Familiar Chemical Elements

Familiar Chemical Elements

Two stars of roughly the same mass will evolve roughly in parallel. But a more massif star will spend its nuclear fuel faster, become a red giant sooner, and be first to enter the final white dwarf decline. There should therefore be, as they are, many cases of binary stars, one component a red giant, the other a white dwarf. Some such pairs are so close together that they touch, and the glowing stellar atmosphere flows from the distended red giant to the compact white dwarf, tending to fall on a particular province of the surface of the white dwarf.

The hydrogen accumulates, compressed to higher and higher pressures and temperatures by the intense gravity of the white dwarf, until the stolen atmosphere of the red giant undergoes thermonuclear reactions, and the white dwarf briefly flares into brilliance. Such a binary is called a nova and has quite a different origin from a supernova. Novae occur only in binary systems and are powered by hydrogen fusion; supernovae occur in single stars and are powered by silicon fusion.

Atoms synthesized in the interiors of stars are commonly returned to the interstellar gas. Red giants find their outer atmospheres blowing away into space; planetary neubulae are the final stages of Sunlike stars blowing their tops. Supernovae violently eject much of their stellar mass into space. The atoms returned are, naturally, those most readily made in the thermonuclear reactions in stellar interiors: Hydrogen fuses into helium, helium into carbon, carbon into oxygen and thereafter, in massive stars, by the successive addition of further helium nuclei, neon, magnesium, silicon, sulfur, and so on are built – additions by stages, two protons and two neutrons per stage, all the way to iron. Direct fusion of silicon also generates iron, a pair of silicon atoms, each with twenty-eight protons and neutrons, joining, at a temperature of billions of degrees, to make an atom of iron with fifty-six protons and neutrons.

The Light. Illustration : Elena

These are all familiar chemical elements. We recognize their names. Such stellar nuclear reactions do not readily generate erbium, dysprosium, hafnium, praseodymium or yttrium, but rather the elements we know in everyday life, elements returned to the interstellar gas, where they are swept up in a subsequent generation of cloud collapse and star and planet formation.

All the elements of the Erath except hydrogen and some helium have been cooked by a kind of stellar alchemy billions of years ago in stars, some of which are today inconspicuous white dwarfs on the other side of the Milky Way Galaxy. The nitrogen of our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars.