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."
Svi opipljivi objekti oko vas su sastavljeni od submikroskopskih jedinica koje zovemo molekuli. A molekuli su sastavljeni od zasebnih atoma. Molekuli se često razlažu i onda grade nove molekule. Sa druge strane, praktično svi atomi sa kojima stupite u kontakt tokom života, oni na tlu ispod vas, u vazduhu koji dišete, u hrani koju jedete, oni koji čine svaku živu stvar, uključujući i vas, postoje milijardama godina i stvoreni su na mestima koja nisu uopšte kao naša planeta. Kako su ti atomi nastali je ono što želim da podelim sa vama. Sve je počelo pre 14 milijardi godina pojavom koju zovemo Veliki prasak, čiji je ishod bio da se svemir sastojao samo od gasa. Nije bilo zvezda ni planeta. Gas se samo sastojao od atoma koji su pripadali najjednostavnijim elementima. To je bilo 75% vodonika a skoro sve ostalo je bio helijum. Nisu postojali elementi kao što je ugljenik, kiseonik ili azot. Ni gvožđe, srebro ili zlato. U nekim mestima, gustina gasa je bila malo viša. Zbog gravitacije, ta mesta su privlačila još više gasa, koji je dalje jačao gravitaciju, koja je povlačila više gasa, i tako dalje. Na kraju, stvorene su lopte gustog gasa, skupljajući se pod sopstvenom gravitacijom i tako su se grejale iznutra. U nekom trenutku, jezgro takve lopte postaje toliko vrelo da se dešava nuklearna fuzija. Atomi vodonika se sudaraju i stvaraju helijum, što prati veliko oslobađanje energije, dovoljno jako da se usprotivi skupljajućoj sili gravitacije. Kada je energija koja se oslobađa nuklearnim fuzijama jednaka gravitaciji koja vuče gas unutra, dešava se ravnoteža. Iz ovoga je rođena zvezda. Tokom njenog života, fuzije u jezgru ogromne zvezde proizvešće ne samo helijum, već i ugljenik, kiseonik, azot i sve druge elemente periodnog sistema do gvožđa. I na kraju, fuziono jezgro se istroši i dovodi do potpunog prestanka. Ovo izazviva neverovatno moćnu eksploziju. koju zovemo supernova. Primetimo dve stvari o tome kako supernove stvaraju elemente. Prvo, ova eksplozija oslobađa toliko energije da fuzija podivlja stvarajući elemente sa atomima čak težim od gvožđa, kao što je srebro, zlato i uranijum. Drugo, svi elementi koji su se sakupljali u zvezdinom jezgru, kao što je ugljenik, kiseonik, azot, gvožđe, kao i oni stvoreni u eksploziji supernove, su izbačeni u međuzvezdani prostor gde se mešaju sa gasom koji tu već postoji. Ovde se istorija ponavlja. Oblaci gasa, koji sad sadrže mnoge elemente pored pređašnjih vodonika i helijuma, imaju oblasti veće gustine koji privlače više materije, i tako dalje. Kao i pre, nastaje nova zvezda. Naše Sunce je rođeno na ovaj način pre oko 5 milijardi godina. Ovo znači da je gas od kojih je stvoreno bio obogaćen mnogim elementima iz eksplozija supernove od početka svemira. Tako je Sunce završilo sa toliko elemenata. I dalje je uglavnom vodonik, 71%, a većina ostatka je helijum, 27%. Ali imajte na umu da dok su prve zvezde sadržale samo vodonik i helijum, ostali elementi periodnog sistema čine dva procenta Sunca. A šta je sa Zemljom? Planete nastaju kao slučajni proces pri stvaranju zvezda iz istog oblaka gasa kao i same zvezde. Male planete kao naša nemaju dovoljnu gravitaciju da zadrže mnogo gasa vodonika i helijuma jer su oba veoma laka. Iako ugljenik, azot, kiseonik i drugi čine samo dva procenta oblaka gasa iz kojeg je nastala Zemlja, ovi teži elementi grade masu naše planete i sve na njoj. Pomislite na ovo: sa izuzetkom vodonika i nešto helijuma, zemlja po kojoj hodate, vazduh koji dišete, vi, sve je sastavljeno od atoma koji su stvoreni unutar zvezda. Kada su naučnici ovo najpre shvatili tokom prve polovine 20. veka poznati astronom Harlou Šepli je izjavio: "Mi smo braća stenama, rođaci oblaka".