Blue Fire in Space

Marianne Dyson, October 2021

As the holiday season approaches, many of us decorate our tables with candles. If we did that in space, they would burn round and blue, not pointy and yellow. Why?

Candle flames heat the air around them. The hot air rises because it is less dense and thus weighs less than cold air. This air movement shapes a candle flame into a cone with a point at the top (see photo). Flames are blue at the base where they are hottest, and yellow above because as the hot air rises it takes some of the unburned wax with it in the form of smoke and soot.

Flames on Earth are cone shaped and blue at the base and yellow at the top because heated air rises. Marianne Dyson photo, 2021.

In space, hot and cold air weigh the same: nothing. Heated air just expands outward in a sphere. Because the air doesn