Chemistry is a science that deals with the composition, structure, and properties of substances and with the transformations that they undergo. The bedrock of modern chemistry is the periodic table of elements. First laid out by Russian chemist Dmitry Ivanovich Mendeleyev, the elements in the periodic table are arranged according to their atomic structures. Hydrogen occupies the first place in the table because it is the simplest of all the elements, consisting of only one proton in its nucleus and one electron revolving around it.

Helium, with two protons, occupies the second place in the periodic table. Carbon has six protons and oxygen has eight. All the elements differ in the number of protons that they contain. Another particle present in the nucleus of an atom is the neutron. Unlike protons, neutrons do not carry an electrical charge: they are neutral in other words, hence their name.

The third basic particle of which atoms are composed is the electron, which has a negative electrical charge. In every atom, the number of protons and electrons is the same. Unlike protons and neutrons however, electrons are not located in the nucleus. Instead, they move around the nucleus at a very high speed that keeps the positive and negative charges of the atom apart.

The differences in atomic structure (the numbers of protons/electrons) are what make the elements different from one another. A crucial rule of (classical) chemistry is that elements cannot be transformed into one another. Changing iron (with twenty-six protons) into silver (with eighteen) would require removing eight protons from the nucleus. But protons are bound together by the strong nuclear force and the number of protons in a nucleus can be changed only in nuclear reactions. Yet all the reactions that take place under terrestrial conditions are chemical reactions that depend on electron exchange and that do not effect the nucleus.

In the Middle Ages there was a "science" called alchemy–the forerunner of modern chemistry. Alchemists, unaware of the periodic table or the atomic structures of the elements, thought it was possible to transform one element into another. (A favorite object of pursuit, for reasons that should be apparent, was trying to turn iron into gold.) We now know that what the alchemists were trying to do is impossible under normal conditions such as exist on Earth: The temperatures and pressures required for such a transformation to take place are too enormous to achieve in any terrestrial laboratory. But it is possible if you have the right place to do it in. And the right place, it turns out, is in the hearts of stars.