What happens when a star goes supernova?

Question posed by Aimee.

Some stars towards the end of their lives go supernova. Supernovas can be seen by us as a brightening of a star over a few days or weeks, followed by a gradual fading of the star's brightness over a matter of months or years. Seen through a telescope, supernovas may be even more impressive, with more detail and various different, beautiful colours being displayed. But why does it happen?

What actually happens to a star when it goes supernova?
To understand what makes a star go supernova, you need to understand the basic ideas behind what a star is and why it burns in the first place:
A star is a big ball of gas that collapses under its own gravity. This collapse provides the energy to start the gas burning*. This burning provides energy that pushes outwards and stops the star from collapsing further. The gravitational energy and the energy supplied by the burning balance out for a while (a few billion years) and the star doesn't noticably shrink or expand.

When most of a star's fuel is used up, the outward pressure starts to fall, gravity takes over again, and the star starts to shrink as the matter it is made up of starts to fall towards the centre again. Sometimes (depending on the mass** of the star) this inward falling sparks off another bout of nuclear burning. This burning may be brief but violent, lasting only a matter of days. The star literally explodes, and the outer layers*** of the star are thrown outwards in all directions at speeds of up to 30,000 km/s.

Such energy being given out in such a short space of time means that the star and the gas it has thrown off can glow many times brighter than the galaxy it is in before gradually fading.

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* By a process called nuclear fusion.
** 'Mass' is how much stuff something is made up of.
*** Sometimes the entire star may blow itself apart.


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