The violent death of a star and why it happens

Supernovae have been observed well before the invention of the telescope, with observational records dating back millennia within many cultures across the world.

The fact that stars exist at all is a result of a delicate balancing act. Stars are giant balls of hydrogen, and gravity pulls this gas to the centre of the star. As it compresses, the hydrogen atoms fuse together to form helium. This process releases a lot of heat, building up pressure which pushes the gas out.

This outward pressure is balanced with the inward pull of gravity, causing stars to have their neat spherical shape. However, if this balance is tipped in gravity's favour, the star will explode as a supernova.

One type of supernova, called a core-collapse supernova, is the result of a massive star, eight times or so heavier than our dun, running out of fuel. Starting from hydrogen, the first and lightest element on the periodic table, the atoms in the star's core fuse together to build heavier and heavier elements. Heat is produced as this fusion continues, until the star is left with a core made of iron.

It takes an incredible amount of energy to fuse two iron atoms together, and this reaction gives off no heat. As a result, the outward pressure from the centre of the star stops. The balance of the star is tipped in gravity's favour, which causes the atmosphere of the star to collapse in on itself and bounce off the iron core, producing an enormous shockwave and flash of light that we see as a supernova.

A more exotic type of supernova explosion is a thermonuclear supernova. These are known to occur from extremely small and incredibly dense stars called white dwarfs. The outward pressure in these stars doesn't come from heat, but rather strong atomic forces.

We can imagine a white dwarf star as a box of billiard balls. We can shrink the box to a certain point, but eventually the billiard balls will push against each other and physically stop the box from shrinking any further. A similar process occurs in white dwarf stars, except instead of the outward pressure coming from billiard balls, it comes from electrons, one of the building blocks of atoms.

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This outward resistance is extremely strong, but not impossible to overcome. The strength of a star's gravitational pull depends on its mass, and if the star weighs about 1.4 times the mass of our own Sun, gravity will be strong enough to overcome the electrons' outward resistance, resulting in a runaway nuclear explosion. White dwarf stars usually weigh less than this mass, but they can be tipped over the line if they are able to pull the gas from a nearby star, or if they collide with another white dwarf.

The fact that stars exist at all is mind boggling, as they require the pull of gravity to be balanced on a knife edge with the outward pressure from the star's centre. If this balance is tipped even slightly, the star explodes in a spectacular shockwave and a bright flash of light.