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The stars are parts of our lives much more than we realize. Essentially, everything that is around us – and in us – was forged a long time ago inside a stellar furnace. All of the chemical elements that are heavier than carbon, the oxygen we breathe, the silicon that makes up the sand on the beach, all originate during the life cycle of stars.
Stars produce these chemical elements through nuclear fusion, the same process that gives them the energy that is radiated in the form of light. Most stars fuse hydrogen atoms into helium, and helium into carbon and oxygen, and end their life as a “carbon-oxygen white dwarf”: a really tightly packed ball of atoms for which the pull of gravity is counteracted by the so-called “electron degenerate pressure”. This means simply that there is a limit beyond which electrons don’t like to be too close to each other; this causes a repelling force that balances gravitational attraction. For the most massive 1% of stars, however, gravity wins over the electron degenerate pressure. These stars go on to transform carbon and oxygen into a cascade of elements, like neon, sodium, magnesium, aluminum, silicon, sulphur, argon, calcium, nickel and iron. This process eventually ends in a big explosion which sends many of the chemical elements that the star had produced in its outer layers hurling into space. For a few days, the dying star shines extremely bright, sometimes brighter than the galaxy that it is a part of, earning it the term “supernova”. The most massive and luminous stars burn their hydrogen into heavier elements at a furious pace, and thus run out of fuel quickly, so that this sort of “core-collapse” stellar explosions happen relatively shortly after a new generation of stars has been born.
There is one other way in which stars can become supernovae. This happens when one of the white dwarfs has a companion star, whose atmosphere spills over with time, bringing the white dwarf’s mass above the threshold that quantum mechanical forces can support against the squeeze of gravity. Carbon and oxygen then rapidly get transformed into heavier elements up to iron and nickel, and the energy released causes the entire white dwarf to explode and disperse these elements into space. This is called a “type Ia” or “thermonuclear” supernova. This process was thought to take a much longer time than a core-collapse supernova, because the white dwarf first has to form from a lower-mass star, and then requires additional time to accrete matter from its companion before the explosion occurs.
Core collapse and type Ia supernovae tend to produce very different chemical abundance patterns, with the former generating mostly lighter elements like oxygen and magnesium, while the latter are mainly responsible for iron and nickel. Intermediate elements like Si and S are produced in roughly equal amounts by both supernova types. We can then hope that, by measuring the chemical composition of the Universe, we can reconstruct the history of how, when, and where each of the chemical elements so necessary for the evolution of life were produced. Was the early Universe very different than today? Are there parts of the Universe that have a very different composition than ours?
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