Alchemy was the ancient art of transformation, a precursor to chemistry. At its heart was the dream of turning a base metal like lead into a precious metal like gold. It doesn't seem outrageous; both are dense, malleable metals of dull appearance in their natural state. Alchemy was proto-science rather than crank science and its imagery and has permeated into popular culture, most recently in the blockbuster Harry Potter series of books and films. Alchemy features in the titles of two of the books, the symbolism in the naming of many of the characters, and in the person of Hogwart's headmaster Albus Dumbledore, who is stated to be a an alchemist of renown, partner of Nicolas Flamel, a real-life alchemist from the 14th century. In literature, alchemy is used as a metaphor for the redemptive power of transformation and purification.
In real life, however, alchemy was doomed to failure. The essence of an element lies in its atomic nucleus, and this fortress cannot be touched by chemical means, which only operate on the outer shell of electrons. Alchemists need only have looked to the stars to find their Philosopher's Stone. Every star is involved in the transmutation of elements. Because stars are large gas balls held together by gravity, the density and temperature rise consistently as you move towards their centers. In the interior of the Sun, where the temperature exceeds 10 million degrees, hydrogen is converted into helium by the process, which is the merging of atomic nuclei to form heavier elements. The protons in atomic nuclei have a positive electric change so they resist each other like tiny magnets. It requires a phenomenal temperature to force them to fuse.
Radiation from fusion is what keeps a star "puffed up." At every point in a star the inward the force of gravity is balanced by the outward force of radiation. Stars aren't expanding or out of control like bombs; their fusion is steady and measured. The energy released from the nuclear reactions reaches us as sunlight. The best technology on Earth can only keep a fusion reaction going for a fraction of a second; the Sun does it endlessly and effortlessly. Each second, it converts a mass of hydrogen equivalent to twenty cruise ships into helium, and it will continue to do so for another five billion years.
A typical star like the Sun spends most of its life fusing hydrogen into helium. After the hydrogen is exhausted, the star loses its pressure support and its core collapses. That compression continues until the ignition a new nuclear fuel creates a balance at a new, higher temperature. Fusion is an unnatural act. It forces atomic nuclei, who have an intense electrical dislike for each other, to merge. The helium nucleus has two protons, so its positive electrical charge is larger than hydrogen, which has only one. It takes a much higher temperature of 100 million degrees to make helium fuse.
The next step is the key to life. It requires luck and a juggler's skill. Two helium nuclei fuse to make a beryllium nucleus, but beryllium is unstable and it decays in a tiny fraction of a second. It's as if someone has a building block in his hands but it crumbles to dust before they can add another piece. Occasionally, some beryllium survives and if the energy levels are just right, a third helium nucleus is added. Carbon is born. This two-step fusion is so tricky that it causes a bottleneck. As a result, the universe has 300 times less carbon than helium.
Now it gets interesting. Like Willy Wonka making cosmic candy, the cosmic element factory continues to build heavier elements. Add a hydrogen nucleus and the atomic number increases by one. Add a helium nucleus and it increases by two. In stars like the Sun, reactions peter out at carbon because small stars can't get hot enough for carbon nuclei to fuse. But in higher mass stars, when an extra proton is added to carbon, it becomes nitrogen. One more, and oxygen is formed. Suitably warmed up, the cosmic element maker picks up the pace. Helium fuses to oxygen makes neon. Helium fuses to neon makes magnesium. Finally, at the awesome temperature of three billion degrees, two silicon nuclei fuse to create a nickel nucleus. Nickel quickly decays into iron-the most stable element. The end of the line.
In massive stars, creation of heavy elements is a crescendo. It takes ten million years to turn hydrogen into helium, half a million years to make carbon, and only 600 years to make neon. Silicon fuses to make iron in less than a day. Imagine the frenzy of a roller coaster ride that goes faster and turns in an ever-tighter spiral until you're dumped breathless and dizzy at the bottom.
The creation of elements up to iron releases nuclear energy because it give a more tightly-bound nucleus. Heavier elements are so unstable that they decay spontaneously in a process called fission. Since it costs energy to make elements beyond iron, stars choose to sit tight and hold their cards. Low mass stars end up with a seething core of carbon, with some nitrogen and oxygen. High mass stars have cores of iron, not the solid iron of a wrecking ball but a bizarre, dense billion-degree gas.