On Earth, a candle flame is orange and shaped like a teardrop. Hot air is light, so it goes up and pulls the flame up. In space, hot air does not go up, because there is no "up."

So a candle flame becomes round and blue. It burns slowly. On the space station, scientists also found a strange flame. It looked like it went out, but it kept burning without light.

Why is a candle flame on Earth shaped like a teardrop? The air around the flame gets hot and becomes lighter, so it rises, and fresh air flows in from below. This movement, called convection, stretches the flame upward.

In microgravity, however, hot air does not rise, so there is no convection. According to NASA, flames in space become round, blue, and produce little soot. Oxygen reaches them only slowly, so they burn more weakly and much more slowly.

In 2012, an experiment on the International Space Station showed something surprising. A burning fuel droplet seemed to go out, but it kept getting smaller. An invisible "cool flame" was still burning it.

The familiar teardrop shape of a candle flame is a product of gravity. On Earth, air heated by the flame becomes less dense and rises, drawing in fresh, oxygen-rich air from below. This buoyancy-driven convection stretches the flame upward and keeps it burning vigorously, while glowing particles of soot give it its orange color. In microgravity, heated air has no tendency to rise, so convection disappears.

NASA explains that flames then become nearly spherical, blue, and largely free of soot. Oxygen then arrives only through slow diffusion. Experiments aboard the Space Shuttle and the Russian station Mir confirmed that candle flames in these conditions burn far more slowly and more dimly. An even more surprising discovery came in 2012 during droplet combustion experiments on the International Space Station. A flame burning heptane appeared to go out, yet the droplet continued to shrink.

Researchers concluded that a "cool flame," burning at temperatures far below those of an ordinary flame, was sustaining combustion invisibly. Such behavior is hard to observe on Earth, where buoyancy interferes. Beyond satisfying curiosity, this research matters for spacecraft fire safety, since small, pale flames and smoke that does not rise may be harder to detect.