Short answer In space, hot air doesn't rise, so a candle flame burns round and blue.
A candle flame always rises up to a point. It's so obvious you've probably never wondered why.
The hot air around the flame is light, so it rises. Cooler air from below flows in to fill the gap. This flow pulls the flame up into a long, thin shape.
So what about in space, where nothing has weight?
This is a candle lit on the space station. Hot air doesn't rise there, so the flame spreads out into a round shape.
The color is different too. On Earth, soot rises, so the top of the flame looks yellow. In space, the flame is bluer and burns better.
On the space station, they also run experiments that set drops of fuel on fire. Once lit, the flame burns like a ball wrapped around the drop.
Guess birthday cake candles would be round in space too
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