For thousands of years, people have been mixing the oxygen-rich air of Earth with an almost endless variety of fuels to produce hot luminous flame.
Flames are hard to understand because they are complicated. In an ordinary candle flame, thousands of chemical reactions take place.
The familiar teardrop shape of the flame is an effect caused by gravity. Hot air rises and draws fresh cool air behind it. This is called buoyancy and is what makes the flame shoot up and flicker.
But what happens when you light a candle, say, on the International Space Station (ISS)?
Recently, Williams and colleagues were doing an ISS experiment called "FLEX" to learn how to put out fires in microgravity when they came across something odd. Small droplets of heptane were burning inside the FLEX combustion chamber. As planned, the flames went out, but unexpectedly the droplets of fuel continued burning. They seemed to be burning without flames.
Ordinary, visible fire burns at a high temperature between 1500K and 2000K. Heptane flame balls on the ISS started out in this "hot fire" regime. But as the flame balls cooled and began to go out, a different kind of burning took over.
Cool flames burn at the relatively low temperature of 500K to 800K and their chemistry is completely different. Normal flames produce soot, CO2 and water. Cool flames produce carbon monoxide and formaldehyde.
It is possible that some unidentified flying orbs/spheres are related to ‘cool flame’ processes? sciencenasagov