THE LIFE CYCLE OF A STAR
A star is born when a cold, dark cloud of interstellar gas and dust shrinks under its own gravity. As the gas is squeezed ever smaller, it gets hotter. Eventually, when the core temperature exceeds 10,000,000°C, nuclear reactions ignite, and the ball of gas lights up as a star.
A star represents a temporary balance between the forces of gravity trying to shrink a ball of gas and its internal heat pushing outwards. The star fuses the cores, or ‘nuclei’, of hydrogen, the lightest atom, into the second lightest, helium. The mass difference between the initial and final product appears as the energy of sunlight, according to Einstein’s famous formula E=mc2. This conversion has an important effect on a star like the Sun. As helium is heavier than hydrogen, it falls to the centre. The nuclei of atoms repel each other, and the bigger the nucleus, the stronger the repulsion. For two new nuclei to stick together and make a heavier nucleus, they must slam into each other at high speed, which in practice means at high temperature since temperature is a measure of microscopic motion. The core of the Sun will only ever be dense and hot enough to fuse together hydrogen into helium. However, this is not the case with more massive stars. Their cores eventually become dense and hot enough to fuse helium into carbon, carbon into oxygen, oxygen into neon,
“A STAR REPRESENTS A TEMPORARY BALANCE BETWEEN GRAVITY TRYING TO SHRINK A BALL OF GAS AND ITS INTERNAL HEAT PUSHING OUTWARDS”
and so on. Such stars end up with an internal structure reminiscent of an onion, with the heaviest elements in the centre surrounded by concentric shells of less and less heavy elements.
The end point of this build-up process is iron. Its creation sucks nuclear energy from the core of the star. This causes the core to start shrinking, faster and faster, until a tiny, ultra-dense ball of neutrons, called a neutron star, is formed. In-falling material bounces off the neutron core, converting implosion into explosion – the explosion of a supernova that’s so bright it can outshine an entire galaxy of stars. But, if the core is massive enough, no known force can stop gravity crushing the core out of existence – in fact, crushing it all the way down to a point of infinite density known as a ‘singularity’. Cloaked in the impenetrable wall of an ‘event horizon’, this is a black hole.