The colour of fire and its temperature are directly linked; red flames are cooler, blue flames are hotter. Acetylene and pure oxygen burns blue, at over 3,400ºC – the hottest temperature readily achievable with fuel and flame. That’s hot enough to melt tungsten, which has the highest melting point of any element.

And while that’s extremely hot on our very narrow human scale, it’s nothing compared to the temperatures achieved by nature. There may even be no limit to temperature as we know it; it could be a bizarre, infinite loop where hot becomes cold.

Fire inside a volcano