What is sun?

What is the first thing that comes to mind when you hear the word 'sun'?

In most basic cases, you look up, right?

We look up at a nearly perfect ball of plasma which mostly looks bright yellow, orange, or red.

We all know it is the most important source of energy for the life inside this nature.

Though it's just a mass of gases and, in my previous world's cases, it was made of mostly hydrogen and helium.

The nuclear fusion reaction at its core was the reason it produces energies in the form of different radiations.

The formation of the sun had happened a long time ago with a gravitational collapse of an enormous dust cloud.

But...

We believe the sun in this world to be part of nature and this 'nature' may probably be the entire observable universe.

Humans know little about anything outside this planet, and there is no need to even know.

There is no way of space travel in this world.

They mostly left the idea of outer space itself out of normal academics.