Note to self: don't node about Moby. You don't like that man. Focus, young grasshopper.

Before the stars existed (that would be a long time ago, when the Universe was about a thousand years old), there existed very few things made of something more complicated than helium. Getting protons and neutrons to get together and make simple atoms and well, the victory of matter over antimatter was more than we could ask for, isn't it? But well, we know that things are made of elements heavier than lithium (some of which was supposed to be 'built' at those stages).

Well then, some clouds of hydrogen and helium were attractive enough to get into a relatively small volume. This produces high pressure and temperature, conditions needed for a lot of interesting physics stuff. We are now interested in nuclear fusion, which melds light atoms to form heavier ones. So hydrogen burns to helium, and later on, helium turns into carbon and we go on, creating the heavier elements. This is interesting to us, because it's about the only reason we have to explain how elements such as iron came to exist.

So where is the We Are All Made Of Stars part? Well, humans (and most things biological) are made mostly of carbon, hydrogen, oxygen, nitrogen, etc. (even gold. There's some gold in us. All of us). You put two and two together. Sometime, somewhere, a star exploded and the little bits that got blasted away came to be you and me. We once were stars.

Note: thanks to all the msg'ers and downvoters who made me remember that I had actually studied this.

I hate to give a vegan credit, but I'm afraid that our little friend Moby is correct.

We are all made of the ashes of dead stars.

Our sweet, little periodic table lists about 115 elements, ranging from the mundane--hydrogen, containing one proton--to the massive--ununoctium, with a grand total of one hundred and eighteen protons. Where did all of these elements come from? Have they always been here? Were they created when the universe formed?

Well, hydrogen and helium are primordial. They have been around since day one. All other elements, however, are a result of stellar nucleosynthesis--the nuclear fusion in the hearts of stars.

Stellar nucleosynthesis begins with the hydrogen proton-proton chain. High temperatures--those over 10 million K--cause the hydrogen nuclei to start a little dance party. The nuclei grow so hot and dance so fast that they can overcome their mutual electrical repulsion. (Similar to a dork like me dancing next to some hot chick at a concert. She doesn’t want to touch me, but she just can’t help herself.) Thus, a series of nuclear reactions occur, and the dance party ultimately creates a nucleus of ordinary helium from four protons of hydrogen. The helium builds up in the core of the star, and the dancers take a break. The core then contracts and heats up again. When the temperature exceeds about 100 million K (much higher than the temperature requirement for the hydrogen-to-helium dancing), the helium nuclei start grinding away. The new helium dance party leads to the triple-alpha reaction, where 3 ordinary helium-4 nuclei form one carbon-12 nucleus.

At higher and higher temperatures, heavier and heavier nuclei gain enough energy to overcome their mutual electrical repulsion. At about 600 million K (a temperature technically described by the term “hot-as-balls”), carbon nuclei can fuse to form magnesium. However for any nuclei larger than carbon, dance parties are quite uncommon. (Kind of like people over forty attending evil death rock concerts. It just doesn’t happen very often.) As the star evolves, heavier elements tend to form through helium-capture rather than the fusion of like nuclei. For example, carbon-helium fusion occurs at much lower temperatures than that of carbon-carbon fusion. Since the repulsive force between two carbon nuclei is three times greater than the force between a nucleus of one carbon and one of helium, a reaction at a lower temperature is possible and much more likely.

As nuclei of many different types accumulate, a great variety of reactions become possible. In some, protons and neutrons are freed from their parent nucleus and are absorbed by another. A new nucleus with a mass intermediate between the ones formed by helium-capture is then created.

Enter silicon-28, the Helen of stellar conflict. A struggle rises between two processes--the continued capture of helium to produce even heavier nuclei and the breakdown of complex nuclei into simper ones. The cause of this “breakdown” is the blazing temperature in the core (described as uber-hot-as-balls). The gamma rays associated with this temperature carry enough energy to blast a nucleus apart--a process called photodisintegration. The blasting leads to another element-creating event called the alpha process—photodisintegration followed by the direct capture of the resulting helium-4 nuclei (also called alpha particles). But the alpha process, which creates new nuclei, ironically leads to the star’s ultimate demise. Any unstable nucleus will continue to decay until stability is achieved. Iron-56, the most stable of all nuclei, builds up in the star’s core and will doom the existence of the star.

If the alpha process stops at iron, how did the heavier elements form? These elements can form through a process called neutron capture--the formation of heavier nuclei by the absorption of neutrons. Adding neutrons to a nucleus does not change the element. Rather, a more massive isotope of the same element is produced. Eventually, however, so many neutrons get piled onto the nucleus that it becomes unstable. It then decays radioactively to form a stable nucleus of a new and heavy element.

Enter the r-process. During the first 15-minutes of a supernova blast, the number of free neutrons increases dramatically. Heavy nuclei are torn apart by the violence of the explosion and splatter neutrons everywhere. The neutron-capture rate during the explosion is so great that even unstable nuclei can catch many neutrons before they have time to decay. Thus, the heaviest elements occur after their parent stars have already died.

The death of these stars causes another phenomena. Making the universe into one giant galactic urn, the supernova blast spreads the elements formed within its core all over space. Then from these ashy remnants, other bodies are created. So everything is made from the ashes of dead stars--planets, moons, water, trees, people...

...even our little vegan Moby.

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