Question: How was the Sun formed and what will happen when it runs out of energy?

Rosie Hayward answered on 22 Jun 2020:

The Sun was formed by part of a giant cloud of dust and gas collapsing around 4.6 billion years ago! This probably also made many other stars. A shockwave from a nearby explosion in space (a supernova) would have compressed the dust and gas, causing it to clump together. Gravity would then cause more and more gas and dust to clump together, forming the Sun. Gravity and pressure from the added mass would have generated heat, which would have eventually triggered nuclear fusion – the fusing together of atoms and molecules (specifically hydrogen into helium)! This reaction releases energy which is why the Sun is bright and hot.

The Sun doesn’t have enough mass to explode, so it will become a Red Giant in roughly 5 million years. All the hydrogen will be used up, and the Sun will get bigger and brighter – it will probably engulf Earth! Late in this stage, it will emit a glowing shell of ionised gas called a Planetary Nebula. When all the nuclear reactions are over, the Sun will contract under gravity and cool down, changing colour. At this stage, it becomes a White Dwarf. Eventually it will stop glowing at all and become a Black Dwarf – supposedly the universe isn’t old enough for any Black Dwarfs to exist yet!