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.
If you could see in infrared, flame would have a much less defined edge. The edge of a flame is the point where you it stops emitting visible light
Fire has a defined edge because the hot gases rise up and away from the flame, carrying heat with them. This creates a boundary between the hot gases and the cooler air.
[This video](https://youtu.be/5ymAXKXhvHI?si=fBkGhrtJfQv9xTqP) does a really good job of explaining what a flame is. It is aimed at children, but as an adult it really helped me visualize the process much better.
Why **would** it “simply dissolve out”?
Flames have varying appearances depending on a lot of factors. For a really cool extreme example, the Space Shuttle main engines and solid rocket boosters had very different appearing plumes. The solids were bright orange and the three main engines were much less visible: https://commons.wikimedia.org/wiki/File:Shuttle_profiles.jpg This is largely because of the composition of the fuel and oxidizer involved. For the main engines, liquid hydrogen and liquid oxygen burn to form water vapor. For the solids, aluminum powder and ammonium perchlorate. Lots of hot solid particles in the exhaust.
The orange glow of that and of a [Bunsen Burner](https://commons.wikimedia.org/wiki/File:Bunsen_burner_flame_types.jpg) is due to soot particles glowing hot. This is called a [luminous flame](https://en.wikipedia.org/wiki/Luminous_flame). As the particles glow, they cool off and become less visible. Perhaps this is what you’re perceiving as a defined edge?
Note though the different appearances of the flame by opening the air vent allowing the fuel and air to mix before igniting. The more complete combustion gives a different appearance.
Fire science actually gets pretty complicated so I’m not sure which part to simplify to try to answer your question.
Anyway, check this video where Veritasium takes a fire into simulated zero-G: https://youtu.be/xdJwG_9kF8s
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