system
Given the following post, respond as if you're commenting on Reddit:
human
Can someone explain in relatively simple terminology why it is the case that the Sun is not on fire?

I know that the Sun works by nuclear fusion, but the sun does look like it is on fire.
gpt
They are both hot.

It is as simple as that, the reason why fires glow is because they are hot. They emit radiation (black body in the case of the orange glow, spectral in the case of the blue you get with gas).

As something gets hotter the radiation it emits moves from infra-red to red to red to orange to white. Both the Sun and a fire are hot enough to emit visible radiation.

The reason that the Sun is NOT a fire is because of the source of the heat. A fire is heated by combustion which is a reaction with oxygen. The Sun is heated by a nuclear reaction.