Wonders of the Universe - Brian Cox [50]
Stars exist in an uneasy equilibrium. Their gravity acts to compress them, which heats them up until the electromagnetic repulsion between the hydrogen atoms is overcome and they fuse together to make helium. This releases energy, which keeps the star up. When the hydrogen runs out, the outward pressure disappears; gravity regains the upper hand and the structure of the star changes dramatically. The core collapses rapidly, leaving a shell of hydrogen and helium behind. Within the shrinking core the temperature rises until, at 100 million degrees Celsius, a new fusion process is triggered. At these temperatures helium nuclei can overcome their mutual electromagnetic repulsion and wander close enough together to fuse – the star begins to burn helium. This transfer from hydrogen to helium fusion has two profound effects: firstly, sufficient energy is released to halt the stellar collapse, so the star stabilises and rapidly swells. This is the beginning of its life as a red giant. Secondly, it fuses into existence the element vital for life. At first sight the fusion of two helium nuclei, each consisting of two protons and two neutrons, should only be able to produce the isotope beryllium-8, composed of four protons and four neutrons. This is an unstable isotope of beryllium that quickly breaks down, but in the intense temperatures of a dying star, as the core exceeds 100 million Kelvin, these nuclei live just long enough to fuse with a third helium nucleus, creating the precious element carbon-12. This is where all the carbon in the Universe comes from; every carbon atom in every living thing on the planet was produced in the heart of a dying star.
Just as in a dying star, the structure of a building and the elements that keep it standing become unstable over time. This prison was given a helping hand to its destruction, but a dying star will detonate itself as it reaches the end of its life, producing spectacular planetary nebulae. It took seconds to demolish the prison block, which is the same length of time it takes for a red giant star to collapse.
The helium-burning phase doesn’t end with the alchemic synthesis of carbon, because during the same intensely hot phase in the star’s life the conditions allow a nucleus of helium to latch onto a newly minted carbon nucleus to create another element vital for life. Oxygen makes up 21 per cent of the air we breathe, is a prerequisite for water, the solvent of life, and is the third-most common element in the Universe after hydrogen and helium. As you breathe in around two and a half grams of oxygen each minute, it’s worth remembering that all this life-giving gas was created in an environment as far away from our understanding of what is habitable as you can get.
Compared with the lifetime of a star, this stellar production line of carbon and oxygen is over in the blink of an eye. Within about a million years the helium supply in the core is used up, and for many stars that’s where fusion stops. Any average-sized star, like our sun, has by now reached the end of its productive life. When our sun reaches this stage, in about ten billion years’ time, there won’t be enough gravitational energy to compress the core any further and restart fusion. Instead, the star becomes more and more unstable, huge pressure points will build up, until eventually the whole stellar atmosphere explodes, hurling the precious cargo of oxygen, carbon, hydrogen, and all, on its journey into space. For at this brief moment in time, no