A candle flame in zero gravity (as on the space station) is quite different than a candle flame down here on Earth. In zero gravity the flame is spherical, blue, and centered on the candle wick.
A typical flame produces light, heat, carbon dioxide and water vapor. Heat causes these combustion products to expand, lowering their density, and they increase due to buoyancy. This allows fresh oxygen containing air to enter the flame, further fueling the combustion process.
Because gravity is necessary for density differences to occur, neither buoyancy nor convection occurs in a zero gravity environment like space. As a result, combustion products build up around the flame, preventing a sufficient amount of oxygen from reaching it and sustaining the combustion reaction. Finally, the flame goes out.
According to Dr. Forman Williams, the lead scientist in the flame extinguishing experiment on the space station: “In microgravity, hot gases do not rise, so an entirely different process called molecular diffusion, drives flame behavior . “
