Why does fire appear to have a defined edge and not simply dissolve out?
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Let’s ask the question – *what is a flame?*
Picture a candle, it is made of wax and wax wants to react with oxygen to create heat and light, in order to do so a molecule of wax must physically touch a molecule of oxygen.
So what happens is you light the candle, creating enough heat to melt and then vaporize some wax. The wax molecule is now airborne and the first moment, it’s just in a cloud of other wax molecules surrounding the wick – it can’t burn because it doesn’t have any oxygen molecules near it. So it travels away from the wick, maybe half an inch, where suddenly, like a wave on a beach, it hits fresh air, Oxygen! boom, it burns it and releases light and heat. What you see as the ‘edge of a flame’ can be thought of as the coastline between the wax-vapor ocean and the fresh-air-oxygen beach.
Since hot air rises flames on Earth get that sort of tear-drop shape as a portion of the wax vapor is pulled higher into the air before it touches fresh oxygen. In space flames form perfect spheres surrounding the source of evaporating wax.
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