The shape of a candle flame is largely the result of gravity.
Combustion generates hot gases that rise. The rising gas pulls fresh air-oxygen to the base of the flame by convection. The fresh oxygen allows combustion to continue and sustains the flame. The result is an elongated flame with different temperatures in different parts of the flame. The coolest part of the flame is the blue at the base, with hotter areas in the middle and near the top.
Hot gases rise because hot air is lighter than cold air. Without gravity, lighter air would not rise, because, well, nothing has weight without gravity.
In the absence of gravity, fresh oxygen would have to feed the flame by diffusion rather than convection. Diffusion is a much slower process. The result is a weaker, cooler flame that is spherical rather than elongated.
The photo compares a candle flame on earth versus on the International Space Shuttle. Few things are as rewarding as a good experiment.
Although the temperature of the flame is lower without gravity, the wax is burned more completely, generating no soot and much less carbon monoxide.
