點須注意 一, 爆炸釋放巨大能源 使融合能力變強 產生較鐵更重的原子 如銀、金及鈾 二,聚於恆星核心的所有元素 如碳、氧、氮、鐵 和超新星爆炸所產生的所有元素 均被排進星際空間 與空間已有的氣體混合 然後前事重演 氣團現在含多種元素 不單只有氫和氦 密度較高區域 吸引更多物質,如此類推 如前所述,產生新星 太陽就在五十億年前如此產生 即是說,產生太陽旳氣團 自宇宙開始,超新星爆發時 吸收過多種元素 所以現在太陽所有元素俱備 但氫仍佔71% 其餘27%為氦 但請緊記 首批恆星產生時 只有氫和氦 週期表中其餘元素 只佔太陽2% 地球又如何? 行星是恆星產生過程的副產品 來自同一氣體 地球等小行星 引力不足以捉緊氫或氦氣 因氫、氦均極輕 雖然碳、氮、氧等元素 在產生地球的氣團中只佔2% 這些較重的元素組成地球大部份 也組成地球各種物體一大部份 試想想: 除氫和氦外 你腳下大地 所吸空氣,還有你 當中的原子均來自恆星 二十世紀前半 科學家有此發現時 著名太空人沙普利曾說 「人與石為兄弟,與雲為表親」