Ever seen white phosphorus? It takes very little heat to ignite, and burns on contact with oxygen. A phosphorus sun is an experiment that involves filling a glass vessel with pure oxygen and lowering phosphorus into it. It doesn't always go well, but at least it's always pretty.
If there's anything you'd want to combine, it's highly-reactive chemicals, heat, pure oxygen, and a lot of glass — so you can see why the phosphorus sun caught on as a science experiment. Generally undertaken by science teachers and anyone who enjoys spending the day manufacturing hazardous chemicals, it has a definite aesthetic appeal.
The phosphorous in the phosphorous sun is white phosphorous. Red phosphorous, generally used on matches, is a placid type of phosphorous that, as a molecule, resembles a warped set of monkey bars. It takes quite of bit of heat to ignite. Black phosphorous looks a bit like a complicated pattern made by outlining a lot of bow ties. It doesn't take as much heat to oxidize, but does take quite a bit of pressure. White phosphorus looks like a group of tetrahedrons, and requires the minimum amount of temperature and pressure to burn. So, naturally, white is the allotrope that everyone's interested in.
To make a phosphorus sun, a spherical bowl is pumped full of oxygen, and a lump of the waxy substance is lowered inside. After that, all it takes is a slight nudge from a warm metal rod, and the phosphorus starts to burn. The smoke from the burn fills the bowl, at first rising up, then cooling and curling down in tendrils. The continued burning of the phosphorus lump lights up the smoke, and the combined effect looks like a beautiful, swirling sun.
The experiment is prized by science teachers and was prized by alchemists — who thought it was a first step towards making gold. The modern day teachers manage to do the experiment well. The alchemists, not so much. If they weren't burned or suffocated, they were often poisoned by the exposure to phosphorus. This is definitely an experiment to admire from the sidelines.
Top Image: Robot Byn
Via Popular Science and Jefferson Lab.