Science Art: Coronal plasma on an ionization tube in operation, by BentaxGermany, 2013

Scientific illustration - well, a photograph, really - of plasma forming around a vacuum tube, by BentaxGermany

Scientific illustration - well, a photograph, really - of plasma forming around a vacuum tube, by BentaxGermanyClick to embiggen vastly
If it looks like a miniature sun, maybe that’s because on one level it is – it’s creating plasma, which surrounds it like a corona around the sun. This plasma is being generated by an electrical current, of course, and not a vast fusion reaction in space.

You can read more about what’s going on in this page on how ionization tubes work in an electric circuit. They’re related to neon lights – fill what would be a vacuum tube up with a particular gas, and an electric current will excite that gas until it starts to glow.

I found this photo on Wikimedia Commons.